FireRed91Z28

ever since ive owned my 91Z the tach has been way off, showing higher RPMs than what the engine is actually doing. how can i go about fixing this problem? i also have a T56 but even with the factory auto, the tach has been off by the same amount...?

speedingpenguin

iTrader: (1)

how far off are we talking?
After installing my autogage tach its pretty obious that the stock tach is a few hundred RPM's different than the autogage (i trust the autogage), but if its REALLY off (500RPM or more i'd say would be REALLY off), i dunno....

kptkid

I also have the same problem, the tach is about dubble what the motor is really at. it would be nice to solve it from redlining when the motor is at 3000rpm .

my 82 trans am

mine does this too, at idle its at 1500 rpm.,when it used to read 900 rpm. i have asked dealership technitions and no body knows.
i was thinking that its maybe due to something in the distributor.
i havnt gotten the chance to tear into it yet but im gonna check it out.

vrtc350

iTrader: (1)

Mine is off even further. It says it idles at 2500 rpm(yeah right!). My car is a 91 also. All I know is that the gauges are electronic so it has to be some sort of sensor somewhere. I haven't messed with it since I'm in the middle of building a complete brand new engine.

speedingpenguin

iTrader: (1)

calibrated for the wrong engine? if its actualy double thats what i'd say.....

Charged350

iTrader: (1)

if ya got msd, its will make the stock tach freak out, mine did, it was shoving 5500 at 30 miles an hour , got a autometer and problem solved

vrtc350

iTrader: (1)

My setup is still all original. 305 with the TBI. No mods nothing, and it is still loopy. As time has gone by it has gotten worse and worse. Now i don't really care cause my new engine is still on the stand about to be dropped here in a week or two.

From what I've heard your electronic gauges don't work once you dump the ECM. I'm going carb so I haven't pursued it. I have a mechanical cluster set out of an IROC or a Z that will eventually make it's way in.

There has to be a sensor somewhere that monitors that stuff.

3rdGenSS

why change the cluster? you can still run your electronic one with the carbed setup... just have to do alittle rewireing so you get signals to it... i mean if you want to modify the dash and all or change it compltly to fit an older style cluster then so be it.. but it is easier ( in my opinoin) to use what you have... then go from there... just my .02

vrtc350

iTrader: (1)

I wasn't aware that you could rewire the cluster to work. I read the tech article on this site to see what I needed to change since I was dumping the puter and going carb. I wanted an idea on what I was going to need to change and as I recall there was a blurb about the cluster no longer working.

I would be interested in knowing how you rewired yours to make it work. Although that still doesn't solve the tach being rediculously off. Not mention I am sitting on a cluster out of an IROC.

Benm109

The stock tachs in these cars are notorious for reading incorrectly. The usual problem with the 82-89 Camaro tachs is a capacitor located on the back side of the tachometer face. The problem with the 90-92 Camaro tachs is in the printed circuit board for the tach (or at least the capacitor in the printed circuit board). The problem is that no one has seemed to find a way to fix the tachs themselves on the 90-92 style.

See if there are any instrument cluster/gauge repair shops in your area. They can usually fix the problem. I also think there are some online sources that accept and repair them through the mail, but I don't know there URLs. You could also try a used cluster, but there's no guarantee that the used one is going to be right either. There is also the option of installing an aftermarket tach, like one from Autometer or similar gauge company, but then you have 2 tachometers in your car. It's your decision.

Just remember, the stock tachs in our cars were really made as trim pieces, not accurate tuning devices.

3rdGenSS

as for wireing the cluster for a carb setup... basicly everything is already wired to work... the main thing you would have to change is the vss wiring so that your cruise, and speedo work... you already have the electronic vss sending a signal from the trans.. and since you havea tbi you should have a buffer box as well.. basicaly all you are going to do is drop the signal from the buffer box to the ecm... and leave the signals going to the speedo and cruise.. the wiring for your sensors.. i.e. fuel, volts, temp, oil are all directly wired to the cluster already.. if you need some schematics i can prob help ya out.

as for your tach,, as benm said, they are noted for being off most of the time.. and it is the board in the cluster controling the tach that is bad.. if you want to keep this cluster then you would prob have to get it fixed or try a used one from a yard or something...

just would seem like more work then nessesary to change clusters to an older style due to having to modify the dash or change dashes cumpletly to fit the older cluster in...

FireRed91Z28

ya mine says it idles at 1500RPM. i am also installing a complete new engine with new computer, accel DFI, and i plan on puttin a custom auto meter guage cluster in there. would all this fix that problem? i dont want any of this bull whith my new engine in.

ManWitThePlan

i bought a 1990 Iroc and the guage cluster worked fine when i removed it, but now the damn thing is buried at 8k RPMs lol at anytime WTF?? i put it in my 1991 RS.

lol
Barry

ascalise

iTrader: (2)

Sorry to bring this post back to life, but has anyone come across a url of a business who fixes these tachs? I am about to drop a new hot engine in my 91 z28 and would like the tach to read properly.

vrtc350

iTrader: (1)

Not any that I am aware of. Well I haven't looked either. Are you ditching the ECM? I did and just plan on making a custom gauge cluster.

I would just get an Auto Meter. It will be more accurate and keep you from blowing that hot new engine.

Bens3rdGen

Had the same problem, grabbed one out of junkyard for $15 and it's worked fine ever since. Check out House of Camaro, lots of good used stuff.

ascalise

iTrader: (2)

No, the car is going to stay fuel injected. I plan on restoring the whole thing after the new engine and trans is in it and want to keep it as original as possible.

ANDYZ28

I have 3 1990 IROC-Zs and all of the tchs are off by at least 500 RPM at idle, more as engine speed increases.

I bet their are many pwoplw who would like to get this problem solved. I have tried several different circuit boards to no avail.

FoxRodder

The tach in my RS I just got reads something stupid like 1500 or more at idle as well. I've been wondering what to do about it. Its not like I need a tach with an automatic but regardless I still watch the RPMs closely on my Lincoln after I added a sunpro tach.

The tach in my mustang is also off, although mostly just when rollin all the way through gears on the gas. What I did was install a Raptor adjustable shift light in my stock dash cluster and it basically looks all stock in there. Tach reads closer to 6k when I set the light to hit at like 5200 I think.

http://www.corral.net/photopost/data...hightLight.JPG

I'm contemplating installing one of those mini sunpro tachs on the Slowmaro unless I could slap an in dash unit in stock location.

maroon88iroc

iTrader: (7)

my rpm gauge was way off when i got it and i just bought another 115 mph speedometer setup and swaped the tachometer and it works like a charm now.

ANDYZ28

I beleive the problem with the '90-'92 tachometers is the small IC board that plugs into the instrument panel carrier. This part is not available new anywhere that I can find.

What signal does the tach generate?

1bad91Z

iTrader: (1)

Run a wire directly from the tach lead on the MSD to the tach sig wire that that goes to the tach. Make sure to use the MSD in-line tach filter inline before the wire gets to the tach.

I prefer my auto-meter lunar series tach though....

ANDYZ28

I don't have an "MSD" to use. Can I simply bypass the factory GM IC board, and go directly to the tach?

FoxRodder

Has anyone measured or seen what it looks like behind stock tach location...in regards to using an aftermarket in dash style tach in the hole?

pilgrim

Hey 1bad91Z, please explain your suggested fix in more detail. I'm not clear on what you are suggesting.

MrDude_1

if you cut it up enough with a dremel, you can fit most aftermarket tach guts in there...

that said, i tried it, had it working on the bench, and broke the fugger before i ever got to put it in the car.

that kinda bummed me out, and i havent looked at it in months.




on a funny side note, my tach is almost dead accurate now... im feeding it a 2 pulse per revolution 5v signal.. (stock LS1 "tach" output..)
its purely coincidental, but since my tach was almost reading double, its pretty darn close now... my tach reads dead on on the low side, and when my 5800 shiftlight comes on, its reading a hair under 6k....

coolram62

iTrader: (2)

The tach in my '83 CFI/700R4 Z28 is way off,like 7000 rpm hard on the gas thru the gears and 1500 rpm idle with the motor warm.On the tach feed wire out of the dist there appears to be either a filter or a capacitor mounted on the firewall directly behind the dist.Which would it be?I have another tach to use but it would be much easier to replace this.

shawn baldwin

has any one thought that mabye just mabye their tach is wrong because the sensor in the distributor is off?
has any one thought about the fact they may have a clone z28 that originaly had a a v-6?
their are several reasons tachs go wild loose ground can send a tach wild
too may pulses for the set up of the tach in its original for some tachs are made and set up for 6 pulses per revelution some with 4 and oh yeah my favorite some with 8 pulses perrevelution a tach doesnt just go bad you eaitehr have a loose ground a bad sensor or oh yeah as ****ty as it is you have a clone that some one has put a v-8 in at some point from a 4cyl or a v-6 check your title run the vin of the car see what it was originally built as was it built off the line with a v-8 v-6 or the gas saver 4 cylinder
just a thought mine was doing the same thing when i built it and it happen to be a loose wire alot of the 90's chevy did this same thing when a bulb burned out in the turn signals could be sitting still and the tach would steadily climb speedo does the same thing

vrtc350

iTrader: (1)

Do whaa???? That was hard to understand.

No clone hear for me and my tach is way off. If you do a search you will find that this is a very common problem with our cars.

shawn baldwin

no need to do a search i have owned 18 camaros in my life from 1972-1998 have found both problems as i stated in the previous statement it could be a number of things some one could have changed dashes and it woud cause this problem being that the one they got could have been from a v-8 or out of a v-8 and put in a v-6 model unless you have owned th car since it was brand new no way of truly knowing this

vrtc350

iTrader: (1)

I find it very hard to believe that so many people with this issue have had the dash changed or engine swapped. I know for a fact that before I removed the 305, it was the original engine and the dash hadn't been swapped and the tach was off by a rediculous amount.

There are at least 7 people in this thread with that issue and if you did a search you would find a lot more people with that problem, with no engine swap. Could you be right for some people? Sure, absolutely that could be the case. But for the majority I believe it is an electric part that has simply failed. That does happen to, you know.

The vin tells all.

And (no offense) try using some periods or something. I had to read the last half of your statement a few times to understand what you were saying.

fb305svs

have no problem here - swapped in a 88 140mph speedo in my bird

shawn baldwin

I'm sorry, i didn't relize this was english 101, and if you put a 350 in you car did you happen to change the distributor back to the original 305 dist. that has the right pick up for your tach,
for those who dont know, their is a differance in distributors alot of times it doesnt matter when you pull the engine out and put in an old truck or old car but camaro was one of the first mostly electronic autos that came out.

but again sorry, forgot this was english 101!and not thirdgen.org
go spend about 250 bucks and have some one repair your tach and tell you it is fine you need to fix other problems first..
or you can pull the cole out of your a**es and turn it into a diamond and spend a 100$ and buy an after market tach

vrtc350

iTrader: (1)

Now that I can understand! You passed English 101!

I am quite aware that there is a huge difference in the electronically controlled dizzy from the 305 and the HEI dizzy I am using in my new build. I have done enough research (using the search button) to know that I will have issues with my cluster when I do my type of swap. That doesn't negate the fact that it was off before I removed a single bolt. Get out of your fantasy land where parts don't fail!

How about I spend a $100 on the aftermarket tach and then pull the coal out of my *** and make $250 diamond and get something else usefull.

This is stupid. I didn't say you were wrong, just stating that you hypothesis is not always the case. Most likely hardley the case. The 91-92 clusters are completely electronic. One resiter goes and there you have it. Or whatever in the hell makes them inaccurate.

vrtc350

iTrader: (1)

Did you swap the whole cluster? I have read that you have to make a lot of modifications to make them fit. I have a cluster out of an 85 or 86 Z sitting in my Dad's garage.

fb305svs

Well, from 86 on firebirds could swap very easily - they all used a plug in type setup - totally different from the camaro's until at least 88 or 89... maybe even later...

however, if memory serves me right, i think that i took the 8k tach and the 140 mph speedo and swapped them into the 91 OE cluster... AND, for those who dont think it's easy to roll back a speedo? your wrong - i did it with out damaging it... the 91 have 81k on it, the new speedo had 110 on it... was able to roll it back with no problem at all

NEEDAZ

Now, now. Shawn, I think that if you searched you would find a number of people that have had this problem, done the trouble shooting, used scanners and after market tacks, have the correct dash/motor and still find that it is the stock tach is off. With that said you are right that these things should all be checked before assuming a bad tach. Most people on here have seen it be the stock tach most of the time though. A side note to help with possible repair, anyone got a schematics for the stock dash/tach? Can't be to hard to figure out what is going bad.

MrDude_1

im only going to say this once.


this is a TECH ONLY website.
no guessing.
no problys.
if you dont know exactly how to help, or have some kind of useful input, please avoid the post reply button.

when you do reply, not only must it be useful, but it must be easily read by everyone. re-read before you post.

i dont care if you are Grumpy Jenkins himself, you dont post what you dont know.

just incase you dont know who Grumpy is, i'll rephrase.

it doesnt matter if you own one of every camaro ever made. you may be a expert.



the fact is, this is a tech site, with 100s of people that know more about one specific part of the car then you do.. everyone has their specialty.. everyone has their own experiences that let them know something you dont.

we're here to share that knowledge.









all that said, because i can tell from what you've already replied.. you are not a electronics expert. you do not know what the exact "signal" coming from the coil is, and you do not know the inner workings of our tachometers, nor do you know how they fail, and you dont know how to fix them.

as of right now, you only have 4 posts. lets make the 5 one be something useful.. either a question, or a reasonable answer.


otherwise, you can ignore me, keep doing what you're doing, and be eaten alive by witty people who know more about whatever specific topic we're talking about... then you become branded as a idiot... and either end up ignored when you have a real question, or banned for posting incorrect non tech info.

just be grateful someone took the 3mins to reply to you, and walk away.
im not going to reply to this again, so dont bother making any kind of witty comeback.

NEEDAZ

Heck, even a bad tach and an LCR meter. From how they fail it's almost got to be a wayward capacitor, and likely the same cap. for all these tachs. One person poking around could help a LOT of people... I'll help with what I can over the net, I just don't have a bad dash (or mutch time, or a working LCR meter any more, they don't like 15KV...).

MrDude_1

im pretty sure its just this one IC..
however, i cant figure out what chip it is... mine doesnt have any markings on it.

NEEDAZ

Let me start by saying that I've never looked at one of these dashes looking for a tach problems and I don't have one with a tach problem.
With that said, based off of other experience (a good bit of experience with industrial control tachs) this don't sound like an IC that failed. More like a capacitor is leaky. If you have or know someone that has an LCR meter it shouldn't be that hard to check the caps on the PCB. Shouldn't be more than a handful on there. I could be wrong, but got a good feeling about this one. I should add that LCR meters are woefully inadequate as 'cap testers', but should work with this failure. Question any cap that is more then 10% low. If you find more then one that is questionable, add a variable cap in parallel with one cap at a time (long leads), plug the dash back in and hold the revs as steady as you can. Twist the variable cap and see if the tach changes (the value of the variable cap will depend on the cap that you're putting it in parallel with). That should be the one that is bad.

alhiggs

I posted some info on this topic a few months ago. I have since then took the fugger apart to see what makes it tick. I bought the car NEW in 1991. The tach started reading higher than normal about 3 years ago. Little more each month. More. More. Then 5000 was shown at Idle, then 6000. Now it just stays at 2000 all the time the key is on. I took it apart and figured out how the 2 coils in the meter can position the needle anywhere (even 360 degrees!) and rigged up a microcontroller 2 channel PWM controller to position the needle, and it worked. I never finished it as far as installing it in the car, I just bought another tach and ignore the one in the dash. I can't believe that no one at my dealership has ever heard of the problem, and I certainly don't think it should be $250 to fix a $20 driver board. I'll try replacing a cap or 2 on mine and post the results.

Austin

1991 Camaro RS 5.0 TBI completely stock; 190,000 miles original engine no rebuild yet but needing one soon!

NEEDAZ

Holly cow man, talk about over kill, I LOVE IT. Take your time and you could probably make the stock tach (your tach controller and stk face) and have one heck of a fast, accurate 'stock looking' setup.
Obviously you have some electronics experience. Do you have an LCR meter?
I need to get a new LCR meter my self, anyone near MD got a bad dash for some checking?

MrDude_1

cool.. you're into microcontroller stuff too?
if you want to fake a tach signal to the stock tach, you can just hold the pin high, and drop it for every pulse... the stock tach works fine on a 5v square wave input..
well... as fine as it works in the car.. lol.

ive tried replacing my caps... checked the resistors... theres this one unlabled IC that im pretty sure is the problem... but without a label, i cant look it up any further.

if anyone takes a cluster apart, and can read it, post up the part numbers on the chip!

Andrew89tpi

I think this is what you're looking for:

MrDude_1

that white IC on the left.. thats the one i cant identify.

Andrew89tpi

That's a resistor network.

MrDude_1

hmm.
thanks.

it seems unlikely that would go bad, but thats what i suspect right now....

thoes values you have written in the pic above, are they the factory values, or are they what your read on your circuit?

Andrew89tpi

That was from an 86 v8. The chip was unsoldered and then measured.