ElectronicsNeed help wiring something up? Thinking of adding an electrical component to your car? Need help troubleshooting that wiring glitch?
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I bought a 100-peice set of 1/2 watt resistors. Spent a few minutes soldering them together and I finally got a perfect 196k ohm resistance. I soldered it onto the back of the chip after cutting those pins, and damn! The tach actually works much closer to normal now.
I know many have said that the factory tach is only a guide and not very accurate. But I don't believe it has to stay that way. So I am going to test my current tach reading vs a computer and adjust my resistors until I get it as perfect as I can. Making my factory tach accurate will be a much nicer setup than adding a second aftermarket tach... it fits with my whole sleeper look.
Also, I had originally started out with a potentiometer. I first used a 1 Meghom one but it was damn near impossible to fine tune... the smallest increment of change would send it way too far in that direction and would make adjustments incredibly tedious. So I use a 100k ohm potentiometer and wired that in with a 120k ohm resistor. That made the adjustments much more manageable. Either way, I had no method of accurately reading my actual RPMs at home. I will do the fine tuning part at my mechanic's shop over the next few days.
Thanks for all the hard work, guys... this is an excellent fix.
I was also thinking about pulling these chips from busted or soon to destroyed cars and wiring them like this to be sold as replacement when other people's tachs act up. Not really interested in making money off of it, but it would realy help out fellow f-body owners who experience this problem.
where have I been .. i read all 3 pages .. and Im still scraching my head.. some of this talk read like stereo instructions. Im good with soldiering but .. Im confussed with where to soildier them ... I saw the pic where it was bent like U shape .. and I saw a pic where it was scraped on the back ..
oh well I've been with out the RPM for this long .. whats a few more days.
Yeah, I was too. But when you pull out the cluster and see the board sitting right under the tach, it is actually pretty easy.
The board in this picture is right underneath the tach, it just pulls right out:
Do you see the connectors that he soldered the resistors onto? I just cut those two pins off (4 and 10) and soldered the resistors onto the board where the connectors went. But to make it easier, I soldered it onto the back side.
I bought one of these: http://www.radioshack.com/product/in...entPage=search at Radio Shack and I added up the numbers to get to around 196k ohm (120k, 56k, 15k, 4.7k = 195.7k) I cut the wires down and soldered them in a line, then I used the voltmeter to check the actual resistance (5% tolerance) and then soldered it into place on the board. I went to my mechanic for an inspection and had him check it against the computer, it is about 50RPM off. So I will switch resistors until I get it perfect.
I wish I could find a topic like this that covers the early tachs from like 82-84. With these cars nearing 25 years old, the tach's are almost always way off. When I got my 83 TA the tach read about double, I got another tach and its was ok, but then I got an L69 tach but now I'm runnig into the problem of reading high again. So I wish I could find good info like whats posted here that would apply to the early tachs since they are completely different in design.
I'm in an electronics class right now, which is in a room with about any electronics testing equipment you can think of, so I figure that if I'm gonna do this, then now would be a great time, especially since I have 2 extra boards to play with.
__________________
1983 Trans Am LG4 305, T5 w/3.73 gears; LT1 VERY soon....oh yeah FS:mildly modded 67 chevy 283 engine, 15X8.5 Torq Thrust D (set of 2)
Hadn't browsed through this thread in a long time, but its good to see alot of members have managed to successfully fix their tach.
By the way, the I'd suggest you guys browse through marry's link in addition to the information here. He has some clear pictures, and added some extra detailed info. If you missed the link before, here it is again... http://camaro3g.administratorz.com/r...htm#TACHOMETER
As a slight addendum to this... I have been having my tach checked by my mechanic whenever I put it in for some work (getting all the kins out and do not feel like messing with all that wiring and time intensive crap). Anyway, I started with 196.01k ohm and it was about 200RPM low at idle... obviously not where I want it. I increased the ohm to 197.21k and now I think it is about 10 RPM low at idle. I am going to tweak it and test it until I can get it dead on at 3000 or 4000 RPM. Then I will measure the actual ohm and see if it does the same result for other cars. Most people think of stock tachs as more decoration, but rather than add another one, this makes it much easier to just fine tune the stock one. I just wish I could hook up my computer to get the actual RPM... then I could get what I want in only a few minutes rather than weeks like the way I am currently doing it.
So if anyone can give me dummy proof instructions on how to measure RPMs on my notebook computer, I would really appreciate it.
You can use Winaldl. It's free and I believe can do what you ask. The interface cable is about 40$ but if you read their site, they tell you how to make your own, for just a few cents. There are other programs out there that can do the same thing, but I think Winaldl is the only free one.
For anyone still viewing this helpful thread, i have successfully repaired my tach (89 Iroc) by clipping pins 4 and 10 off, and soldering two 100k Ohm resistors in a series to bring me to 197.1k Ohms. Soldered them onto the board and tested it against my autometer tach. 99% accurate, revved in 200 rpm increments to 4000 rpm and it's dead on.
Resistors used were Radioshack
100 Piece
Carbon-Film Resistor Assortment
1/2 watt - 5% tolerance
...1/2 watt and 1/4 watt. The only ones I saw in 100k or similar were the 1/4 watt ones. So now I have a package of 5 1/4 watt 100k resistors... so if I solder two of them in line, and then cut those pins and solder the ends in place, that will fix my tach problem? I am just no 100% positive which resistors to use.
Thanks, and I apolagize for my question being so simple.
1/2 and 1/4 watt shouldn't make any difference in this situation. Take two of them, solder one of each of the leads together to make "one" resistor, and then solder the two ends to the pins. Should look like this:
---|ooo|---|ooo|---
Connect one end to each pin on the chip (there is no backward/opposite way with resistors) and you're done.
everyone of the 115 mph v8 speedos Ive had had the tach problem
i upgraded to a 145 cluster and the tach is dead on even used a external to check the most it goes off is @5000 it reads 5500
but idle is exactly on i can even set min idle with it
but my factory tach read 3000 at idle and shot off the scale if you revved at all
then i got a 2nd 115 and was ok for idle about 300 rpm off but then above 3000rpm engine speed it read something like 4400
so far both 145 units worked close to dead on
maybe they used a different tach board for the 5500 red line tachs?
i know my v6 115 speedo was also dead on
seems to only be a 115 v8 with the 5000 rpm red problem
needless to say I'm glad to see they found a fix finally
I would have to atleast partially disagree here. I have a 145 mph speedo in my B4C package camaro. The tachboard was the same as the one in all the pictures posted. Anyone with a malfunction tach in their 145mph board should be able to fix it just the same.
I would have to atleast partially disagree here. I have a 145 mph speedo in my B4C package camaro. The tachboard was the same as the one in all the pictures posted. Anyone with a malfunction tach in their 145mph board should be able to fix it just the same.
Merry Christmas and Happy New Year to all!
Yep, I had a B4c until recently and its tach was malfunctioning just like all these others. It got worse and worse until it just pegged the top end range as soon as you put the key in. I sold it before I fixed it, but I know this fix would have solved the problem.
Hello, all. My tach definately reads high and I was excited to see about the solution suggested, but I have a quick question before I go snipping away on my circuit board. Let me first admit that I don't know how to use an ohmmeter, but here's what I did. I set the meter to 200k ohms and measured the resistance across pins 4 and 10 on the suggested circuit. When I have the positive lead on 4 and the negative on 10 it reads 42.5 and when I reverse the polarity (black on 4, red on 10) it then reads 112.5. From what I have read above, anyone with my issue had a reading much higher than the around 186k it's supposed to be, so does anyone know why mine would be reading so low or am I doing something wrong? I appreciate your input, thanks! --John
John, I'm guessing you're probably measuring the resistance of the part still connected in the circuit. Once you snip at least one "leg" of that part off the board, you can measure the resistance precisely, without the whole circuit influencing the measurement. That's just one of basic rules of using a multimeter.
Hope this helps.
Lou
__________________ '91 Z28 convertible w/305TPI WC T5 (no more auto)
"Nothing screams more of poor craftsmanship than wrinkles in your duct tape!"
Add me to the list of very happy customers ! I had purchased another cluster off Ebay a few months ago to fix this problem. Guess what ? It didn't fix a thing. This repair cost me $0.99 plus tax ! Now I will fix the one I bought and resell it with a working tach.
A BIG thank you for sharing the repair info in this forum.
Ok I hope I am missing something here. I followed the instructions. I had 2 tach drives that were way off. Snipped the 4 and 10 pins and soldered 2 100k Ohm Radio Shack 1.4 watt resistors together inline. checking the resistance yeilds 195k Ohms. Soldered them to the backside pins of 4 and 10. Plug them in and now when I turn the car on the tach zeros out, but it never moves. When I shut off the car the needle relaxes and falls to below zero. Same results for both tach boards.
DOH! Nevermind. I'm an idiot. I bought 100 Ohm resistors not 100k Ohm.
My multimeter is auto ranging so that kinda screwed me up and far as not seeing it right away.
I double checked tha package after reading one of 92blue's early posts and sure enough I did the same thing.
Oh well, I'll grab the right ones tomorrow.
I just bought a 91 RS V6 with the tach reading about 400 rpm high at idle. You gentlemen have done a lot of work and research, but do not mention one of my symptoms. All of my electrical gauges read high. The 91 came with an extra instrument panel. I notice a lot of Camaro insturment panels for sale on eBay. I think the problem is not on the instrument panel, but in in the ground or power circuits or some other circuit that is off the insturment panel. I am guessing that a good instrument panel would read high. Just a thought. Thank all of you for your efforts and postings.
ok so what is the point of cutting the two pins from the board, i just recently soldered two 100k resistors to the 4 and 10 pin and the tach is working better but i can tell its off, and i remember someone mentioned that for a v6 i need 280k so i will have to try that next, but i was just wondering if there is a specific reason for cutting the pins, because i soldered the resistors to the pins without cutting them
Last edited by 91Camaro_RS; 07-21-2008 at 01:10 PM.
I believe the point is that you can correct the tachometer by adjusting the pins and resistors, but the real problem is somewhere else. I could understand it if the tach and other gauges were reading low. However, I do not know what to look for that would make the gauges read high. I suspect it is a very common problem. Hopefully somebody will get a fix.
ok well i just read thru this entire post and found nothing on Firebird tachs... the board in my 90 Firebird tach looks nothing like the Camaro and doesnt have the black and white chip that everyones lookin for on the Camaro board. anyone have any insite?
OMG Original Poster (and all who helped him) I owe you a hearty thanks and, should you ever find yourself in Birmingham AL, a tasty meal and a brew. I will be using this info to fix my tach this weekend as well as attempting to get winaldl and making a serial cable this weekend to check my work as well. Maybe my poor 91 rs isn't as hopeless a cause as I thought.
Hopefully when I pull my entire cluster this weekend all my problems will be as easily fixed.
I've put up with a bad tach in my daughter's 91 RS Camaro since we got it 2.5 years ago. I finally got around to pulling the dash and going after this repair. The car is a 305 audomatic with TBI. Indicated idle before this fix was about 2500 RPM, and as soon as we accelerated the tach was basically pegged all the time.
I've actually had the critical directions and photos from this thread printed for more than a year, but I hadn't gotten around to using them. Dang.
The sense I made out of this thread is that the best bet is 196K to 200K ohm resisance added between pins 4 and 10. Radio Shack is my best resource for parts, so I bought a .99 cent pack of 100K ohm 1/2 watt resistors - couldn't find smaller ones that would let me stack them to 196K...and I could only find 1/2 watt.
I had finite resistance, so I clipped leads 4 and 10, then carefully removed the pin leads from the board. I soldered two 100K ohm resistors in series, and then attached them from the back of the board - pretty easy when the existing pins were removed. I used a magnifying class to examine the work and found I had soldered between pins 4 and 5, but I scraped off the trace of solder and achieved a clean job.
With the way the tach PC board mounts in the cluster, there is LOTS of room to add more then one resistor. Very nice!
I re-installed the tach and fired it up. The first idle indication was about 900 RPM, although it climbed to 1200 or so - and since the car was cold, that didn't surprise me. But it is MUCH closer than it was.
My daughter loves the car and with an automatic, she doesn't need the tach to be accurate enough to make power shifts at 5000 RPM...she just wants it to indicate something like the real RPM. This is definitely close enough.
I think the 1/2 watt resistors will work, but if they burn up it won't be any big deal to order some 1 watt resistors, pull the dash again and replace them.
any chance anyone has an unmodified 90-92 tach they wanna sell me? i dont wanna mess with the one already in the car...id rather fix a different one then swap it out
Just remember, any replacement tach you buy will be as old as the one you're replacing. I've tried another replacement tach and it had the same identical problem.
TRUST ME on this - if you go back through this thread from the start, print out the critical directions and the photos of the actual circuit board, you will have the info you need.
My daughter's car has been running fine for two weeks with two of the 1/2 watt 100K ohm resistors from Radio Shack; therefore I believe they will work fine. Just buy a packet of them- they're 99 cents for a packet of 5. To make a 200K ohm resistor, solder two of them end to end so you have one continuous unit. Then you're ready to take out the dash panel.
The ONLY really tricky part of this is soldering the resistor leads in place. I used a 30W soldering iron and I have a fair amount of experience soldering.
Be very careful, because you CANNOT afford to melt the other leads. I clipped the existing pins with a pair of tiny cutters I bought at Radio Shack ($5), then heated the solder and got as much of it out as possible with a solder sucker (handheld device that sucks hot solder away), then loosened the old pin tips in the circuit board holes. I carefully worked them out with a tiny screwdriver so I'd have an open hole in which to mount the resistor leads. When re-soldering, I got solder to slop over onto an a neighboring pin, but I used a magnifying glass to carefully cut through the sloppy solder so the circuits would not be connected.
You CANNOT - I repeat, CANNOT - do this with a big Weller 200W automotive soldering iron. The tip is too big and it genereates too much heat. You need a small iron with a small sharp tip that will allow you to do the fine work required. Again, Radio Shack can provide this and it's worth it to fix the problem - and you'll have a new soldering iron, too!
Make sure to do the resistance test and know whether you have finite or infiite resistance between pins 4 and 10...that tells you whether you need to cut the leads or just add on the resistors.
If you don't have any clue about soldering, find out if there's a local repair shop that will do it for you. If you have a friend who dabbles in electronics, they may be able to do it as well. It's not that hard but it requires steady hands, patience and attention to detail.
Couple of questions.
a) First, I tested the pins with my multimeter set at 200k ohms, and got no reading, so I did the infinite resistance method. Later I went back and set the multimeter at 2000k ohm and got 360 reading. Should I have done the fix with the finite resistance method?
b) I soldered in two wires with ~156k ohms of resistors in them and then a 50k pot, which gave me the ability to adjust from 153k-196k ohms. Problem is, that total adjustment only changes the tach reading by about 100 RPM (700-800 RPM indicated). Datalogging tells me this is still about 200-250 low at idle. Reading the first page told me that increasing resistance raises RPM. Do I really need to go higher with my resistance or is my problem elsewhere? I will test the accuracy at higher RPMs tomorrow when the wife and baby aren't asleep. Maybe it will be accurate in the mid-high RPMs where I really want it.
Well, higher revs just multiplied the error to the tune of 600 RPM error at 4k. While I'm seriously considering your offer, Lou, I did throw another 100k ohm resistor in there (98.4k measured) just to see what happened, and it seems to have done the trick. But I am a little confused because that gives me a total of 251k to 294k with the pot adjustment. Me thinks I should have clipped the pins.... or maybe I'll visit BBLou.
Nevermind, fixed it. I used Marry's method. My problem was I should have done the finite resistance method as I later learned I did have 475 ohms across those two pins. The effect of what I did was ended up creating a parallel circuit, which at 270 ohms or so that I added actually did give me accurate RPM indications, but the tach needle did not have smooth operation. It was kind of jerky. I went ahead and soldered in that 50k pot so I can dial it in, then take it back apart and mix and match resistors to give me whatever I had with the pot.
BBLou, I will. Just had a baby girl, so life has changed a bit for me, but I'm getting back to my Camaro. I discovered that the jerky motion of my tach was just my tach, not the tach board. This led me to pull a tach out of my 91 RS, swap my z28 5500 redline tach face over to that one, and put it all back together.
Things I have learned over the past few days that may or may not help anyone.
a) Each tach will require different resistance to get the right indication. For example, I made two tach boards which were right on in my Z28, but well off in my RS (V8). I do not attribute this to the fact that one was a Z28 and one a RS, but to the amount of error in the components used by the factory. I even swapped the tachs, and I can guarantee that for the best possible accuracy, you must use a potientometer ($3 at Radio Shack). I put about 175k ohms of resisors plus a 50k pot which I was easily able to dial in both tachs.
b) If needed, you can actually swap Z28 (5500 RPM redline) tach faces with RS ones (5000 redline). It takes a lot of time, patience, a razor blade and some clear glue, but it can be done.
c) It's unlikely that you will get 100% accuracy from idle through redline on your factory tach. If you use the pot, then you can 'dial' in exactly where you want it to be accurate. On the two that I fixed, I 'dialed' in the ohms that gave me accuracy at 3-4000 RPM, but left me showing about 150-200 RPM high at idle (750). This is fine for me since I care about when to shift, but with the pot, you can dial it in for idle if you care more about low RPMs.
d) This fix can be done for about $20 if you have a multimeter. $8 radio shack resistor pack listed above in this thread and $10 radio shack soldering iron. $3 pot-radio shack-50k ohms. couple of bucks for solder.
e) Personally, I like Marry's method as you don't have to worry about trying to clip those pins and make sure you avoid touching them with your resistors.
Why don't one of you electronics experts write a tech article on this fix as I'm sure close to 100% of 90-92 camaro owners have a faulty tach, whether they know it or not. I would, but I think credit goes to you guys, not me. I noticed over on tpiparts they have one.
ok well i just read thru this entire post and found nothing on Firebird tachs... the board in my 90 Firebird tach looks nothing like the Camaro and doesnt have the black and white chip that everyones lookin for on the Camaro board. anyone have any insite?
I am with you I have a 91 305tpi t/a and my tach is reading very low with a slow response....I also have a T-5...dont know if it matters...but I have a cluster out of a v-6 car laying around and was wondering also if I could make it work with my cluster...
I am with you I have a 91 305tpi t/a and my tach is reading very low with a slow response....I also have a T-5...dont know if it matters...but I have a cluster out of a v-6 car laying around and was wondering also if I could make it work with my cluster...
ive searched everywhere i cant find anything on the internet about the Firebird/Trans am guages. im still interested in making this work too but i cant find anything on them
ive searched everywhere i cant find anything on the internet about the Firebird/Trans am guages. im still interested in making this work too but i cant find anything on them
I'm gonna keep looing and if I stumble across something I will let you know bro..just do the same for me..looks like we may have been left in the dark..haha
i used 220k a it worked good within 1% checking with scan tool idle. the first resistor i used was 180k and reading was over 350 rpm off from scan tool.
then i used 150k and it was off, more about 500 rpm
ill post some pics if someone would like to see them?
i install leds in my cluster this really wakes up the whole dash assy! pics are coming soon
I would like to say thanks to everyone for there input on this i fixed my tach with the info here. I used 2 100k in series for a total of about 196.8 with a meter and it works great and is just about 800 at idle. Again thanks!!
Now only if i could get the speedo to work i would be in like flint :} i was wondering if the board that is on the speedo side could have the same type of problem as i have replaced everything other then the speedo and the little board under it ? something i will look at and if anyone has any ideas please let me know.
Last edited by Fordkiller; 11-09-2008 at 02:45 AM.
Although mine was different than the above pics because its an 89 iroc, the resister was the same. Mine was nearly double the actual rpm. I put 2, 100k 1/2 watt resisters in series (on the back side of the board) after removing 4 and 10 pins and its dead on the money accurate. Thanks to all who took the time to find a fix to this.
is this the "pot" resistor i need? to do a quick fix tot he tach? i just want to be sure before i do this....??? im so confused by this i have to know everything before i attempt
No it is not. Why use a pot anyway??? All you need to do is buy a package of 100k-ohm resisters and run two between the pins. It doesnt matter what way you run them as resistors do not have pos/neg ends. I used 1/2 watt resistors on mine and it works perfectly. If you want to be able to adjust the gauge with a pot then you need to "test and tune" how much resistance you need. It seems most everybodys car needs roughly 180-220k ohms of resistance. So then you would run 2 resitors equal to 180k ohms and then between them run a 50k ohm pot. This allows you to adjust 50k ohms to completely dial in your tach.
And before you ask......no you cannot just run a 250k ohm pot because it is wayyyyyyyyy to touchy to actually dial in on the correct rpm.
So, this is what I and many others used> 2 100k ohm 1/2 watt resistors soldered together between the pins.
Mine is acurate within 100 rpms at 4000 rpms. Good enough for the girls I go with.
No it is not. Why use a pot anyway??? All you need to do is buy a package of 100k-ohm resisters and run two between the pins. It doesnt matter what way you run them as resistors do not have pos/neg ends. I used 1/2 watt resistors on mine and it works perfectly. If you want to be able to adjust the gauge with a pot then you need to "test and tune" how much resistance you need. It seems most everybodys car needs roughly 180-220k ohms of resistance. So then you would run 2 resitors equal to 180k ohms and then between them run a 50k ohm pot. This allows you to adjust 50k ohms to completely dial in your tach.
And before you ask......no you cannot just run a 250k ohm pot because it is wayyyyyyyyy to touchy to actually dial in on the correct rpm.
So, this is what I and many others used> 2 100k ohm 1/2 watt resistors soldered together between the pins.
Mine is acurate within 100 rpms at 4000 rpms. Good enough for the girls I go with.
Well.. like everyone here my z28 1991 has the same rpm problem....
I have bought several months ago a used cluster on ebay but dunno from which car he comes from, i just tried to put this one in my cluster to replace the electronic item but it's worse ! about 2750 Rpm instead of 750/800 idle. The genuine one is mistaken too but shows something like 2000 rpm if i remember right...
the electronics are not the same as you can see on this pics :
i understand i must put 2x 1 K ohm 1/2 Watts ... to correct the problem but because English is not my maternal language, i woul be sure i understand well please.
