VetteBabe1

Just wondering if anyone has any thoughts on how to fix a tach that is terribly inaccurate. When I first got the car, we thought that it had gears or something because the tach was about 1000-1500 rpms off (too high). Then when we had it on jackstands and spun the wheels, it came to 3.33, normal rear. When Lee had it hooked to the datamaster software (I think that's right), the computer told us that the tach was AT LEAST 1500 rpms off, and actually got increasing off as the engine spun up - to the point of being over 2000+ rpms off at like 5000!!! :eek: Is there any way to fix this and return the tach to normal?? It is really annoying to not really know the engine rpms. Hopefully someone has encountered this before? Thanks so much! :cool:

scorp508

Hmm... normal rear for an '89, not a '90. Correct? It could be possible that the tach filter has taken the stairway to heaven. Especially if it is getting worse the higher you go.

VetteBabe1

What are the normal rears in 90 then? I thought 3.33 was a possible choice, I guess I just need to read the codes under the console lid to figure out which it is. This was just the approx # of turns of rear wheel. But at any rate, the tach is such that I would have to have a 5.+ gear ratio! Do they make tach filters anymore? I know that the older ones (70's) are discontinued from when I worked at ZIP. Thanks!

VetteBabe1

Ok, I found the gear ratios on a website for Corvette RPO's, non-year specific, just a list, and this is what it said.

G44 3.07 REAR AXLE RATIO
GM1 2.59 REAR AXLE RATIO
GM3 3.45 REAR AXLE RATIO
GH0 3.54 REAR AXLE RATIO
GT7 3.33 REAR AXLE RATIO
GU2 2.73 REAR AXLE RATIO


My car has the GT7 option code, just went out and looked. So it is the 3.33. Maybe that will help, maybe not :). But at least now we know. :)

LPDesRoche

Jen I already had this discussion on here.:D :D :D Its the cluster. I have it narrowed down that far. Its acutally more than 1500 off. It gets worse as the revs increase. At idle its only 1500 off, but say at 3500 it reads well off the screen way over into the speedo section. Kinda funny.

scorp508

This surprises me emensely.:yesnod: We had a thread going the other day that stated only the early 1989 cars with the ZF6 got the 3.33 gears, and that later in the year they were equipped with 3.54s for a while before going back to 3.45 for 90-96.

I would of expected you to have GM3 3.45 REAR AXLE RATIO on your RPO sheet. The VIN matches the car on the sheet?

LPDesRoche

Scorp,

My 90 had the 3.33's as well. And I know my good friends(HJV11) 94 coupe has the 3.45's. Not sure of the rhyme or reason as to why the 89-91's got this rear or that. I know some of the 90 ZR1's had the 3.54's so who knows.

scorp508

:D :crazy: :crazy: :crazy: Too many discrepencies. :D :crazy: :crazy: :crazy:

VetteBabe1

Yep, I checked the VIN and everything. It matches! I sure wish it had the 3.45 gears though. Although not enough to really make a difference I guess!

Anyone have a spare dash cluster then for a 90? Preferably with low miles :D ... or less than 130k LOL

ENJ

Hello VetteBabe:
I had a set of 3.45 ring & pinion gears sitting in the race shop. Hopefully Lee told you about my rear end problems. Gear has a chip on one of the teeth on the ring gear. Now, I have 4.11's in. Have to get the electronic gear calculator to correct speedo or go in the trans to change gears. I'll do the electrical gear calculation. With your current situation with your car, (not going to get to specific), your trans my be from a 3.45 ratio car & placed in line with a 3.33 rear gear housing???? Just a thought.

VetteBabe1

Hmmmmm, would that make it that far off though? It gets less accurate at an increasing rate as rpms go up. If it were that I would think it would be consistently off. But who knows. Would I be able to see the VIN on the tranny if it were on jack stands? I know my 87 had the VIN on the 700R4. But couldn't see it until removed. I have heard about your rear end woes. A bunch of us are planning to go to VMP on March 22 if you can have it ready by then an whoop some behind. :) This is one of the last problems we are working on, thank god. That and the throttle body is worn out and hangs open, so car revs way up when you push clutch in, while shifting, etc. So if anyone has a 52mm+ TB I need one of those too! It never ends!!! :rolleyes:
Hopefully that will be it for the fixing and we can being with the modding. :D

We need to get together sometime, I have not seen the new addition to your car in person yet! Let's do dinner one weekend or something??

ENJ

Hello Jenn:
Yes, I would think that if the rear in the car is off from the program in the computer, tranny gear, the higher the speed the more off the speedo reading.
Car's back together & tested it Staurday on the expressway. Speedo is way off as expected. While my speedo will be higher until calibrated, my tach was as recorded from the gear calculator on this web site that was listed on this forum. Check this site out & compare the different gear ratio combinations to check yours. http://www.smokemup.com
By the way, 4.11's really push you back in the seat on every shift.

Gene

Dr. Evil

OK, now how did I know that it was going to be a 90' model car just from the title :rolleyes:
This is a VERY common problem specific to the 90 corvettes. Allmost every 90 model owner has seen this it seems. It is NOT the tach filter, been there done that as have many other 90 owners. The dash and instrument cluster changed in 90 and that is partly to blame. Somehow or another the signal wire is picking up some radio interference, thus, the faster the motor spins, the more rpms the tach is off. With the new dash/instruments in 90, they're more suceptable to this interference, since in 90 we still have the distributor instead of the optispark introduced in 92 with the LT-1. The Optispark doesnt seem to have this radio interference effect. I have heard of several 91 owners having the same problems with the tach but not as many as the 90 owners. I think first year introduction of the dash caused a couple of 'bugs' or 'gremlins' that were either worked out in 91 or not affected by the LT-1.
For future reference, another problem you ARE going to run into since you have a 90' is the everpresent INFL REST light. We got airbags in 90 with the new interior. The sensors for the airbag, located on each framerail, get bad ground conections over time causing it to set a code, INFL REST. They used a different sensor in 91 which solved the problem. This is something fairly easy to fix yourself. Theres a proceedure on how to do it in the tech tips section.
I am using an aftermarket tach. I've tried alot of things but no luck yet. Sorry I cant be of any more help on that issue.

As for the gears, you've got the 3.33's. In 89 you could get 3.54's but in 90 only the ZR1 got the steeper gears. 3.33 was the steepest gear you could get in a non-ZR1 1990

BTW, nice looking car ;) :D

NX1991

Welcome to the club. I have the same problem. Off about 1100rpm until 3000 and as rpm increases it pegs off of the scale...its an ongoing battle. I think the problem is in the cluster or the resistance of the wire has dropped to much. Thus giving you an erratic meeting. I am in the process of troubleshooting this... I will let you know when I find out more..and you do the same

VetteBabe1

So would getting a new dash cluster fix the problem? Is there anywhere that does repair on it? I hate not knowing what rpms I am running! I don't want to have to put a big, ugly tach on the dash. I guess I might just have to live with it huh?

As for the INF REST light, it comes on when you first start the car, but goes off. Have not had that problem yet, knock on wood. But who knows if the car actually has an airbag or not? Any way to tell? Or would the light not test correctly if not?

Thanks!! You are really helping me out guys.

David-F

I guess I need to start taking a look at this tach off it seems to be a growing concern. I have not come across this in many years and I can not remember just what I found at the time but there are a few things I would like to do to see what is going on. As I see it there is two possibilities here either the tach is having a problem(maybe an internial ground problem) or it's picking up EMI of which I would suspect would be coming from the bulkhead connector but what I would like to do first is remove the dash and start the engine and use a seperate tach and see what RPM that tach reads ate the terminal at the back of the dash. Even if this does read the correct RPMs it may not prove a dash problem, but if it shows to be off then you have proven the problem to be somewere else. Another thing I just might want to do is make a jumper wire to go from the tach filter to the terminal on the back of the dash there by by-passing the firewall connector and see what happens. I have seen many strange things happen because of currect being passed through the grease used at the firewall wall connector. In 84 the tach would shut off because of this when the A/C was turned on. Had to take the tach wire out of the connector and by-pass it to solve the problem and there is a chance that could be happening here. Now if that did not help then another place to look would be the ground for the dash which is behind the left door sill cover just about above the left front speaker, removing it and cleaning everything up and seeing if it makes any difference. I wish I could be of more help at this time but I do not know of any cars in my area with this problem if I do I will see what I can find out and pass it along, but I have given you a few things to look at just let me know what you are able to find out.

David Fulcher

VetteBabe1

Thanks for the suggestions. When Lee had that datamaster on the car, it showed that the computer was definitely reading the correct rpms...so it's somewhere in between! I feel better that it's a common problem but it also sucks that it's common, you kinda need your tach to work right when you are drag racing, etc. I think we are going to hide a shift light somewhere for racing purposes so at least I'll know exactly when to shift.

Thanks! :)

David-F

The ECM get's it's RPM reading from a different circuit, so I would expect it to be correct.

David Fulcher

torchred96

Well, it's not a vette, but my 90 iroc has the same tach problem and also the infl restr. light. Kind of cool seeing that tach neele go past 8000 or whatever rediculous number it gets to.

You might try thirdgen.org and do a search for this. I remember this is a common problem with 90 irocs too. Those camaro guys are really into their motors, so they probably have this one licked too.

Good luck.
Sam