Chevy86 IROC-Z

iTrader: (17)

Get a couple variable pots, snip pins 4 &10. Solder a lead to both 4 & 10 on the circuit board with a pot inline. Turn on vehicle, read timing with a timing lift and adjust the pot to the RPMs on the timing light. once your around the range, measure resistance on the pot or pots and get a matching resistor and solder in place on where pins 4 & 10 would go. This thread is dedicated to the procedure that I just explained.

BASSr

Hope you don't mind a quick question so I understand a bit more here.

Why between 4 & 10? Isn't that for R4 which seems to show a reasonable resistance but R3 & R6 don't?

R1: 4.28
R2: 3.84
R3: 1983
R4: 3.71
R5: 4.31
R6: 2947

Chevy86 IROC-Z

iTrader: (17)

Well it's a trial and error situation. Almost every issue with the tach maxing out has to do with the pins that I noted. If it makes it any easier for you, visit and online auction site and search for junkyards. Then contact the junkyard and ask if they have a Camaro with a cluster gauge. Ask them to pull the circuit board. Then you'll have a back up in case you mess up. But if you feel that the pins you noted are causing problems with your reading, by all means, jump in and geetter done. I gave you my advice. Now it's really up to you which route you wanna take. But it's clear to me that there is an error.

BASSr

Hope I can ask for some more guidance here as I am not very electronically inclined!

I bought a 1m pot at Radio Shack. I cut leads 4 & 10 on the circuit board but before I solder it in, I wanted to test it to see what resistance I would get from my ohm meter.

As per the pic attached, I put my ohm meter leads on the outer solder points on the pot. My ohm meter read ~2-3 ohms of resistance when I connect the two leads together not connected to anything so I assummed that the basic reading (variable switch on the pot turned completely counter clockwise) I would get a little larger reading, which I did ~3.8.

Now when I turn the know clockwise I get various inconsistent readings. I would have expected the readings to go up smoothly.

What am I doing wrong?

jtsk

iTrader: (3)

BASSr, the reading you are getting may be within the +/- 20% of the device. Be sure you zero your ohm meter before you take any readings. As for the smooth rise in readings, I am not sure, but I think as you do on it that it should be a fairly smooth rise.

Hunter0075

Is there a coverter to make the tach read right

jtsk

iTrader: (3)

You mean to read right after you soldier in the pot?

BASSr

OK, so I tried to do this process but seems like I still need some guidance here!

I soldered leads to the outer connection points on the pot (not sure what the center one is for??). Soldered those leads to pins 4 & 10 after I cut them from the onboard resistor pak. Leads are soldered directly to the back on the circuit pak on the printed circuit board.

Put the circuit board back in the cluster, installed it in the car, started it up and the tach read 4.0k right off the bat. Ok, better, but when I turn the pot know, it makes no difference in the tach reading. According to my tach/dwell meter the car is running at ~1200 RPM (still cold).

So why is the pot not making a difference in the tach reading?

What am I doing wrong here? :-)

jtsk

iTrader: (3)

The 3rd point looks like it could be for a switch as I look at your drawing. I don't know why your pot wont lower the RPM any more unless you do have other resistors bad??? If so, it's not a total loss because 4-10 should read 196k, so it's a matter of just soldiering in that value across 4-10 and it's fixed. I never read any of my resistor values before the fix because it looked like the majority of the fixes on here were for the 4-10. If you can't get the pot to function as desired, I would advise soldiering in 196k of resistance across 4-10 to see where you stand. I got all my resistors at Radio Shack for like $5, but I know you can get them elsewhere too. Can you post pics of what you have so far?

BASSr

Thanks.

I'll just try adding some resistors in line until I get the tach RPM to read the same as my tach/dwell meter.

This is the posting I made earlier in this thread from the test I did before I started:

R1 - 4.28
R2 - 3.84
R3 - 1923
R4 - 3.71
R5 - 4.31
R6 - 2947

As I stated earlier, what is confusing to me is that R3 & R6 are very high, so not sure why I would be screwing around with R4?

BASSr

Hey, BTW, I thought there was a post here somewhere with a URL so you could get the color coding of resistors? I looked and did not see it anywhere?

Anyone know where to find that? I have a ton of resistors from my Dad, so I probably don't need to buy any (they are probably better than the ones ones you buy at Radioshack as well, came from his stuff over 50 years ago!).

85irocinjected

^^^ Here ya go..
http://www.hobby-hour.com/electronic...calculator.php

85irocinjected

Also I want to say thanks to 92blue for his work and everyone else. The schematic of the resistor network was a great help. I just yanked the resistor network off my tach board and soldered all new resistors on in place and my tach works GREAT now. Finally!

The car is a 91 RS v6 and the tach was all the way tached out at all times. Whether the car was on or off. Now it works great!
The resistances I used where as follows.

R1: 4.62k
R2: 3.80k
R3: 1.90k
R4: 186k
R5: 4.30k
R6: 2.95k

I have not gotten to verify how accurate it is yet because I literally just put the cluster back into the car but on a cold start the idle was sitting at 1000rpm which I think is pretty damn accurate.

Thanks again guys for the work. Finally a REAL WORKING fix for the tach. Now to get to work on my speedo so I can see how fast I'm going too..

85irocinjected

All your values with the exception of R4 look fine. your R3 is 1.92k and your R6 is 2.94k which is perfectly fine. Your R4 value should be around 186k and its only at 3.71k. Fix R4 across 4 & 10 and you should be good.

zenish

i ended up using a 100k and two 47k resisters.according to my scanner my tachs now right at idle,100 rpm low at 2000 and 200 rpm low at three thousand.close as i could get without using a variable pot.thanks to whoever figured this out.i also fixed my trip odometer and my ac vents this month with advise from this site.

drunck1

So I have been reading about doing the fix but I had one question I havent been able to find the answer too , My tacho is bad on my 91 RS and am going to do the fix my question is though that I am going to do a LT1 Swap on my TBI 5.0 but leave the original 700r4 tranny in is there any modification I have to do to my clusters? can I use the original gauges and tacho from the 91 RS (110MPH) or what is the best solution. any advice would be of great help everyone thanks.

Chevy86 IROC-Z

iTrader: (17)

The best place is at the tranny board. Just copy your post and post it in a new thread over in the tranny board.

Chevy86 IROC-Z

iTrader: (17)

Look how bad my tachometer is....
http://www.youtube.com/watch?v=xhUc3qEJW6k

BASSr

Zenish, seems like that is the range of the resistors required and I found the same thing with mine.

I tried a 180k resistor on the 4-10 points on the board and the tach RPM was a little low, added a 12k with not much improvement. Changed that out to a 210k resistor and at an idle is perfect, but at the higher RPM's seems to be off a bit, maybe 400-500 rpm at 4k.

My R6 resistor initially read very high resistance so I am not sure what the other leads do as I have not found that in this thread yet either.

Anyone know how to fix/make the tach calibrate correctly all the way through the scale range (idle thru high RPM) ??

You said you worked the trip odometer on yours. I had the opportunity to break the trip odometer reset shaft off. Have another speedo but has much more mileage on it than my car does so I need to swap the trip odometer out. What size are those 2 little torx screws that hold the front of the plate onto the speedo? I pulled the speedo needle off, the speedo range decal and now those two little torx screws are keeping me from swapping the trip odometer.

Bruce

BASSr

OK, figured out those 2 torx are T5's. Got them out and put it all back together.

Thanks to all for the help and advice! Tach works pretty good now!

Bruce

crx350x1

I'm not very good at the Forums so I'm just gonna say thank you! On my 91 1LE I soldered two 100k resistors between pins 4 and 10 and scraped away the resistor connection going to pin 10. The tach matches the Auto meter tach perfectly!

jtsk

iTrader: (3)

crx350x1, welcome aboard! Don't worry about being good at the forums, with time you will be. The important thing is to learn what you need and share what you know, it's as simple as that.

Mr. Knight

Hey guys, new here,I have this issue.
when car is running the tach maxes out to 7000rpm.
The biggest resister I can get a gold of is 33k.
tested it and the rpm drops to zero.
Can some one spare a 200k round about resistor with a 45cent us stamp?

Also I had an issue with the fuel gauge but after fiddling with the resistor it seems to work now

Mr. Knight

I found 2x 100k from some old junk laying around. Paired them in series and the tach works ****ing perfect!!!
pardon my frenchwa.


You are a genius!!!

jtsk

iTrader: (3)

Mr. Knight, that is good to hear. By the way, you aren't the first to use that foreign language on here.

Mr. Knight

I wish i was that good of an engineer.

any one wanna teach some?

duffer26

just did this repair today thanks everyone for the info i just did the 2 100k inline set up and it is a whole lot better than it was




will know how close it is next week when i get my MT2500 from the strapon guy
but it is now verry close compared to what it was

Sanse

and at the other side of the big pond (netherlands) the fix worked for me too:

pin 4 of the thick-film-network cut




solderded 2 resistors of 390 Kohm parallel at the back of the printed circuit between pin 4 and pin 10




printed circuit back in instrument cluster



the tachometer works fine again in my '91RS. thanks.

Ticeblue

Well here is my story to add.....Tach used to read 4100 at idle....but now it's reading within 50rpms throughout total RPM range. I used two 1/2 watt 100k ohm resistors.

Thank you for posting this all those years ago it has helped a lot of people I'm sure

zenish

it would be nice if an admin could change the title of this message.this fix works on most thirdgens including my 89.

bobby hill

iTrader: (1)

Awesome info in this thread and on this site.

Long time lurker - new member here. I have a 1990 Iroc 350 automatic with the tach issue. I used a 180k and 18k resistors in series (198k total.) I also had to repair the tach signal wire under the hood. That took a while to find. The factory tach is now very close to the Autogage tach I had in the car.

bobby hill

iTrader: (1)

I believe you should use the center terminal and one of the outer terminals on your pot.

CHEVYFAN77

I am new to this, I just bought a 91 Chevy Camaro RS the main problem is the tach it pegs at start up. Sense i have read all the posts i am pretty sure I will be able to fix mine. Thank you to all who have worked to resolve this problem. I am glad I have seen about this because i was told it was the ignition control module ?

CHEVYFAN77

can you drive car without tach board?

Chevy86 IROC-Z

iTrader: (17)

Why would you wanna do that? It's cheap to test it out. Or you can go to the junkyard an pull a cluster. Test out the board. If it works, then your set. If it doesn't, cut the necessary pins and solder 2 long wires. Reinstall the cluster and solder in a pot (potentiometer). Turn Camaro and test the RPM's via digital timing light. For best rpm reading, disconnect the EST wire so that the RPM's won't fluctuate. Then note the RPM's on the timing light and go turn the pot left or right until your RPM's read as close as possible to the noted RPM's of the timing light. Once you get it dialed in, you can trim the wires and leave in the pot, or measure the resistance across the pot. Then solder in as many resistors needed to match the value of the pots reading.

Phew. Am I missing anything guys?

SolarGoldRaptor

I just did the repair, I used a 180k and a 16k ohms resistors. When I power it, it goes straight to 8000rpm wherever I force the needle to stay. The odd thing is that, if I force the needle below 2000rpm, it will go straight to 8000 but CCW :/ If it's above 2000, it goes to 8000rpm as it should.

I haven't tested it before doing the repair though (at least, not out of the car). It was showing totally random rpms. Staying above 8000rpm most of the time.

Is there any sure way to test the tachometer out of the car (my car isn't running at the moment, and won't be for a little while).

Chevy86 IROC-Z

iTrader: (17)

Testing it "outside of the car" is a while new can of worms. But you can test it out in an other car, if someone is willing to pull their cluster gauges out.

BigBadLou

iTrader: (3)

Raptor, as mentioned above, it is not easy at all to test the tachometer out of the car. You need the right dash connectors, power supply, digital pulse generator with variable duty cycle, scope etc.

I have a jig set up for that. I can test a tachometer in the cluster or just the tachometer driver board (that you fixed). If you want me to take a look, PM me, I can try to help out.

Lou

Cottle7

Hi, ive looked all over and can not find the answer to my question. when i turn my 92 rs on the tach goes all the way to the other side of the gauge and stays there. Any suggestions???

Chevy86 IROC-Z

iTrader: (17)

Have you considered reading the thread? The answer is here. I seriously don't think anyone is gonna take their time to type up a long post just to satisfy your question. The answer to your question has been answered many many many times in this thread. Now it's up to you to read and piece together what you need to do. I'm not being rude or anything, but I'm sure that any other member here in this thread would reply in the same fashion as I just did.

Crazyace909

Awesome and very fix. I did the whole thing in less than an hour. Thank you for all the hard work on this fix with the pics it makes this fix very easy. Thank you again.

tarling17

Fix worked for me today also.
90 iroc l98
used 2 100k ohm 1/2 wat 5% resistors
tested all 5 and used the ones with 98.xk ohms
solderd them togeather and got 196.6k ohms
now reads within 50rpms

fastlane1414

I did this to my 91 z28 and it didnt work.

Spaceboy1980

iTrader: (6)

This thread is still fixing tachs. Repaired my 90 rs v8 using the instructions provided. It would peg the tach and I used two 100k resistors on the board after cutting 4 and 10.

Thanks!

tsev

Works perfectly, used 2 100k resistors and no problems, thanks

lizardkng

I think this is such a popular issue that it should be written up as a "Technical Article" on the main page. And I suggest that because I have the same problem, but I dont know much about electronics on this level, and I found it difficult to sift through the posts and figure out the solution.

I looked through this page but didnt see it.

https://www.thirdgen.org/technical-articles

tsev

Right, it should be expressed somewhere at main page. I am also not an expert in this field but it is very easy to do. Just look at the pics in the post number 163 on page 4 of this thread and everything will be clear. The most difficult or time consuming process is getting to the board by removing the cover of the cluster. After that all you have to is cut two legs from the board (you will see on the pics exact location) and then solder two 100k resistors at the back of the board..

lizardkng

Im sure I can do that too, thanks for your point in the right direction.

I noticed that several people here had slightly different problems, some tachs didnt work at all, some read too high, some stayed pegged until they started the car and then went down and back up again, thats my problem, mine is pegged even when the car is off...hope this fixes it

Thanks!

tsev

No problem, glad I can help. Yes I had exact same problem, rpm gauge was pegged even with car off and keys out of ignition and this fixed the problem.. If you get stuck somewhere feel free to ask, but I dont think you should have any problems..

rodford

thanks for the help. 196k worked great this is off a 86 monte carlo im using it on 81 el camino got it at the junk yard pegged the tach the first time i started the motor