Hi I just fixed the tach in my 91 rs 305 using this method. i got a 180k 1 watt resistor and a 16k 1 watt resistor however i found a slightly easier method of attaching them. after you snip the legs on the chip solder the resistor's on the back side of the board instead, this will make it alot easier and less of a chance of soldering any of the other legs on the chip together. Then i used a piece of double sided tape to secure the resistors to the board to help keep vibration from knocking them loose. By the way this should be a sticky since it seems so many tachs are messed up this way.

 

Ditto on the soldering on the back. I did the same thing. However the total resistance I used was ~147.8k. I verified that with pot and a scan tool.

It seems that each resistor that I tried had a actualy value slightly less than what it was rated at and each was a little different. Thats why they give the +/- 5% accuracy. But I actually found a combination that gave me 147.8k of resistance. The resistors were rated at 100k+22k+22K+5.6K. At any rate im not sure why my tach needed 148k instead of 196K. My car is a 92 z28 with a 350. I tried 196K of resistance(again I had to check each resistor for actual resistance to find the right combo) but it made very little difference. The original resistance was at 214K. Im just happy that it works now!

 

all i have to say is that you guys are hard core lol... your work is very much appreciated and i have just finished fixing my tach because of this thread and saved myself some money for another cluster.... also i had a question, im guessing that there is no sort of supplier for new boards for the back of these gauge clusters that are brand new and are a dierect replacement correct?

 

Did you check the resistance while the car was running? In my experience there is some variance in the measured resistance across the pot or resistors if the car is running. Could be the source of your difference. Additionally, you may need a different value if the original resistor is not completely disconnected(infinite resistance).

 

How do you measure resistance of an element that's in a live circuit (engine running)?? *scratch* *scratch*
If using a plain ohm-meter, resistance can be measured only with the element (e.g. resistor) out of the circuit, with no outside voltage present. Or do I need to return my EE diploma?

Lou

 

You can give it to me so that I wont have to take any more classes for it...

And I snipped the pins completely then hooked up the pot.

 

All,
People fail to realize that a stock tach is there only used as an indicator and they are not accrete. If you have the tach move up as the RPM is increased then they are doing the job for which they were designed. I don’t think you can tune a Camaro with one. Must of the tachs in Camaros are either don’t move or they are pegged. The 196k resistor gets the tach working if your tach tach was pegged.
Swap350tbi
John
florida

 

"tachs in Camaros are either don’t move or they are pegged". Mine moves and it's not pegged!

I'm convinced my tach is not working properly. I have a '91 V6. Sometimes the tach will read normal idle, sometimes it will drop to 0 at idle and stay there until I give it some gas. Sometimes the tach will read normal at a shift point, sometimes it will read about 2K too high.

Any ideas?

What happened to the theory that the resistor mod was a temp fix for a failing capacitor?

I know it hasn't been long since you guys figured out the resistor trick but is it still working? Have you noticed any change in your "calibration" since it was done?

 

You are right. That principle is what i was trying to explain to 92z. He was saying he came up with a different value for his resistor that made it work other than the 196k that seems to work adequately for most. I was trying to tell him that because the ohm meter measures resistance by sending a set voltage across the resistor and measuring its "change" trying to measure while somehting else is also passing a voltage through the circuit will change the reading. When i measured mine while running it read lower... or higher than 196k; different either way. So was trying to explain that the car atleast needs to be off to check the resistance. I dont have an EE degree, im an ME, so my understanding is a little sub-par i think.

I forgot that he had said his resistor still measured 214k, thats prolly why. Not completely sure tho cause a 214k needs about a 2.3Mohm resistor in parallel to come out to 196k.
In your case it might not be a bad idea to check into distributor problems, Maybe a bad signal wiring coming from it, or maybe a bad ground somewheres. It seems like a low probability to have an intermittent problem in the guage module. I would say pull the module, check out the resistors and make sure they measure what they are suposed to and clean off the dust and the contacts. If you know how to check caps and have the equipement you can do that too. Then check your grounds and signal wires on the distributor.

Good Luck,
Tanner Powell

 

tanner you have helped me alot with my tach thanks alot for your electronics advice. I have a 91 z28 and the tach has been wacky since 2002. it's great to finaly have one that works. i set the potentometer to 196k and it works just like it used to. also we share the same last name, where ya from, if your dont mind me asking?

-Allen Powell
Ft. Lauderdale, FL

 

Don't know if anyone is still into this topic or not, but I've read the whole darned thing and have one question:

"My tach works, but reads roughly double and from what i gather soldering a 196K either across pins 4 and 10 or in lieu of pins 4 and 10 should solve the problem. I'm not huge into electrical, so I don't own a volt meter or other fancy tools to measure infinite or finite resistance, but I'd just like to know which would be my most likely course considering how my tach reads? Cut the pins and replace or solder right on top?" (I'm assuming, since it's neither pegged nor dead, that the resistor is just wearing out and I'll have to cut and replace)

 

f_bodyman, you are correct. Since your tach is not pegged, it doesn't have infinite resistance across pins 4 and 10. You will need to cut the #4 and #10 leg off the resistor network chip, and solder in your resistor across pins 4 and 10.

Don't recall how long it has been since the fix, but I have been driving the car almost daily without any problems. The accuracy has not changed at all.

I believe someone had asked about the original idea that the problem lies with a failing capacitor. Originally, I was under this impression too. I have seen other tachs get fixed with a cap change. However, I ran across a manufacturers data sheet for a tach driver board similar in design to GM's. The datasheet specifically stated that the needle fluctuation on the tach is controlled by a single resistor. There is a cap involved in the same circuit as this resistor, but it has a different purpose.

Testing several boards (or the resistors anyway, I didn't test caps) showed that the difference that the boards had in tach error coincided with varying resistance across the boards with this specific resistor.

By the way, this same method can be used V6 models as well, not just V8s. However, those of you with 6 cylinders will need to play round to get the right resistor value.

 

Gentlemen, I have just stumbled onto this thread (and the preceeding "waaaaaay too high" thread). I have a 92 305 B4C, nearly all stock, and over about 5 years I have watched it gradually go from roughly accurate to reading higher and higher to now being pegged at all times when running. I greatly appreciate all the work that's been to help solve the problem. I will have to wait a couple months (my tools are in another state!) to work on mine, but now I will know exactly what to do. I just wanted to add another data point about these failing tachs and to thank the folks that have worked out a fix. I'll post results when I get mine fixed, too.

 

alright i bought some resistors and put them in the way you guys said and it didnt help any exept instead of being pegged all the time it pegged on startup oh and i used a total of 175 k olms i hope i just want it to work so im not worried about being dead on but do you guys have any ideas

 

I just tried this fix on my 91 Camaro RS 3.1L V6 and it works great. The correct resistance for the six-cylinders is somewhere around 280k ohm. I cut pins 4 and 10 on the resistor pack and soldered in a 1M-ohm pot from radio shack on the back of the control board. The 1M pot is super sensitive to adjustments so once mine's calibrated against a timing light I'll probably but some glue on the pot. I didn't check my original resistance except on Pins 4 and 10, and that was 874k ohm. Previously my tach would show 3000RPM at idle and 7000RPM at a steady 55MPH cruising speed.

 

I have a 91 with a tach that would bounce between the correct reading and 0.

I pulled a card from one at the junkyard (reading was 780), put a 150, 22, and 22 resistor on the back (series) It read 190 rather than 194 (I guess that 5% accuracy). Replaced the stock one and voila, no more bouncing tach.

The readings look right/normal now. Plus, I now have a spare.

Thanks for the help . This has been bugging me for years.

 

I just put a 1 megaohm pot in mine and it works now. I havn't had a chance to see how accurate it is through the rpm range yet, but I will in the next few days.
BTW, mine was reading double and then a little more. Between pin 4 and 10 I was reading 468k before clipping the legs. I put the pot on about 8" leads and it sits behind the cover under the cig lighter. After I confirm its accuracy and set it correctly, I plan in gluing the **** on the pot in place.
Thanks to everyone who contributed to finding this fix. They could have been greedy and offered to fix everyone's board for money and made a killing, but chose to be helpful instead. thank

 

Oh, my reason for using the pot was my radioshack only had 100k, 10k, and less. So unless I was starting with 200k, I would be using many resistors. If the pot does not hold its adjustment well, I will measure it and swap wit resistors of the correct value.

 

First of all thanks for the good info guy's. I tried this fix in my 84 v8 firebird and it seems to work pretty good. I used 194k for the resistance haven't had a chance to see how accurate it is but it is alot better off than it was. I think I've also found out what the C2 capacitor in the diagram the controls. I thought that i'd try and replace mine but it didn't work as planed. I put a 47 uf capacitor in and the tach worked fine was accurate but it was extreamly slow responding. It took 15 seconds or so for it to adjust to the actual rpm. I put back in the old cap and it responds fine. I now reallize that it should be a 470 nf capacitor but it would appears that all that capacitor does is regulate change in the circuit to keep the needle steady from quick changes.

 

OK found this post about tachs..Usefull but gets beyond me a lil bit.N/P.Can anyone direct me to info for this problem...car started tach is pegged with lil or no deflection(pegged to the right).Car not running..tach in normal dormant non active position to the left.The car is a second hand 90 camaro with 3.1 v-6 mpfi.(i'm dealin with someone elses work and the resulting gremlins.Unsure is the engines original to the car or not yet only had it for few weeks.theres other issues but 1 step at a time and right now this and the rich butt exhaust are the things at this time that bother me the most.any suggestions ,help ,information and or direction to such of that nature would be appreciated.I plan to hunt all thru yer site here for what ever usefull or potentionaly usefull tech I can find soon as i have time down from work.Thanx Bill

 

take a while and read this thread in whole, there is a lot of knowledge to be gained.
In simplicity:
-remove guage cover clear plastic and pull the board in the bottom right corner out- there are some pics earlier in the thread.
-Get out your digital voltmeter and check the resistance on the pins of the white and black 14 pin chip.
-Pin 4 and 10 are the ones to be concerned about- see earlier pics for pin #'s
-Your tach is pegged upon startup, prolly infinate resistance(open)
-cut or unsolder the 4 & 10 pin and solder in a pot or correct value of resistors betweeen the 2 pins( if you aren't pretty good with a solder gun, don't even try unless you have a spare board, get someone else to do it)
=if there is infinate resistance, it is easier because you do not have to cut the pins, just solder to them
-you have 3.1l, so read back and find what someone said the correct resistance is, and if you trust that value, use that amount of resiatance, if not, use a pot to find the correct amount- the correct resistance for a v8 is about 200k give or take 5% depending on who you talk to
- reinstall and check it out
- connect to a scanner or somthing to read the true rpm of the motor and compare against your tach at several diff rpms, if you used a pot, dail it in until correct and either glue the stalk on the pot or measure the resistance and replace with correct value of resistors
Total cost= less than $5

 

OK OK basicaly ya sayin stop bein lazy.Read it and the recomended additions thru the thread and original one.Time ta make another salvage yard raid and trip ta the shed with my 5 yr box's or faster route RADSHACK for assorted elec parts.Also unfortunately quite familiar with the smell of solder and burned fingertips.Never could stand usein forceps or anything else to hold stuff when solderin.Shoot I must have near 50+ cb radios out in the shed and not to mention the crap that i have pulled outta boards and such outta couriosity.Unfortuneately my excuse isn't stupidity it's laziness and lack of motivatioin.Seen what ya all have so far and should be able to fix it.Hey???Any ya all in TN??Might be neighbors.

 

The caps are basicaly to smoth out the current (high and lows)so the needle doesn't bounce.The resistors are there to basicly scale said current to correct manageable voltages/values to move the needle according to RPM/pulses for scale on display.As to the white circuit board..One could basicaly replace it with resistors if one actually had the values.It would be rather bulky and not nearly as stream lined nor as cost efficient to mass produce.(which is why companies use I.C.'s instead of individuale components)However having the individuale components visable should make it easier for joshmo to recognize diagnose and potentialy repair.Now I know that this view is rather cromagnon but am I in the ball park?

 

Yup, you're in the ballpark.
The main reason for the white DIL package of the resistors is that the two of them that are plain visible from the top, are shaved in the calibration process.
That means that a person in the test dpt at the mfg plant hooks up the tach to a signal generator and by grinding down (shaving) the resistors, they slowly get the resistor values to where they need to be so that the tach shows correctly. Then they just spray it to seal the surface and it goes out into the world.

Another benefit of the shaved resistors is that they hold their value much longer than a trimmer potentiometer would.

Hope this helps.
Lou

 

Cool guess it's time ta break out the ole books and try ta build this puppy and see what I blow up this week.

 

I have 2 clusters and BOTH have a tach w/ a completely different board that you guys are talkin' about. Neither of them work on the LT1...totally DEAD...it HAS juice and ground and signal. I put an old school tach on it and at least it MOVES so I know the signal is right. Plus, my alarm accepted the tach signal, so I'm good. Any ideas? I don't know why these two clusters are the same, but diff. from urs...1 is a '90 V6 bird, the other is a 89 V8 bird.

Thanks!

 

Maybe different because the boards being discussed are 90-92 camaro?

 

If it's only related to Camaro's, any fix for the Bird's or know why it's totally dead?

 

I say the board is camaro, but might be similar to the bird. Got a pic of the board? It might be different, but should have similar components.

 

The boards shouldn't be all that different between the years. I fixed the tach in my 84 firebird. The board was a different layout from the one pictured here but the important thing is that the board has that white and black chip. That is where the calibration lies. You may find it helpful to look back at the original thread it has a link to the circuit diagram that you could use to find the differences.

 

That's the thing, it doesn't have the white/black chip like the one described. I'll have to get a pic on here to show it. It's just weird that both gauges have the same board, and neither of them even MOVE from 0. I'll post a pic. I'm going to try a few alternatives to still make it work, but be more accurate. If all else fails, I'll be back...Thanks

 

Does your board have the same plug in? Post a pic, I am sure someone has the answers.
BTY, I repaired another 1 today for my new cluster- my odo bit it and speedo was intermittant, been that way since before the tach fix though. I used a pot on the last 1, but put 2x 100k resistors in the new 1. Remember tat +-5%, both together gave 198k, shoud be about perfect. I will fire it up and check for accuracy when i put the dash and column back together.

 

I need to get a pic on here. It doesn't have a plug-in board like that...the motor is soldered "below" the board. It's kinda hard to explain. Anyway, I've looked all across the archives and havn't seen anyone refer to this type of tach. I'm just surprised it's totally dead. If I put juice right to the motor, it shoots all the way to max, so the motor works.

Anyone have any idea on what I'm talking about, w/ this board? lol...it's totally frustrating. The easiest thing that SHOULD work w/o mods is dead in the water.

Btw, I bought an aftermarket tach just to test the feed and everything, and that tach is dead on. I don't want to use it though b/c I want the cluster to all be the same, and not have different types of gauges. My total last resort is fabbing up a whole different gauge cluster w/ all aftermarket gauges, although I don't want to run new senders, etc. for all the other gauges that "work" already....ugh

 

I finally got around to doing this fix... it seems to have worked very well.


I used a 150k resistor in line with a 50k pot and it gave me a very fine adjustment. ended up dead on at idle (compared with my dfi view screen) with something like 187k. reads a few 100 off at 6k but I'm sure that's as good as factory gets.

thanks to everyone that contributed to this fix.

 

Sooo, WWDD?? I have to believe that a bunch of guys brought these back to the local Chevrolet dealer under warranty. What Would the Dealer Do?? was there a service bulliten on it?
I know this issue is resolved here thanks to great minds at work ( Thanks!!) but I would love to know what happened at Dealer end of it. Many cars have this issue ( thousands)

 

well the warranty was only 3 years and it happened slowly, my guess is not many got replaced under warranty. if so i'm sure they just popped in a new cluster.


plus mine was fine for about 10 years of normal, not till i put the msd on it, did it start to read higher.

 

My tach on my 91 will sweep from right to left and lays past redline whether the car is on or off. I'm going to be tearing down the cluster to see what the problem is. Luckily I'm an electronics tech in the Marines so if its that chipset gone bad micro soldering is an easy task.

 

i have to thank the people who figured this out. i just fixed mine and put it in and i have a tach again. i do have a problem though. i have a green light blinking between the two bottom gauges. i never had this before. what is it ? the light is square.

 

sorry .i figured it out. i didnt have the cover on the cluster. i had the 4 ways on. i guess i was excited about having a tach

 

Dayuum, this has been a very interesting thread as I have an '88 IROC-Z with a tach that reads like 1.5x the true value.

I've used my AutoXray to scan true RPM vs tach reading so it should be fairly simple to get the scaling resistor trim dead on.

BTW, is the tach board on the lower right after I remove the display front panel on the '88 IROC-Z too??

 

On your 88, you want to look at the link at the beginning of this thread for 89 and older tachs. Morley had posted a fix for the 89 and older units.

 

Hello, guys. I succeeded in repairing tachometer of my '90 Camaro RS by valuable information here. I really appreciate folks sharing information here.
Below is my summary to repair this kind of faulty tachometer:
http://camaro3g.administratorz.com/r...htm#TACHOMETER
Thank you again from Tokyo, Japan.

 

Hi everyone. I joined today, just to pull the pics from this thread, so I could be sure I had the right parts/pins for this fix. In went my resistors, and my tach now works. As stated in this and the other thread on this topic, it may not be exactly on, but it's close enough to get the basic information needed. I just want to thank everyone involved in figuring this out. I appreciate you guys. Thank you.

 

Hey guys, great work on the newer tach's. Has anyone worked on older tachs? I have a cluster off of 87. It reads correct at lower RPM and is way off in the upper RPM's. Reading about 6500 when actual ia 4800. The board on the back of the tach has a simaler chip like the one described on the drive board from the 91-92 tachs. 92Blue, do you think that the resistor between the 4-10 pins could be the same? The chip looks the same. The resistance I measures is 250k. It needs to be around 180-200K from what I have read.

Thanks,

 

I would recommend using the potentiometer like suggested earlier in the thread. I wanted to, but the only one I had was no-where near as sensitive as it needed to be, and it was sunday, and too far to go to the nearest radioshack (about 40 miles). With the potent, you can "dial in" the correct resistance, and either just leave in the potentiometer inside the dash somewhere that you can access it, or dial it in, then measure the resistance from the potent, and then put in a resistor(s) to achieve the same value.

 

Indy, it kinda sounds like your resistance there is a bit high. At some point the tachs seem to srat to get sensitive to changes and they show it more extreme up high. The potentiometer with the help of a more accurate tach will help you dial it in. I would recommend cutting the 4-10 resistor if you find you're able to get a better response with the potentiometer adjustment. I've been lazy and the potentiometer is still taped up in my dash, but it results in my tach twitching sometimes. Best of luck to you

 

Thanks Elephantismo,
I cut the 4-10 resistor and installed 2 100k ohm resistors. Final resistance was 195k. Seem to have done the trick. I have a S/W G.U.S. tach in the piller pod and they are almost identical after the fix. What kind of potentiometer did you use? I checked Radio Shack, but was not sure which one would work. BTW, What part of Texas are you in? I am in the Valley.

This is a great thread 92Blue, thanks for the research and keeping me from spending a grand on C2 Cobalt Autometer gauges, which were my next option.

 

Wow, It works like it used too!!!
Good work and research guys.
I used a 200k and it looks about right or atleast close enough like it was originally.

 

Hey guys, I came across this thead after finally experiencing the reading way too high problem only a few short months ago. My mechanic couldn't fix it, so I came back here.

I was just at Radio Shack, the second one actually stocked resistors (that first one was really shitty). Anyway, I know absolutely crap about resistors and electronics in my vehicle. While I was looking through the resistors I saw they came in 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.

 

Need to go to a real electronics specialty store. RadioShack doesn't stock that many.