92blue

iTrader: (1)

As many of you guys know... barely anyone has a 90-92 gauge cluster with a tachometer that reads properly. They almost always read too high. There has been a multitude of posts regarding finding a solution or a fix for this, but to my knowledge, no solutions so far.

For those of you wondering about calibrating an 89 and earlier tach, Morley has an old post that outlines the repair, which is a simple capacitor change.

For those of you that haven't already read it, you might want to read the following thread as it has tons of useful info regarding this issue: https://www.thirdgen.org/forums/elec...aayyy-off.html

ANDYZ28 has been nice enough to mail me a couple of tachometer boards. I have done some research with his boards as well as a couple of others that I have picked up from a local junkyard.

Before I get into what I have found, here is a little background info:

A tachometer works by converting pulses received from the vehicle's ignition system into needle deflection on the gauge. A pulse is generated each time a spark plug fires. On an 8 cylinder engine, there will be 4 pulses per revolution. On a 6 cylinder, there will be 3 pulses per revolution. Inside the gauge cluster, there is a small circuit board which is responsible for controlling the tachometer's needle deflection. Naturally, there are some differences between a 6 cylinder tach board and an 8 cylinder tach board. The 6 cylinder board needs to consider 3 pulses to be equal to one revolution, while the 8 cylinder board needs to consider 4 pulses to equal one revolution.

According to this articleTachometer System by Oliver Sholz, the calibration for the tach board depends on one resistor and one capacitor, both located on the tach board. Calibrating a tachometer that reads high would just be a matter of locating the resistor and capacitor on the board, identifying which has failed, and replacing it.

With that in mind, I've attempted to find the location of the resistor and the capacitor on the board. I am fairly confident that the resistor is part of the "thick film resistor chip", or "resistor network" across pins 4 and 10. This is the large white chip with black markings on the top left of the tach board. The capacitor mentioned previously would be the one on the top right of the board.

92blue

iTrader: (1)

The resistor network consists of 6 resistors. Here is a schematic:

92blue

iTrader: (1)

Here are the resistances for each resistor within the resistor networks for 5 tach boards that I have tested:


Tach Board 1
This is a board that I picked up a long time ago, and don't know if it was from a 6 cylinder or 8 cylinder application. It pegs the tach as soon as the car is started.

R1 = 4.90k
R2 = 3.81k
R3 = 1.90k
R4 = no continuity
R5 = 4.28k
R6 = 2.94k

Tach Board 2
Board was mailed to me by ANDYZ28 (thanks!). It is supposed to be from a V8 application, and does not work either. It pegs the tachometer instantly on engine startup.

R1 = 5.46k
R2 = 3.83k
R3 = 1.92k
R4 = no continuity
R5 = 4.33k
R6 = 2.95k

Tach Board 3
Board was mailed to me by ANDYZ28. It is supposed to be from a V8 application, and it reads high. It reads 2100 RPM at an actual 800 RPM idle, and reads 3750 RPM at an actual 1500 RPM.

R1 = 4.09k
R2 = 3.84k
R3 = 1.92k
R4 = 475k
R5 = 4.32k
R6 = 2.96k

Tach Board 4
This was pulled from a 6 cylinder application. It reads 2500 RPM at an actual 800 RPM idle, and reads 5000 RPM at an actual 1500 RPM.

R1 = 4.62k
R2 = 3.81k
R3 = 1.90k
R4 = 495k
R5 = 4.29k
R6 = 2.94k

Tach Board 5
This is the original board from my daily driver. It is for an 8 cylinder. It reads high, but not as high as the others. It reads 1200 RPM at an actual 800 RPM idle, and reads 2450 RPM at an actual 1500 RPM.

R1 = 4.64k
R2 = 3.81k
R3 = 1.90k
R4 = 302k
R5 = 4.29k
R6 = 2.93k

92blue

iTrader: (1)

Both of the boards that peg the tach have no continuity across resistor 4. Also, regardless of the number of cylinders, all boards seem to have the same resistance values across resistors 2,3,5, and 6 (with the exception of resistor 1, although most boards are relatively close within 4.6k-4.9k ohms).

Interestingly enough, resistor 4 coresponds to the resistor on Oliver Scholz' article that is responsible for the needle deflection (in other words, its responsible for the tach calibration).

If you notice how much error is between boards 3 and 5 (the only 8 cylinder boards that work), the higher the resistance across R4, the higher the error. Board 3 is relatively close to board number 4 which is for a 6 cylinder application.

The error on board 3 seems to be linear.
800/2100 = .38 and 1500/3750 = .40

The error on board 4 seems to be linear.
800/2500 = .32 and 1500/5000 = .3

The error on board 5 seems to be linear as well.
800/1200 = .67 and 1500/2450 = .61

I really should have taken more RPM error samples at several higher points, as 2 points isn't really enough to determine if the error is linear or not.

92blue

iTrader: (1)

I'll be posting later with some math on what the resistor values are supposed to be. I don't have time to work it out now. If anyone has any ideas on any tests they would like to see, just mention it. I don't know how to test the capacitors, so if anyone knows how, I'd like to know (I should have paid more attention in physics class...). Otherwise, I'll pull out a physics textbook and do some reading.

At this point, I am going to solder in a resistor across pins 4 and 10 on the resistor network (R4 basically) . That seems like it will be the fix for the tach boards. Haven't worked out the math yet, but the proper resistance for 8 cylinders will probably be around 200k ohms.

NEEDAZ

Great work!
On testing caps, well the best way would be a Sencore Z-Meter. They aren't cheep. Some DMM have a capacitance function, but these are really only good for saying the cap is bad, not so good at saying it's good. What i mean is most give you an idea what the caps value is, but don't give you ant indication on leakage or ESR. They are sometimes referred to as sorting meters. There good if you need to sort caps, but so good at testing them. The good part is most leaky caps will read a lower value when checked. You can also get an idea about leakage by checking the resistance of the cap. If discharged when you connect the ohm meter it should start are a low resistance and 'ramp up' to an open. How fast it 'ramps up' will depend on the value of the cap. Don't have you fingers on the leads. But this is still just an indication of leakage. A cap may measure open, but still end up being leaky. An old trick is to jump in caps in parallel with the cap when testing and see where things change. This can be dangerous though. You can go taking a cap, getting it charged on one part of the circuit, then taking it charged to other parts of the circuit. Discharge it EVER time, just short the leads. Any of this help?
Also, take the cap in the corner of the PCB out, and take the cap under the net-res out. Just need to lift one lead to remove it from the circuit. See if any of the resistances change.

92blue

iTrader: (1)

Thanks for the advice on caps. I think I get what you are saying. I just realized it may not be necessary to mess with any caps on the tach boards though. I had only skimmed through it before, but I sat down recently and read the datasheet for National's LM1819 air core meter driver and found some more details.

According to the datasheet, "needle deflection is always proportional to R2 (pins 4 to 10 on the thick film resistor), and ripple is proportional to C2 (capacitor I had mentioned before)". I did a search online for ripple, and all I understand is that there is some relation to input noise of some sort, kind of like having a rough sine wave (kind of like a stock market fund graph with a bunch of small peaks and valleys). Does that sound correct?

Later in the datasheet, ripple is mentioned again. It mentions that a typical tachometer pointer is about 3 degrees wide, and acceptable ripple ranges is 3 to 10 degrees.

Well, I was battling with a couple equations to calculate the necessary resistance instead of going with a trial and error method. I worked on that for a while, but found myself assuming too many values, as I had alot of unkowns to work with at once.

I'll post again when I have some time, but for the moment I jumped at the trial and error approach. According to some rough math (don't ask me how I got this number, don't recall), it would be 186k ohms. I bought some resistors at radioshack, and soldered one in. It was aiming at starting low, at 150k ohms first. Tried it in the car with tach board 1 (pegged on startup previously), and the needle stayed at the bottom of the tach, didn't move at all. Turns out instead of soldering in a 150k ohm resistor, I used 150 ohms.

Anyways, thats a sign that this project is on the right track... just got to buy new resistors of the right value...

swap350tbi

92blue,
I have been reading your notes on the 90-92 tach board, can anyone tell me what the signal looks like at the coil connection, I don't have a scope at home. I would think it would be positive going spikes, if this is true then you should be able to run this signal through a one shot and make a square wave. Once you get your square wave you should be able to run this through a F/V converter, apply the voltage to the tach meter and make the thing work you would need to know what full scale voltage is. My tach started reading about two times what it should, now when I start the car it will go full scale.
swap350tbi
John
Florida

NEEDAZ

I think I said in the other post we could work out a DIY drive. But it still would be nice to get this worked out. It's got to be a common problem. I'll look over that datasheet for the LM1819.
----------
Is the LM1819 isn't whats on the PCB in the other post. Is that whats on you PCB? What's the IC part# on your PCB?

swap350tbi

Needaz,
I looked up the LM1819 chip, it is not just a F/V converter, the data sheet calls it a function generator/driver. The meter movement that is used on the tach is an air core meter movement. It has two inputs, a sine and a cosine signal, one signal applied to one coil and one applied to the other. I tried to find a replacement part for the LM1819, NOT. I tried to find a circuit to drive a air core meter, NOT. The only thing is if you change the meter driver circuit then you will need to change the meter movement also. I did find out that Auto Meter tachs use the air core movement in their tachs. I think if someone can get this problem fixed he could sell them like hotcakes, everyone that I know has the same problem with their Camaro (90-92).
swap350tbi
john
florida

NEEDAZ

But is the LM1819 what's in the 90-92 tachs? If not don't bother with that just yet. It will be much easier to fix the driver PCB then DIY a new one. If needed there are other ways to move a air core movement. Someone in the other thread had a PWM driver going for one.

92blue

iTrader: (1)

I'm aware that the LM1819 is not on the tach board, but the design of the tach board is similar to it. Thats the reason why I've been giving some importance to the LM1819 datasheet. It has an explanation of how the circuit works, which I've found quite useful considering my lack of experience with electronics.

swap350tbi, I'm sure with a good background on the subject, it shouldn't be too difficult to fabricate something to drive the tach. However, thats beyond my abilities, or atleast I think it is. I don't know what the signal at the coil would look like, I don't own an oscilloscope.

Still haven't picked up some new resistors to try. I'll stop by Radio Shack soon, hopefully I'll have some good news to post afterwards.

swap350tbi

92blue,
When you took the resistance reading of the resistor network did you first remove it or did you take the reading while it was in the board? If it was in the board the resistance readings may not be the network resistor values, by the way they are used on the board there could be another component across the resistor, this would change the reading. I just removed my board and I will start some reverse engineering to see if I can get a drawing of the complete board. It may take some time but I will start tonight.
Swap350tbi
John
florida

swap350tbi

To whom it may concern,

I worked on this thing for about 3 days and I think I got everything on the board connected. I made a Microsoft file of the drawing one is a drawing of the resister chip and the other is a drawing of the tach driver board, by the way the pin 1 on the resistor chip is on the upper right-hand side. Found this out after looking at the pictures of the chip removed from the board. All the lines that are in red are on the back-side of the board. I talked to two KSC engineers with no luck but I will continue. Look at the drawings and tell me if you see something wrong.
swap350tbi
john
florida

Elephantismo

The tach has to have a single powered output right? Doesnt the needle respond to a change in resistance on the chip? So as resistance goes up the needle goes up? Put a potentiometer on it. adjust the **** till it works right, then measure the resistance across the potentiometer and and plug in a resistor with that value.
I'm tryin to get a spare cluster to play with. I'm also looking at programming a chip to take the input signal and output the signal the tach uses. Some guys around here already have burners to burn chips, theyll just need cradles for whatever chip works out best for the job. If i manage to write a prog taht works ill post it up and mention the chip i burned it on.

NEEDAZ

Swap350tbi, i don't see all the parts. There look to be 6 caps on the PCB and only 2 in your 'schematic'. Am I missing something?

swap350tbi

Needaz,
I did not remove any of the parts from my board, I did not know if they were resistors or capacitors. Here is a different drawing with the capacitors in and the resistors taken out. If someone has the values of any of the components that would help a lot. I know what the banded resistors are and large cap. In the middle of the board but that is about all I can come up with. I know that there are regular diodes and a zener or two in the 12 volt side of the circuit.
swap350tbi
john
florida

NEEDAZ

That looks better. The caps you're unsure of, there should be 3 number on each one. What are the numbers?

swap350tbi

Ok Needz,
I took one end of each cap off the board so i could read the numbers.
Looking at the board with the resistor net/work, top/left hand side. The first cap down K2G 221K FC1 9028, the next cap down on the left side K1G 222K CX3 9024, the cap right under the IC K1G 221K FC1 9028, the next one down 0,068µF ±10% 250V, now on the right hand side top cap R47UF (I think the U is a µF), and the one under it is K5R 104M CJ5 9027, I think the 90XX number is the manufacture date year 90, week XX, but I can not remember what the other number are.
UP DATE
Needaz,
Here are the values I came up with.
221K = 220pf
222K = .0022µf
221K = 220pf
104M = .1µf

swap350tbi
john
florida

NEEDAZ

Those numbers look right. You could drop the letter off the end as it's the tolerance, don't really need that. It would be helpful if one was better then the others. That's a good sign that it's important, or that they had an excess of parts in that tolerance in stock. But there's just one that is a wider tolerance.

Did a quick search and found this: Capcitor Value Codes
They'll sell you an ESR meter too...

GameRS

I'm curious as to whether anyone has made more progress in fixing our excellent tachs. It seems like one member was making great progress with using a new resistor in place of an old one which looks to be the problem with basically all our 90-92 cars. I think I speak for many owners when I say I would be eternally grateful to whomever finally cracks this case wide open! I'm not great with tiny electronics but would be glad to help however i can.

joe3931

Something to Try: You May whant to add a 10 Turn (or More) 500K ohm potentiometer in Parallel with R4 (Solter one end of Potent. on one side of R4 and whiper on other). Set Potentiometer Reading to 250K before installing (so it is about half way). Turn Engine on and measure RPM on Engine. Then Turn Potentiometer till you get same reading on Tackometer. Then Rev Engine UP and Compare Engine RPM with Tack Reading (To validate Linearity of Tachometer). If it matches; Turn Engine Off and Remove Potentiometer. Read the Resistor Value of potentimeter setting you needed to get Linearity. Install a Resistor with the value obtained above. If you get a lower RPM reading even if you set the Potentiometer to Max (500K); then you would need a Higher value Potentiometer to Get the proper Value.
As I think about it, It may be better to start off with a 2 Megaohm Potentiometer (10 Turn) since this would guive you a wider range of options.
Hope this helps Guys and thanks for the effort.

pilgrim

I have a spare dash cluster for a 1991 RS with 305 V8. Both the tach in this cluster and the one I'm running in the RS have the typical tach problem of reading much too high. I've been following the various threads on this problem for some months.

If anyone is close enough to an answer that using this cluster as a test bed would put them over the top to a solution, please let me know. I am willing to ship the cluster to someone who's close to a solution if it will help them solve it.

email: apowell@gocougs.wsu.edu

swap350tbi

All,
I do not think the resistors would go bad (may be wrong). In most of my electronic work I have seen the caps. go west but very few resistors. I am contuning to try to find a fix for this problem or come up with a complete new control board. I will be talking to a NASA eng. this week. I'm not going to give up.
John
swap350tbi
Florida

92blue

iTrader: (1)

Woohoo!!! Got the tach fixed....

I finally took out the wrong 150 ohm resistor, and soldered in a 150k ohm resistor. Its not 100% dialed in, but good enough for me considering how much it was off by. I'm too lazy/busy right now to try another slightly larger resistor.

At an actual 850 RPM, the tach reads about 800 RPM. At an actual 3000 RPM, the tach is reading about 2800 RPM. Thats roughly 7-8% error on both ends. I still think the proper resistance is going to be near the 180k-190k range. I really suck at soldering and it takes me a long time, so I'm going to let it stay as it is.

92blue

iTrader: (1)

Those of you that want to fix your tach.....

Take your cluster apart, and gently remove the tach board. It will be the circuit board located underneath the tachometer. You will see a large 14 pin white and black chip on the tach board. This is referred to as the resistor network.

Take a look at how the pins are numbered in the picture below...

92blue

iTrader: (1)

Disregard the picture above, the numbering for the pins is too hard to see. This one should be better....

The picture also shows the tach board with the newly added resistor.

92blue

iTrader: (1)

Grab a voltmeter, and set the scale so that you can read over 100k ohms. Check the resistance across pins 4 and 10. If you have infinite resistance (no continuity), then the fix will be easier.

Infinite Resistance:
All you will need to do is solder a resistor across pins 4 and 10. I used a 150k ohm resistor, and the tach reads 7-8% lower than actual RPM. I believe the proper resistance will be close to 180k-190k ohms (More resistance makes the tach read higher, less resistance makes the tach read lower). Trim the resistor ends a bit to shorten the length, and then bend it in a U shape. Be sure not to get solder onto the adjacent pins. If you are not comfortable soldering, practice a little first before trying it on the tach board. Once you solder the resistor on the board, you can reinstall it into your gauge cluster. You might need to bend the resistor slightly to the side for adequate clearance once you go install it. Start your car, and you should notice your tach reading properly now.


Finite Resistance:
If you have resistance across pins 4 and 10, you will first need to isolate a portion of the resistor network. Using nippy cutters (available at Radio Shack), cut pins 4 and 10 very close to the circuit board. Carefully pry the pins upwards, away from the board. At this point, you can solder a resistor (again, I used a 150k ohm resistor which reads 7-8% low, a 180k-190k ohm resistor will be better) onto the contacts on the board which used to attach to pins 4 and 10. Be very careful not to get solder onto the adjacent pins. Also, the pins that were cut/bent out of the way must not touch the resistor that you installed. Once you soldered in your resistor, you are good to go. Install the tach board back in the car, and enjoy having a tach that actually works.

Note: The resistor values mentioned will work for a V8 application. I don't know what the proper values will be for a V6, but the fix should be the same.

GameRS

alright, when I fix mine I will try something a little bit higher and tell everyone how close it is. Thanks alot 92blue! I think your little write-up should be a tech article.

Elephantismo

Option two: instead of using a resistor at all, solder in a couple leads to those terminals and run wires to a remote spot and solder a 1M ohm potentiometer (adjustable resistor basically) in. Put a timing light with tach on the car and adjust the potentiometer until the tach sits on the correct rpm. Idle is suposed to be around 800 if you just want to ballpark it.

You can then leave the potentiometer in or solder a resistor or resistor series that matches the resistance measured across the pot in the location where the pot was. Potentiometers are availible at radioshack in the bins with resistors and such.

The resistance marked on the potentiometers and trimmers availible is the maximum resistance the device can produce. of the three terminals availible on most pots and trimmers, only two are necessary to set this up. Use a voltmeter thats got a resistance circuit to play with it and see which terminals to use.
Good luck to you.

GameRS

okay everyone, I soldered 4 resistors together across pins 4 and 10, and the total resistor value ended up being 185.4k. I can tell you right now that its still too low a value, because it says the tranny shifts at about 4700 rpm at wot now, but I'll have to check that again tomorrow. I have a tbi so I think its supposed to shift at 5000. Ill try something close to 200k soon.

92blue

iTrader: (1)

Yeah, using a potentiometer would be good to get the exact resistance you need. The only one I saw with enough resistance at radio shack was huge and rather heavy (heavier than the tach board itself).

How did you wire the resistors, in series or in parallel? If in series, then the resistance is additive. If its parallel, it works out differently.

Also, don't gauge how far the tach is by your shift point. Theres too many variables to get it right that way. I would suggest hooking up a scantool or laptop, that way you can compare the tach with actual engine rpm at a point higher than idle. The higher the RPM at which you analyze the two, the more accurate you will be.

GameRS

Well, the 185.4k resistor actually made it shift at 4300 rpm, and I have now found out that my tbi car should shift at 4500, not 5000 like I said earlier. So after some trial and error I think 201k is very close. I agree that it probably isn't the best way to calibrate the tach by using shift points, but I have no way of reading the actual rpms of the engine. I have a laptop but no way of hooking it up. Also, I wired the resistors in series, one into another using 2 100k resistors and a 1k resistor.

MisterZ

New member here...I just came over from J-Body.org to continue investigating a solution to my tach problem on an '88 Z24 (V6): Pegs at start up, then falls to zero. I will check my pin 4-10 resistance this weekend. The DIP resistor-pack looks the same, the circuit board footprint a little different. I still hope it falls in line with what you guy's are all coming up with. I'll let you know my findings after I get the meter out. The cluster is already setting in the back seat. Thanks to those invoved in this thread!

MisterZ

Okay, I just checked out the resistor network chip. I appears I have an open (measures infinity) R2 (3.8K) resistor. R1, R3, R5 and R6 all measure as others previously posted. I'm not sure about the notorious R4. Reversing polarity, it measures 27.5K one way and 82K the other so I'm obviously reading through other circuitry. If shunting R2 with a replacement resistor doesn't fix things, I'll nip legs 4 and 10 and recheck R4. Again, this is for an 88 V6 j-body (Z24). The circuit board is a little different but the resistor network appears the same. Also, for anyone interested, the Meter Driver chip on this board numbered: 9555A CS8816 J.

swap350tbi

Everyone,
I cut the land on the back of the circuit board and checked the resistance on R4 resistor and it was open. and in the above notes someone said that with no resistance for R4 the tach would peg. Will that is what happening to mine. I'm going to find a 200K 10 turn pot and wire it into the circuit in place of the dip resistor. I will put long wires on it so I can adjust it with the tach in place. will let you know what I come up with. You can look at R4 and see that it has been eaten away with time.

Swap350tbi
John
Florida

Elephantismo

i used a 1 m ohm pot from radio shack and ran wires through the vent slots in the back of guage cluster and set the pot in the opening behind the dash bezel under the headlight switch. The exact value i measured across the pot after calibrating the tach with a Sun invesitagtor unit(very big expensive machine, usually very accurate) was 196k. The tach was exactly correct at idle, 1500, and 2000 rpm. I have not soldered in a 196k resistor cluster yet cause the pot works perfectly atm. On my cluster i measure inf. resistance across pins 4 and 10.

Good luck all,
Tanner Powell

pilgrim

You guys have done great work. I'll make time in the next couple of weeks to try this on my daughter's 91 305 convertible. Sounds like resistance in the range of 196-200K ohms will be close enough for practical use - especially since she has an automatic.

GameRS

196k appears to be the right number, but my 201k is basically right on, and good enough for me. 5k doesn't seem to make much difference in these tachs.

MrDude_1

you cant go by shift points.
your transmission is NOT computer controlled. its based off several things, all mechanical.

if you want to scan your car, you just need one transistor, some wire, and a serial cable connector... the cable is really easy to make. then download winALDL for free, and you can scan and log your exact RPMs.



if you want to be able to bench test this, you can build a 555 timer circuit that makes the proper signal. i can dig up a scimatic or somethign if you need it. ive built one before to test and play with, but i havent worked on any electronic project in several months... btw, the signal is always high ( +12v) and is pulled LOW (to ground) when the engine fires, not the other way around.
once you have a signal generator, you can easily test the tach at home and compare it to a second meter.. like another tach, or a 'scope, ect..


in anycase, good job. im glad someone got on it...
i kinda lucked out on my car for the moment... its dead on when i feed it a 4cyl output... and that happens to be what my LS1 outputs stock.... but im sure eventually, i'll need to fix it properly.

swap350tbi

McDude_1
what is the pulse width. I know you get 4 pulses per rev. on a V8. I have a circuit for a 555 and a transistor to pull the coil to ground (snock's high voltage page). I don't want to put more than one pulse out at a time, but I want the pulse to be wide enough.

Swap350tbi
john
florida

Elephantismo

i would imagine the pulse width would be very narrow since it is only pulsing for the duration which the ignition is firing. It would have to be on(down) for less than 1/400 of a second @ 6k rpm in order to not have overlapping signals.

yuel

i swapped my 91 cluster out last summer and had the same problem with the tack, then i realized the problem and whalla its fixed. here is my solution and seem to be heck of easier solution and it worked for me. i just did it two weeks ago... i swapped out the old board off my 110 cluster and put it in my 145 cluster. done. but maybe that just worked for me??

GameRS

I think you just got lucky there. Mine has the 110 cluster and I had the tach problem.

MrDude_1

since its triggered by the edge of the signal falling, its really not critical.

swap350tbi

OK everyone,
Here is the latest on the tach fix. I found the resistor that I needed and cut the legs on the resistor network (pins 4 and 10). I replaced it with a 196KΩ resistor, replaced the dash in the car and it works. I ended up using a 160K and a 36K in series but that works OK. The problem that I was having with my tach was that it was pegged at idle speed. Hope everyone can use this information.
John
swap350tbi
Florida

BigBadLou

iTrader: (3)

I learned that duty cycle of the signal does matter a lot. At least on my tach. I hooked it up to a function generator and it would accept only certain limited range or duty cycle, otherwise it would just drop to 0. Especially at higher frequencies (RPM's).

That tells me that it has a lot to do with the conditioning condender on the signal input. I'll try to post a diagram of the chip later.

Lou

Elephantismo

Right. The pulse would have to be as short as possible as you approached higher rpms. I'm not really familiar with the exact dimensions of these signals but i can see how a longer pulse would begin to look like zero as the rpm got higher. The longer the pulse, the lower the max readable rpm.

92zcamaroperson

iTrader: (1)

Ok so my tach reads high. I took everything apart and have a finite resistance across all of the resistors. Between connectors 4 and 10 there is 214k of resistance. So hooking up a potentiometer in parallel would not work in my case right? I would just have to go ahead, snip the pins, and replace that circuit with a resistor with a value of 196k? Or is the only way to know exactly how much it would take is cut the old circuit and hook up a pot?

Elephantismo

it will still work, but you will need a potentiometer capable of a resistance below 214k (prolly more like 156k) to make the resistance neccesary.
resistors in parrallel add like to (1/((1/x)+(1/x)+(1/x)))= resistance where x is the value of the resistors you use. you can add as many as you like, just follow the same formula.

....................1
--------------------------------- = Resistance
1 .......1 .....1 .....1
--- + --- + --- + ---
X ......X ......X .....X


Good luck,
Tanner Powell