You cannot, do the resistor mod or send it to a member on the board for a fix.

 

No need to replace anything man. Start reading. starting at page 1 man.

 

*deleted - wrong thread

 

I've had the stuff to do this for a few months and finally got around to doing it. Worked out well. I used a single 196k resistor that actually measured 195. I removed the cut pins from the board and soldered from the opposite side as they were from the factory. While I had the cluster out (which you do not have to do for this repair, tach is held in from the front by 2 screws after you remove the lens) I did the trip odometer repair. Totally functional again! Props again to those who did the research on this. I bought a couple dozen of these resistors, I'll mail you one for $1.50 if anyone is interested, PM me

 

I just did two of these. One was a 8K L69 tach and one was a 6K V8 tach

Both used a .47uF electrolytic cap and a .068uf Mylar cap. The 8k used a ,0047uf cap on the tuning circuit and the 6k used a .0068. I then used a 250kohm pot and tuned them with a frequency counter and glued the pots.

 

Thanks so much for the research. I had been driving way off and had no idea. I feel like I have a new car now! Here are my readings:

Tach Reading- Actual- Difference
1200 700 500
2100 1200 900
3000 1800 1200

So its clearly not linear. I just measured 302k ohms across R4 and soldered a resistor and pot in parallel to avoid cutting. I aimed for 200k ohms based off this threads finding so 1/200 - 1/302 = 1/R. Thats about 600k in parallel. Put in a 550k and a 100k pot and adjusted it in the car then left it. Was dead on at 600k total so the resistance of the board must be somewhat negligible. Took an hour at most. Thanks again for all the help.

 

Hopefully the resistor network does not get worse or blow out causing you to take it all apart again.

 

so i just picked up a 91 Z28 and my tach is pegged at idle!!!! ??? it seems as if all of you are simply having issues with the tach showing higher than real rpms (1400RPM when really 800 etc..) but my issue is the second i start my car the tach is fully pegged whether its at idle or driving!??? am i having the same issues or is my problem something else?

also the guy i got the car from claimed he "rebuilt" the distributor... could that be a problem like maybe he did something wrong or used incorrect parts??

 

Could be either but most likely an open circuit between pins 4 and 10

 

I am getting ready to fix a few tachs. I measured the values for all resistors on the chips on all 4 boards and here is what I found:

Cluster from my 91 RS (cluster is for a V6, but I don't know if it is accurate since I did a V8 swap):
R1 4.31k
R2 3.85k
R3 1.94k
R4 292k
R5 4.34K
R6 2.93k

Cluster from a junkyard RS V6 (unknown yet if the tach is right for a V6, but it reads too high for a V8):
R1 4.61K
R2 3.81K
R3 1.90k
R4 260.7k
R5 4.28K
R6 2.94K

Cluster from a junkyard RS V8 (tach pegs instantly on start):
R1 3.95k
R2 3.84k
R3 1.94K
R4 3.84M
R5 4.33K
R6 2.96k

Cluster from a junkyard Z28 (tach reads about 3000-4000 RPM too high)
R1 4.41k
R2 3.84k
R3 1.92k
R4 832k
R5 4.32k
R6 2.95k

The magic number for V8's seems to be around 196k for R4. I'm going to try using about 190k resistors and a 10k pot in series to dial it in and check it against my multimeter that has a tach function.

I haven't seen a specific answer yet for the R4 value for V6 tachs, but I think it is somewhere around 260K like the JY V6 tach, and that would make sense since there are 4 pulses per rev on a V8, and 3 "PPR" on a V6, so the ratio of V8 to V6 1.33. Assuming the magic number for a V8 is 196K, then 196k * 1.33 = 261K. I don't have a V6 car or a function generator to test this on, but I plan to test this theory on my V8 and set my meter to V6 tach mode to see if a tach with ~261K across R4 will match the meter.

Based on all of the values I've read in this topic, I think the correct values seem to be (within about 5% tolerance):
R1 4.6k
R2 3.8k
R3 1.9K
R4 196k (for V8), or 261K (? for V6)
R5 4.3k
R6 2.9k

Only R4 seems to be critical to tach function, but the resistor traces for both R4 and R1 are exposed at the top of the chip, and those 2 resistors seem to have the most deviance, the other 4 are usually close in all of the measure tachs. I am wondering if R1 is off badly enough if could it also have an adverse impact on tach behavior? (just thinking out loud here).

I will follow up with the V6 info as soon as I get to test it out, probably early next week.

 

There is actually an RC constant that plays into tach calibration. I have a 8K V8 tach and a 6K V8 tach. They use the same resistor R4 value but have a different Cap for the swing range. I think I used a 200K multi turn pot to dial it in. Fortunately the caps tend to live quite well.

I think one was a 0.0068 uf and the other was a 0.0047 uf.

 

That's a good point. While I'm working on them I"ll check the caps to make sure they are still within tolerance and check their markings to see what value caps should be on each board.

 

The electrolytic should be replaced on all boards due to age, it controls how fast the needle sweeps.

 

I tried this fix. My tach was reading high around 4000 or so at idle. I measured all the res chip values everything was good except R4 was at 860k. I clipped the pins 4&10 and soldered 196k res. Tried it and I got no movement when revving the engine. I then resoldered the res this time from under side and tried again, got same. I was worried my tach was dead so I removed the 196k res and soldered the pins back to its original and plugged it back in and now it works but reads 1000 rpms low now.

 

Too high of a reading on the tach is usually caused by too high of a resistance value on R4, and the tach pegging instantly in cases where the R4 value is extreme (like a few Mega Ohms) or infinite (open circuit). Too low of a reading would be too small of a resistance value, or a short circuit.

Double check that nothing shorted out, especially if you had to re-solder the pins back on to the chip. It would be easy to accidentally solder-bridge something and not notice. I would also re-test the values on the chip to see if they have changed any. R1 and R4 are exposed on the top of the chip and are particularly susceptible to physical damage. If the resistor values are all good, I'd also check the capacitors on the board, because as Midias pointed out, it it the resistors and capacitors together that control the tach function and behavior.

 

I have checked all the resistors and they all check out. Solder connections are good. I think it might be the cap that's drifted in value.

 

That is very possible. Given the age of these clusters, I am a bit surprised that capacitor failure hasn't seemed to be that prevalent of an issue since caps usually tend to be the first components to degrade. The big capacitor in the center of the board is a Mylar 0.0068uf 10% 250v cap. The one that I believe has the biggest influence over the tach behavior is the tantalum capacitor at the top-right of the board (opposite of the resistor network chip, marked in the pic attached to the very first post in this thread). It is a tantalum 47uf capacitor I believe. I had trouble deciphering the markings, but it said "T110 K10% R47uf 15v 981A". I would check this one especially since I believe this capacitor works in conjunction with R4 on the chip to control the tach (someone please correct me if I'm wrong).

Midias, I checked a V6 and a V8 board, and those 2 capacitors mentioned above were the exact same between the two different boards. I have not been able to figure out yet what the values for the other 4 capacitors on each of the boards are, but I still suspect that the difference between a V6 and V8 tach driver board is just the value of R4 on the resistor network. All 4 tachs I have (2x V6 and 2x V8) go to a max of 7k rpm, with the only difference being where the redline is marked. Does the cap value come into play only if the max RPM marked on the gauge is higher/lower?

 

I think the V6 and V8 tachs use the same values for caps but different resistors. I only had 2 V8 tachs to look at. One I think from a LG4 that ends at 6k and another L69 that goes to 8k.

I THINK they V6 should be about 280k and the 4cyl would be 420K

Then to test the tach RPM = (((input in Hz) * 60) / number of cylinders) *2

 

Having a similar problem with my tahc pegging as soon as its given power. I tested my board and came up with; (for a V8)
R1 = 3.76k
R2 = 3.85k
R3 = 1.92k
R4 = x
R5 = 7.28k
R6 = 2.94k

R4 is obviously a problem and I plan on trying a 196k resistor in there as it seems to work for others. Not sure why the R5 is so high though. It seems much higher than everyone elses tests. Any thoughts?

 

That sounds very similar to something I saw when I measured R5 across the wrong pins on the chip. If you measure R5 across pins 7 and 9, your result is the measurement of R5 and R6 in series, which would mean the values of both would be added together. Your result for R5 looks like the sum of the values for R5 and R6. Make sure that you are measuring R5 across pins 8 and 9, and R6 across pins 7 and 8. I think you'll find that R5 will be within a reasonable range, and R4 will be the only one that is grossly wrong (pretty common). R4 is almost definitely the cause of your problem. Open circuit (infinite resistance) on R4 will result in instant pegging of the tach.

 

yep that is exactly what I did. R5 is 4.3 Much better, so hopefully that resister ion R4 is all I need. Have to get out to the store to pick one up and give it a try today.

 

It is an axial tantalum capacitor - T110 series, K is 10% capacitive tolerance, R - represents a decimal(.47uf) 15 volts and manufacture date code 981A(1981?)
http://www.datasheets360.com/pdf/-8928185433789013845

It is a hard to find capacitor.

 

Well, I cut the pins to R4 on the 2 V8 tachs and changed them to resistors in series that add to 196k. ~800 RPM at idle is a much more reasonable number! Unfortunately, the automotive multimeter that I bought does not seem to get along with my distributor. When I set it to V8 tach mode, I get unstable readings (the numbers flash too fast but seem to hover in between 10k - 20k RPM!). Even if I try to read voltage from the tach wire to ground, or the power input to the dizzy to ground it freaks out, but the meter will take stable voltage readings across the car's battery terminals or the pins for an indicator light... *head scratch*

Anyways, I was not yet able to confirm the exact R4 value for a V6 tach without another tool to verify it against. I will have to find something else to test it with, but I am thinking something in the 260k-280k range is probably correct.

 

Hello guys hoping someone will respond to this old thread gonna have my father inlaw an engineer tach in a resistor for me but rather than 2 100k ohm resistors thinking 1 200k ohm resistor might be better but looking for one noticed there are different watts? above someone had 2 1/2 watt 100k resistors any idea what watt for 1 200k ohm resistor??? and also see some people cut pin 4 on the top side and another cut pin 4 & 10 is this required?? I only want to to this once trying to get all the info I need cause I was asked what color bands and how many etc there are a lot of choices thank you for all your help!!!!

 

The wattage is essentially the max power the resistor can handle. The current of the circuit is so small most any resistor will be fine. 1/2 watt, 1 watt, 2 watt, whatever watt. Cutting it is necessary to remove the bad 4&10 resistor from the circuit if you are going to get to the ~196k ohm resistance. If you don't cut the pins, the bad resistor will be in the circuit and affect the resistance you want. So cut em, solder in a 200k to the board, and you should more or less be good to go.

 

if he is an engineer have him install a 1w pot and tune it so the needle reads 900 rpm at 60hz 12vpp sqare wave for a v8 tach. should be about 196kohm and cut 4 and 10.

 

He only has my chip no access to my car no idea what a pot is or where but I did already tell him a 196k ohm resistor between 4 & 10 think I can get away without the pot? and should I cut the top side of pin 4 & 10??? any pictures of this pot if you have already done your home work to simply tell me what to buy and where to put it with out trying a tune I'm really new to this was hoping he could just order this resistor and tach it in and be done 8) and maybe what watt resistor or pic of the color bands around it so he can identify it exactly! thank you for all your help really!!!!

 

What kind of engineer is he? A potentiometer is a pot. It is an adjustable resistor. Using a POT is not required but I used one for accuracy.

I think I used a 1W 250kohm pot then when I was done I glued the adjustment wheel down.

You can just tack in a resistor after cutting pins 10 and 4. Get a bunch of 200kohm leaded resistors and measure them. Find one that is close to 196K and put it in.

 

Thank you Midias I will tell him what to do with it and I'm not sure but he builds things and works with circuit boards all day and just thought he could do a better job with it and might even have a 196k resistor made to be exactly 196k ohms everything he does I know is from scratch and he has a lot of his own patent's so hoping I get this working!!! but if you have any pictures of this job you did I'd like to see this "POT" you are talking about as well.

 

Fairly sure I used this one

http://www.digikey.com/product-detai...4LF-ND/1088556

 

Hey Midias all I could find were 100k ohms one pack 1/4 watt and one package 1/2 watt but looking at this.. http://contact.ebay.com/ws/eBayISAPI...actUserNextGen would this work? all these carbon film, metal film 1&5% tolerances etc and don't even want to ask about the 3rd wire on the pot lol ill stick to just a resistor. What specific's of a 196k ohms do I need and is the listed one above ok to use???? thank again buddy!!!! "196K Ohm 1/2 Watt 1% Precision Metal Film"

 

Midias just ordered 100 of these hope this does the trick thanks again for all your help!!! 8) http://www.ebay.com/itm/221272677255...84.m1439.l2649

 

Should do the trick. 196 seems to be about the correct value. The circuit uses a Resistor and Capacitor combination to make the tach accurate. The problem is both items have a tolerance and that can cause small amounts of error on the tach. That is why some people opt to tune it with a pot

 

How do I find the number 4 and 10 pin on 88IROC tach
?

 

 

196 resistor soldered over 4 and 10 worked on my 91 Z28 too. I had to buy 100 of the little buggers on amazon.com. Hardest part about this job is finding the right angle to take out the gauge cluster over the steering wheel. I'm so grateful to this message board. You guys make this too easy. If anybody in the Pittsburgh area needs one, I have 99 left. It might cost you a beer.

 

I have a 91 RS with the 3.1L V6 and I noticed my tach was reading about 1K high recently. When it was my daily driver (10 years ago) it was reading accurate. I guess over time they can get off.

So I take out the cluster and the reading I got from pins 4 and 10 was 297K ohms. This was pretty close to a number I saw in an earlier post (280K). I cut the pins and soldered a pot to it and "guestimated" the idle at 650rpm (warm) and then measured the resistance of the pot and got 180K.

Does this seem low? I am going to wait until I can get an accurate reading of the actual rpms before making the permanent fix. What is the best/most accurate way to get the actual rpms?

 

You can get the rpm reading from an inductive (does not require external power) timing light that has a digital tach option.

 

another one fixed, easy to do

 

Mine did not work

Tach pegged startup and running. Did the 196K resistor and it showed idle rpm but rpm on revs would not go over 1400 rpm and actually swung back the other way to 0. Then some smoke came out from the OTHER board below the Speedo??!! Are those two circuits related? Anyonehave a diagram of the speedo side board to see what burned up?

 

Glad to have my tach fixed after seeing it lay pegged for so long.

Thanks for figuring out the fix! I bought a five pack of 100K ohm resistors and used the two with the least resistance to get close to 196K. On my initial test it pegged clock wise and went all the way around to zero ( which I thought was odd). However the fix works great and is more accurate then the aftermarket one I had. Testing the tach with a timing light the difference was negligible, its about 50 rpms off on the low end and at higher rpms its never more than 100 off.

 

Update on 82-87 tach reading too fast. This is my 1987 tach and I looked everywhere for that year and only found a site for monte carlos here and basically used that information to find the two traces that need to have two 100k resistors soldered to it. I had to take the whole facing off to get to pin 10 so I could snip it but it was reading approximately 1250 rpms at idle and now it reads 750 rpms at normal temperature(which may not be exact but it's very close).

 

More than one way to skin a cat I guess. I just desoldered the pins after cutting them and poked the two ends of the resistor through the holes and hot glued the resistor to the resistor pack. Soldered and clipped the excess wire. Single 196k resistor.

 

Definitely more than one way. Your way is more professional but all I have is a soldering pen and my skills are next to nil. Wish I'd have went to college for this kind of stuff. It just fascinates me and gives me such a well accomplished feeling to fix something like that so cheaply when a repair shop would charge so much.

 

I just wanted to say that I finally got my resistor in today (196k exact) and soldered it into the board and stuck it into my car today and it worked perfectly. Before i did this, the tach was reading 1k higher than normal readings, and would max out pretty quickly when you attempted to touch the gas pedal. I cut the 4 and 10 lead pins on the IC and soldered the resistor on the bottom of the board and measured the resistance, it came out to around ~183k. Im guessing thats accounting the resistance through the circuit of the board itself. But this worked out perfect of me. The pack I ordered came in a set of 2, so I have 1 left if anyone wants it, I will send it out to them for them to be able to fix theirs. Also as equally great, if anyone sends me their board, I can solder it in for them, but only if its a 90-92 board. I work in a factory where I use a soldering iron all day long so I am a professional.

Great job on finding the solution for this problem. I am super excited about it.

 

Awesome thread. I used the info here to fix the tach in my 1980 Z28. The resistor chip is exactly the same. Instead of just plugging in 196KΩ worth of resistors, I opted to buy a 500KΩ trimmer pot from Frys. Two for $1.99. It made adjusting the tach a snap and fixed my "high-read" issue.

This thread is a tremendous service aid!!

 

Hi everyone-

First post but long time user since I seem to always find what I need by doing a !!

Anyways, i realize this is an older thread, but wanted to post my fix just as an FYI to other readers.

I, too, had a problem with the tach reading waaaay too high. This is on a '91 Camaro, 305 TBI. My problem also turned out to be the R4 resistor between pins 4 and 10. Prior to the fix my approx readings were:

R1 4.6k
R2 3.8k
R3 1.9K
R4 793k
R5 4.3k
R6 2.9k

I then used a 100k 5w potentiometer in series with a 100k fixed resistor between pins 4 and 10....this gave me a variable resistance of 100k-200k between the pins. I then fired up my laptop with the WinALDL software and hooked up the ALDL cable. I adjusted the pot until the RPM reading on the laptop matched the reading on the dash tach. This turned out to be about 195k (pot turned almost all the way up). I then removed the pot and soldered a fixed 200k resistor between the pins and retested...all is well! Put dash back together and am now enjoying my now accurate tach!

This forum is awesome.

 

its there between the 2500 and 2750 RPM

Just wanted to add my twist to tachometer fix, it became clear for me that a trimmer potentiometer is the way to go especially if you have a V6 like mine with a different R4 value than the V8. Trim pots are small meant to be adjusted then left alone. But where to mount it? My 86 has the circuit board mounted to the tach no easy acess to the adjustment screw. Being lazy and wanting to adjust when mounted in the car I came upon the idea to drill a tiny hole in the tach face for the trim pot screw then epoxied the trim pot to the back of the tach face. Its not noticable unless its pointed out. It made for an easy adjustment. Now you may not want to do this to you your tach but I like the idea of being able to adjust in the future if it should go futher out ouf wack.



I cut the trace and soldered to the back. 86 board is different fron the 90-92 but same idea




I used a 500k multiturn trim pot . you only need the center leg and one of the other legs for your connections

 

OK this is one thread I think may explain why I see 2600ish RPM at 70mph in my 91 RS with the 305 TBI and 700R4. I only thought about the odd RPMs now that looking at speed and RPM calculator to see if my torque converter was needing replaced or what manual transmission I wanted swap for. I will post my findings and repair later. By the way I find thread funny also, because I current in college for an AS in electronic engineering and did 16 years electron repair in the military. Thanks for keep this thread relevant.

 

Sorry to join in late! just got my third gen car running about a week ago. When it came to set timing I unplugged the white tach wire, not the tan EST wire. Quickly realized my mistake, and now I've got a tach stuck at zero. Nada. Wont move. Plugged the wire back in, still nothing. Car was sitting a while, so I put in a coil and dizzy, but still no tach. Its a Firebird 5.0, and I notice this is mostly Camaros. I'm going to pull the board today and see the resistance, Although it looks like most were pegged in the opposite direction from mine.