94Z07

Hey everyone, I believe I have a unique issue here that I can't find on the forums anywhere unless i'm searching for the wrong keywords.

A while back I had an issue with my LCD going out on my dash cluster so I repaired it and it lasted a while until it finally went out. So i picked up a dash cluster out of a '95. I put it in about 6 months ago and everything seemed to work perfecty fine.

however as of late I've noticed my speedometer is reading 5-8 MPH faster than i'm actually traveling. No wonder the cops aren't pulling me over for speeding. Also my 94 will not pass emissions due to high C0 readings so i'm not burning the fuel it's dumping.... when the last year it passed with flying colors.

So i replaced all 02 sensors, new denso plugs new denso wires and that helped a little bit.

Do you guys think because 94 was when they 1/2 transitioned from OBDI to OBDII that by putting a 95 dash cluster in it's causing issues? I know OBDII has to do with the ECU but I don't know if data is stored in the dash cluster as well that is unique to OBDII.

I have not ruled out catalytic converters since the car has 150,000 miles.

And maybe this is a long shot, i just know that my grandfather used to unplug his speedo on his 95 ford truck to keep the miles down and he had to go through a weird sequence to get it to pass emissions such as driving a few miles at 18mph turning it off and doing other things to reset the ECU to re-calibrate. If he did not do that, it would not pass emissions.

Maybe i have to re-calibrate in a similar fashion?

Anyways thanks in advance!

WVZR-1

Changing only the cluster didn't create or have anything to do with any of your issues. It's very, very unusual for a failed VSS to display erroneous values so if it were correct at one time then I'd think it still should.

If this is an A4, since the '94 is a 4L60e and the VSS is very inexpensive and a DIY replacement I'd just go ahead and do it. Is the rear axle ratio and the tire sizes as built OR have either been changed from it's original configurator?

If this car is an M6 then the VSS isn't quite as inexpensive.

Your VSS needs to read consistent values in percentages. I'd do a speedometer/odometer check with a GPS with the dash values at 45, 55, 65 & 75 MPH and note the GPS results. Post back. There should be no references to varied values like you mentioned 5 - 8 MPH off.

94Z07

The rear wheel configuration has not changed. And my speedo has always been right on the money until i swapped out the Dash Cluster.

I did a GPS check and if i'm doing 82 MPH the gps reports 76 MPH

50 reports 44 mph

65 remports 58

It's roughly off 6 mph.

VSS = Vehicle Speed Sensor i'm assuming?

I'm not getting any error codes thrown as well.

leesvet

wvzr IS pointing you in the right direction.

The dash cluster has nothing to do with ANY engine controls or function...its simply a display and not a control element.

Your cats are bad. 150K almost assures that. I'd guess the single thing that contributed to the cats death is/was the injectors failing. Ohms them if you must, but that car needs injectors if they have not already been done. High miles, age, alcohol in the fuels...yeah, it needs some contemporary model (Bosch-III) injectors.

The VSS is not a big deal...easy to get to from under the car. You might get lucky and clean the wire plug to the VSS and find that its dirty and sending some inconsistent signals to the dash. Look at the wire harness to the VSS as well...There was a service bulletin many years ago from GM about THAT section of harness laying across the trans in such a way that it rubbed thru the insulation and caused electrical shorts and other misc problems. These wire harnesses are fragile and they do not allow for much change in the signal values they carry. A dirty connection can cause the engine to misfire, use more gas than necessary, and even fail emissions... IIWM, I'd clean them ALL, and plug in with a good coat of silicone grease to seal off the plug from water/dirt. The silicone grease is hard to find and expensive but it's worth the work.

94Z07

Thank you both for the excellent posts. For the car being 150,000 miles it runs absolutely awesome. I had the original brake rotors on it and just barely swapped them out about 1,000 miles ago.

I agree on the cats. I remember one morning starting the car and hearing a loud "slam" come from under the car..and then my exhaust had a rattle that finally went away. So i'm sure the substrate finally broke.

I should have replaced the injectors when i had my intake off, had to replace a bad lifter.

Thoughts on Eastern Catalytic Converters? Rock-Auto has them for 150 ish..after freight i can get each one for around 170 a piece.

Any connections on the Bosch-III injectors?

Good thoughts on checking connections etc. I'll go through those and sort it all out.

WVZR-1

Is this car an A4 or an M6? I'm going to guess that it's an A4 because of the erratic behavior, the M6 would be a consistent error I believe because of it's mechanical drive. The M6 usually either works or doesn't.

The % of error isn't consistent so I'd go ahead and change the VSS after I confirmed the connector to the VSS. Does the cruise control work? Does the Radio Speed Compensated Volume work?

You say there's no DTC's so there not really any direction to go except the VSS I believe.


There's maybe a possibility of maybe a bad board in the DIC or maybe the CCM but it would be a very unusual situation.

Have you done any measured miles to check the odometer aspect of the cluster. I would do that also. There's measured mile markers nearly everywhere. I15 should be in your back yard. Post back.

ToniJ1960

Did you verify the gps as accurate? With the difference being so consistent I wonder. It should be a consistent percentage or ratio, or if erratic where is that consistency coming from?

WVZR-1

Change both the DIC and your GPS to METRIC KPH and do your comparisons again. Post back. Oh and mention A4 or M6.

I'm thinking that maybe it's a failed electrical component in the DIC switching board maybe. A sure diagnostic I believe would be to replace the VSS and if the error was still present then it's either a CCM error or a DIC error in the switching board or whatever you want to call it. Another possibility is a PCM error/failure since the PCM is used in the conversion of the VSS signal.

leesvet

...

Cliff Harris

The output from the VSS goes to the ECM, which sends it to the dash and the HVAC system if it's a C68 digital system. You can go into diagnostic mode on the HVAC system and find out what the ECM thinks the speed is.

WVZR-1

Yes that if you have C68 should be the next check. If you have the FSM see 1C-3 for procedure to use the C68 control head. If you don't the procedure has been posted several times and this is one of the "search" results. The link should open at post #3 but if not just see post #3:

https://www.corvetteforum.com/forums...post1577429729