Need to have my speedo calibrated. Factory service manual states gear in tranny feeds VSS mounted in side of tranny which feeds CCM which feeds digital readout on dash but no mention on how to calibrate it if the reading is wrong.

Vehicle specifics: 94, 6-sp manual tranny, running stock size factory rims and tires all the way around.

Took car to a speedo shop, after a run-in with the law. Speedo read accurate up through 30 MPH but at 40-80 it was a minimum of 5 and maximum of 7 MPH slower than actual speed. Shop personnel were not familiar with how to calibrate a Vette speedo.

Any suggestions and/or recommendations other than to paste a yellow sticky on the dash saying "subtract 5 mph from reading". Don't think the judge will accept that method as fixing it.

Thanks in advance

 

Speedometer ratio adjustment is done by reprogramming the PCM.

You need to find out _why_ it is off. The nonlinearlity of the error means a simple ratio change in programming isn't going to fix it.

Does it show RPO MN6 on the console lid?

 

Thanks for the response Vis Croceus.

RPO code MN6 is on the console lid.

Here are the numbers from the test just in case it could aide in troubleshooting:

Speedo 20 30 40 50 60 70 80

Actual 21.63 31.88 46.76 54.04 65.17 75.88 87.08

If the PCM requires re-programming, can that only be done at a Chevy dealer?

Any help is greatly appreciated.

 

What rear tire size do you have, and what gear ratio? Has anyone ever done a gear swap or changed the trans for some reason?

It's reading consistently 8% low except for the 40 mph reading. Decreasing the tooth count on the Speedo Sensor driven gear by 3 teeth will turn the speedometer about 8% faster.

Answer the above questions and give me all your RPO codes that start with "D" and I'll give you an exact answer. The PCM doesn't need any programming. You can change the speedo gear by jacking up the car and spending $8.

 

I'm running the stock Rear Tires P285/40 ZR17 on factory sawblades. According to the RPO code (GM3) I have the 3.45 rear axle ratio.

To my knowledge there has been no modification to either the tranny or rear end.

The only two RPO codes listed on the label that start with "D" are DL5 and DL8, neither of which should have nothing to do with this problem unless I'm missing something.

Thanks for your response...awaiting your reply.

 

I guess they don't put speedo RPOs on the newer ones.

You should have a 42-tooth green gear on your speedo sensor, if you do in fact have a 3.45 rear. Pull the speedo sensor out at your next oil change and count the teeth. Whatever it is, head to your GM dealer for a gear with 3 less teeth. And of course, report back!

There's no way to be positive what rear end is in there unless you do an rpm test on the freeway. I don't recall what the engine should be turning at a given speed though. A previous owner could have replaced the rear diff.

 

Thanks for the advice CentralCoaster. I'll do what you suggested, have the speedo shop run the test again and let you know the results. Appreciate your advice.

 

Anybody with the software and cable can do it. If you post in your regional forum you can probably find someone near you to do it for free. (Listing your location in your profile would help here.)

Or you can send it off for a mail-order tune for about $100.

Need to find out whats' up with the two readings that are not matching the 8% of the others.

To get an 8% increase in mph from a 3.45 would require a 3.19 rear, and there is nothing available near that.

Since you also have the stock tire size, it is not a simple case of wrong rear or wrong tires, so I'd want to find out what is broken.

 

According to the FSM, as it came from the factory, the VSS should produce 4 thousand pulses per mile.
The best way to recalibrate the speedometer when making gear changes is to select a VSS that produces 4K pulses per mile with those gears -- this way, things are standard and easy to trouble-shoot for the next person that buys the car.
Other ways, with the '94 and up C4, are to reflash the PCM or buy an adjustable unit that corrects the VSS output, but I wouldn't use them....too many variables that way.

If someone changes rear gears or tire size, that will change the number of pulses per mile the VSS puts out.

So, a way to test it would be to simulate a sine wave signal in place of the VSS signal at the same amplitude as original.
4 thousand pulses in 60 seconds would be 60 mph.
In other words:
66.6 hz output from a sine wave generator should be 60 mph.
33.3 hz output from a sine wave generator should be 30 mph.
etc.

Another thing to try is compare the speedometer digital readout with a scan-tool reading of the speed.

If someone changed the VSS, recalibrated the PCM, or installed a pulse adapter/modifier, it is anyone's guess.


Since you are saying the speedometer is accurate at the low end, but off at the high end, my "wild" guess is you have a poor connection in the wiring from the VSS or a defective CCM (I believe the CCM, not the PCM, actually "drives" the speedometer digital readout).
The PCM, radio for speed controlled volume, and the cruise control receive a raw VSS signal.
The CCM receives a raw VSS signal and converts it into the signal to drive the speedometer digital readout.


Tom Piper

 

Go to this website http://www.atrol.com/era.htm with this unit you can set your speedo to read whatever you want or set it to read your perfect speed. I use it in mine because my wife ( a.k.a. speedcop ) freaks out when I go too fast so I set my speedo to read 74 when my actual speed is 90. she feels much safer hey I didn't marrie her for her brains. this unit installs pretty easy you just put it between one of the wires that come out of the trans speed sensor and cost about $100

 

Speedo Calibration Update

While I received a few varied responses to my problem I first went with CentralCoasters advice as it seemed to be the easiest and cheapest way to go first.

I did find a green 42-tooth gear attached to the VSS. I also have a complete C4 parts manual so checking it I found that a black 40 tooth gear was made for 94-96. I purchased the gear from a local Chevy dealer ($6.95). Installed it and headed for the calibration shop for a second pass.

From 20-80 the speedo now reads within 1 mph of actual speed. Needless to say I was pleased with the results. Took a bit to get the VSS out, (pretty tight quarters with the exhaust pipes running right blow that area of the tranny but am glad I did it myself instead of paying a shop a labor charge to do it.

Appreciate the various suggestions submitted by all.

 

 

Set it on metric, that'll really knock her socks off. You could take it up to 200+ kph on your next freeway trip.

 

I have the opposite problem. The day I bought my vette, my wife wanted to be the one to drive it home. I said sure, just drive it easy for a while to make sure everything is good and safe as all I did was a 10 min test drive before I bought it. She pulled out of the parking lot and floored it, and held it to the floor till she was out of sight. I called her on the phone and asked what was wrong with her. She said the only thing wrong with her is that her daily driver car is too slow. Now she wants to know how to use the nitrous.

 

good idea

 

Hi
I checked recommended by you web site and want to try ERA unit. Could you tell me where on my C4 do I hook up this device and could you describe the location of it (possibly with pictures). Also wiring instructions would be very helpful.

Thank you

Mike
install

 

I'm having a speedometer misreading. It takes a second or 2 for the digital readout to catch up with reality. When I'm coming to a stop and then I fully stop, the speedometer is still counting down to 0. And my PCM is reading that value because the idle doesn't drop until the readout says 0. 92 6-speed.

 

I got it and trying to install it in / under the dash. Can you tell me what colors are the wires and where are those?

Thank you

 

"vettteset8087" likely hasn't been around for years.

It appears you've purchased an Abbott and rather than attempt to help you blindly in a C4 I'd suggest you call Abbott. I've used Abbott a few times but several years ago. I've never done an ERA/DRA in a C4, I've always done mechanical corrections. Do you have the FSM for your car? There's component locations in the 8A-201 section that can be used to guide you to placement and the wiring. I likely can but I need to study.

Call Abbott direct .. you've done the buy it seems from them so they should be very helpful. They likely have a tech sheet for the C4 and only need to know if it's '89 and earlier OR '90 and later with a CCM. That will change the install.

All contact information is here:

http://www.atrol.com/speedometer/era.htm

What rear axle ratio and tire size are you running currently?

 

Hi
Thank you v. much .
Do not have FSM (1991 C4 ZR 1 ). If you could point me to where and what the wire colors are would be top notch. Abbott just knows what to do w/ their box

Thank you
again