sorry i just realized my last post was pretty much worthless. from what I understand with the increase in resistance caused by bad resistors comes an increase in tach RPM displayed. my tach just moves about 1/2 inch at start up then reads ~300 RPM no matter what. Could this be resolved by the proposed solution or does it sound like something else is going on. I wouldn't think my tach would magically lose resistance and need it added

 

when I was repairing mine I changeedout the capacitor I then replaced resistor to 196k ohm and tested in garage but only at idle, all seemed ok. when I'd done a few other things I took car for drive and found rpm would only lift to ~1500 . So I had gone from ~ double(too high) to a very low readout. My solution as with others before me was remove 196k ohm resistor and in its place connected a 1 meg ohm pot. Took car for drive and when in OD adjusted pot to have tacho indicate ~1900rpm at 60 mph. Close enough for me.

 

Just out of curiosity, is there anything besides a faulty tach that can cause the tach to read abnormally high? I'm runing a Richmond Gear 5 speed and 3.23 gears in the back. According to the calculator on their website I should hit 120 mph at 5000 rpm. Instead I hit 70 mph at 4k and 90 at about 5500 on the tach. Those number would put me as having around a 4.56 rear. Talked to Richmond to get their 2 cents, and they said that since I know what gear is in the car, and that the tranny is shifting into all 5 gears, everything is operating fine ( eliminating the possibility it was the transmission). No matter what the car will have higher rpms due to the nature of the Richmond tranny, but it is a bit unnerving to be seeing 5500 rpm at 90 ( even if it is possibly only around 3757 ( what it is supposed to be reading). With a full SLP exhaust, it sounds like I'm doing around 7k.

 

To find out if your tach is reading correctly, you'll need to get a timing light that has a tach on it. Connect the timing light and compair the reading on the timing light and the RPM's on the Camaro's tach. If you want a more precise reading, go to a mechanics shop to connect a diagnostic computer such as a SnapOn computer. It is so much more precise than a timing lights rpm reader. As for what else can cause a bad reading, I dont know much about the area.

 

Going off what you describe, no. Since there's only one wire required to get a signal to the tach, it's not exactly difficult to screw up. Typically if you know the speedometer is right and the tach is reading wrong, odds are it's the tach itself.

 

I concur, especially given the history of tachometers in our cars. As a betting man I'd be put'n a bet on the tacho. As a matter of interest what is it reading at warm idle?

 

tach reads about 1k while idling

 

I stumbled upon this thread after searching because my tach on my 1992 B4C 145mph cluster wasn't working. I even swapped my original 91Z tach into the 92 cluster and same problem.

Turns out that circuit board in the 92 cluster was bad; Swapped it with my 91 and VOILA! My tach now works fine. Seems my 91 board doesn't have the resistor issue; YET.

Thanks a ton; w/o this thread, I probably would have gone postal.

 

Add me to the list of fixed rpm gauges, if its not correct its purdy damn close and way better then pegging out at 30mph.

Use 2 100k ohm resistors on the chip, total cost was like $9.60!!!. That was one pack of resistors and a all-in-one soldering gun/pen. The cheapest and most rewarding fix I think I have ever done.


Went down the road and had the car chirping tires in 1 & 2 gear, all this time(3 years) ive been shifting early cause I wasn't sure want rpm I was running. With the loud exhaust 3k sounds like 5k so I always shifted early.

Thanks for this great thread!!!

 

I'm bringing this thread back to the top, and I'm also going to make it a sticky. It's something I should have done a long time ago.

Anyway, the tach on my 91 Corvette is reading too high, and not unexpectedly the only repair advice I got from the corvette forum was "replace the cluster". Lots of help.

This thread is like gold. I'll post back my results once I get a chance to yank the cluster and start screwing around.

 

I can't stand responses like that... people not willing to go farther than just turning a few nuts... Does your 'Vette have an analog cluster?

 

Yeah, my vette is the "newer" C4 style with the digital speedo and analog everything else. From what I've learned in this thread, it uses the same style resistor chip as the gauges discussed here. I might take a 1/2 day tomorrow so that I can pull it apart and screw around with it.

As for the Corvette Forum responses... that's a typical response there. 95% of that forum's "tech" board is questions about wax and floor mats.

 

"Where's the best dealer to get my oil changed?"

 

Update:

I fixed my tach in the corvette.

As usual, it took 5x longer than it should have, but at least it works now. I was about to throw my DMM across the room a dozen times... I measured across the 4-10 pins, and measured an open circuit. With the leads reversed, I measured 69k. There's no reason that my DMM should only work with the polarity a certain way during resistance measurements, but I digress...

I cut pins 4 and 10 with a dremmel, and then removed the remaining pieces from the board with my solder sucker. I soldered in a 100k pot with a 100k resistor in series. I put it into the car, and much to my surprise, the tach was now reading way too low. I soldered another 100k resistor in series, and in the process managed to get solder on the sweeper of the pot, so it would no longer turn. In my attempt to remove that solder, I melted the pot. Back to Radio Shack I go.

Later on, I soldered in the new pot, went back to the car, and with the pot cranked, I now had an accurate rpm on the tach. I was surprised that this cluster needed 300k instead of the 200k that the f-bodies need. Regardless, I removed the pot & 2 resistors and soldered in 3 100k resistors in series. I re-installed, verified accuracy, then pulled the cluster back out to button it back up. Yesterday I moved the car and had no tach.

I opened it back up, and diagnosed that by installing and removing the stuff on the board so much, that I managed to melt the copper off the board surrounding the hole, so I had to relocate that side of the resistor pack to an alternate position on the board.

Reinstalled and checked... seems to be working nicely now. Thanks to those of you who did all the hard work figuring this out!

One thing that I'm finding a bit suspicious, is that based on my experiments with the pots & resistors, the higher the resistance, the higher the tach reads, which completely flies in the face with my original diagnosis in which my tach read too high and my resistance across those pins was only 69k.

 

I did some searches and came up with nothing, so I will ask here. Does anyone know the value to use on an LS1 motor with a stock 1991 cluster? Thanks!

 

Off the top of my head, I think I recall that the LS1 uses a "4 cylinder" signal. Chances are it's either 100k or 400k (half or double the standard 200k), but the best solution is to wire in a potentiometer and find the correct value for yourself.

 

I just want to say thank you all for this knowledge. Today I fixed my 91Z using 2 100k resistors on the backside of the board for a clean look. I bought the car 3 years ago and from the beginning it was reading extremely high. A couple months ago it redlined and stayed there. You guys are top notch in my book.

-Morse

 

I attempted to fix my tach on my 91Z tonight - I got a 100k, 68k, 22k, and 4.7k resistor together and soldered them up in series. I measured like 191k. I checked the leads on the board and 4 across 10 showed open.

Soldered the line of resistors on the backside of the board (very easy) and started the car. Much to my joy, I had my tach back! (it was pegged). Unfortunately, in my case, I think I need a little less resistance. My tach showed me idling at like 450. Its too dark to do anymore tonight, but I am curious; any guess as to which one I should take out? I'm thinking about removing the 22k and maybe putting in a 10 or so. I just want to try and get a good baseline to start from, instead of spending hours soldering mix/match resistors and hoping for semi-correct.

Thanks!

 

If anybody has pictures of how to do this and some good instructions can ya email them to fdphoenix92@gmail.com

Thanks

 

ok so originally i had a 3.1 v6 and my tach was bad, did the tach fix and that worked, now i did a swap to a 305 and changed the resistor value to v8 and it worked off and on for awhile, and i eventually needed a new ecm, so i got one and the tach worked awesome for about 3 weeks and now it does this:

http://s164.photobucket.com/albums/u...Video-0010.mp4

anyone have any idea of what is going on with my tach and how to fix it?

currently i have 196k ohm resistance soldered to pins 4 and 10, and in the video im coasting so foot is off gas pedal

 

another v8 tach fix here. follow the pictures and directions and take your time. i used two 100k ohm resistors and idle is a little over 500. i can live with that and will adjust later.

 

once again, another extremly happy third gen owner, who's tach is working properly! went the potentiometer route with a timeing light....PERFECT! thnx for all the help everyone!!!!

 

Thanks to everyone. After reading nearly every post and reply on here, I used the info here to finally fix the tach in my 1991 Z28, 350 (5.7 liter) automatic. This site is truly one of the best sites I have ever been a member of. I think I used some 100k ,47k and a couple 10k. I cant remember the exact combo b/c I tried several until I got the results I was looking for. Once again thanks to everyone, your info really helped me. Could not have done it without everyone here.

 

i have an 85 trans am that the tach spins on startup up but while running it just lays down way past red line and stays there. is mthis my problem? or can anyone help me?

 

Just did this fix on my 91 z, works great, once again a huge thanks too everyone who takes the time to figure these kinds of things out. MUCH APPRECIATED!

 

I had a tach that was always pegged all the time as well, Last night I pulled the tach board out and soldered in 2 100k ohm resistors in series(actually ended up giving me 197k for some reason) and my tach now works perfect!
The information on this site is a great tool!

 

this thread is a live saver, ill be trying this meathod soon

 

Just finex my 91 RS (305 Auto) Tach. It was showing ~ 1500 idle and pegged before it shifted.

I used a cheap breadboard from radio shack along with a 100K & 47K resistors in series with a 0-100K Pot/Trimmer. I also put a 1K 15-turn Pot in for fine tuning. Used 12" lead wires so I could relocate the new board to easily access for adjustments (if and when necessary).

Thanks for all the info! Tach v2.0 is now working!!!

 

YES! Another Tach fixed! Thank you guys for figuring this one out! my tach never did work, it was fragged when I bought the car. I was planning on replacing ALL of the guages and was trying to figure out how to do that and make it look like it never happened... that was going to be very expensive and a ton of work! But now the original equipment works! How cool is that? & for $0.99! thanks again!!!! You guys ROCK!

 

Well, a minor setback. I don't know if I got the stock board too hot or if the resistors or pot weren't of a high enough wattage. Gage worked great, then started pulsating and then dropped off to 0. Going to test the pot (the trimmer was only a .1W where the rest of the circuit was .5W or .75W). I'm thinking it cooked it and fried it into a closed circuit position. If not, then I'm back to the drawing board... But at least I know where to look now!

 

guys, you have been incredibly helpful to me during my recent expidition into third gens and lt1 swaps. 92blue and his knowledge of these tachs has been most helpful also. thanks so much to all of you and here's to being first on race day! LOL

 

where can i buy the resistors at and does anyone have pics on how to do it. i suck at electrical stuff.

 

Radio Shack sells the resistors cheap. In this thread you can see a lot of pictures showing how it's done. It's very easy.

 

I used to suck at soldering electronics, too but I found that a pencil iron and a supply of flux helps a lot. dip the wires into the flux or put a little on the contacts where you want the solder to flow and get it hot just before you touch the solder to it. It flows where the flux is and looks almost professional. get a little spare wire and practice first if you can.

Just remember not to get the electronics too hot. some can handle it and some can't. better off NOT finding out which one can't the hard way.

 

Did the 4-10 soldering today with two 100k resistors from radioshack. I didn't have anything small enough to snip the pins. The tach reads in the ballpark, but it's really erractic. Wanna make sure that it should be normal once the pins are snipped (before the tach was completely pegged). When idle comes to a near stall, the needle bounces off the stop at 0 and up to 4k and back down and it goes nuts at extremely low RPMs

This is an 86 with a terrible soldering job btw :P

 

I have a tach board from unkown year v8 camaro, it has the big old square board behind it that's almost as big as the tach face, if anyone has the know how to set it up for a v6 please PM me with a price, electronics isn't my forte and I have more than enough other much bigger issues with my car to foray into this stuff.
Alternatively if someone can give me very specific instructions I might be able to do it myself, but I don't have time to read through this thread to find and or learn the necessary information....I did notice it has a resistor bank that looks exactly the same as the 90-92 ones for whatever that is worth.

 

Did anyone ever start a thread for older models like 82-89? I've got an 87 with what looks like the problem solved here, but I couldn't find any related thread dealing with the older design via repeated seaches.

 

it aplies to the older models too. How do I know? Look at other members who have successfully fixed the issue. Look under their avatar. It'll say what year of vehicle they own.

 

Excellent work by all. Thanks for making this easy.

 

I think its time to fix my 86 IROC tach. Before I get into the Instrument Cluster, I would like some info first. Do I have to completely remove the Instrument Cluster? Ive been reading the this thread as well ads the initial thread of 89 and below models. But I havent found any info on whether I need to remove the cluster. Thanks guys.

 

Yeah I was sorta thinking it might apply to older models too but there were several posts of people saying they were gona start a related thread for the older ones which lead me to think things were different.

It seems like a pretty cheap fix, i'll have to just order the resistors and try it next weekend, if anyone has info about anything being different for say 82-89 please share this week.

 

This pot should work well and its available at most Radio Shacks.
http://www.radioshack.com/product/in...97#tabsetBasic

 

Just got mine working on my 91 Z. Very rewarding feeling starting it up and seeing the correct RPM. I used a 120, 56, and two 10's and it is basically perfect (+-50 RPMs). Confirmed with a USB ALDL cable from moates and TunerPro.

 

It's not perfect yet, still zeroing it in, but you can add one more fixed tach to the count.

 

I have one question. Does anyone know what kind of tachometer is on a stock 91 rs 305cid. Automatic trans. I'm trying to hook up an msd sci ignition box p/n 6300. And its asking me what kind does my car have. Its showing Autogage, Autometer, Mallory, Moroso, Stewart, S.W. & Bi Torx, Sun, and, Vdo. I have no Idea what to look for. Thanks in Advance.

 

anybody priced out a what a new tach board costs, to just swap boards?

 

Thanks to everyone who has helped out with this. Tons of great information! I snipped the 4-10 pins and soldiered in 196.2 K ohms of resistance. Tach now reads 750 rpm at idle instead of the 3500 rpms it used to read. The hardest part has been getting the !@$# headlight **** to seat back in the switch. If I ever get it back in, I will consider this a success.

 

Finally.... after many tries, cuss words, prayers, attempts, holding my breath, holding my mouth, hammering, stammering, and general discontent, the thing just snapped in like it should have the first time. Took the car our for a spin, and the tach works like it was new! Thanks again to all of have posted on this subject.

 

another winner here in the uk . top info thanks guys it has been very usefull

i dunno if it is dead accurate but appears to be normal and not off the end of the scale like it was before and idles around 550/ 600 good enough for me

again thanks from the other side of the pond

 

Hey guys,

Need a liitle guidance here.

I have a '90 IROC that has the tach peg all the way over when I start it.

Took the circuit board out and got the following readings:

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

Looks like R3 & R6 need to be replaced. Suggestions on what resistors I should get for each of these?

Bruce