Please respond if you have ever experienced an issue with your MPH gauge fluctuating. I can Idle at a stop light see 65 MPH shown on the gauge, it kinda bounces around randomly. At speed I can be @ roughly 45 MPH and see 125 on the gauge.

I have converted my 87 with a ZF6 and it was reading correctly. I recently installed NGK solid copper core plugs and suspect the non-resistor pulse could be playing havoc?

Any other opinions before I start tearing in to the car? I do not think 87 CPU adjusts parameters based on vehicle speed but please correct me if I am wrong.

Thanks in advance.

Mike

 

on my 87 , if the switch that changes it from mph to kph isnt all the way set , it will read all over the place.

 

Thanks, I will check the switch first.

 

Faulty Vehicle Speed Sensor.

I was creeping along in 1st gear but my speedo was reading....



Btw...be sure to use an AC Delco VSS. Myself and others have had issues with cheaper brands.

 

haha! I was reading 200 MP and my range showed 9999.

Thanks for all the input.

 

I am still battling this issue on my 87 corvette. Please see my video on youtube.com of this problem in action.

http://www.youtube.com/v/o5IMCMIhepw?hl=en&fs=1

 

As a "Shot in the dark" I replaced the ECM with a new one, this made no difference in the way the dash is acting up. When I have my car on my scan tool, I show no Trouble Codes and MPH reading are correct in my scan tool software.

 

The ECM has nothing to do with the VSS signal the cluster gets for your year vette - no need to look there.

Fuel gauge working? Any other wacky problems?

Seems pretty clear that the NGK solid copper core plugs need to come out of there since the problems started directly after the swap.

 

Thanks for the reply. Fuel Gauge is working however here are the symptoms:

• Trip calculations are incorrect
• Instant Mileage are incorrect shows (99)
• MPH Readings increase with throttle
• Sometimes all gauge parameters go crazy like 4th of July
• BLM's at Idle are 160 (way lean) Not sure of ECM adjust A/F mixture based on MPH readings

I have been reading about the 90 + C4's and it seems they use a CCU which shares VSS signal?

Thanks for the extra help on this one. If you want to see what I am referring to check out my you tube link above.

Mike

 

I watched the video yesterday before I posted.

The only thing I really didn't see was if the odometer was running at the same time. I'm not surprised to see 99 mpg since the ECM thinks your going way faster than what you are.

Correct - the 1990+ VSS circuits are totally reverse and have more components as compared to your '87....there are many, many differences between these years so it is not the best idea to compare them.

On your year - your ECM sees the VSS after the signal has been "clipped" by the cluster....so yes MPH will have an impact on the functions of ECM.

The only other easy thing I would look at is the DIC switch panel (thing just above the radio with all the switches). The DIC panel basically loops the cluster's information back to itself using different variations (functions). As mentioned above ensure that your on "english"(mph) and not "metric" or somewhere in-between. Additonally, you may want to exercise all 5 of the DIC switch panel's switches as they could be making poor contact(s)/connection(s).

Sometimes (on occasions - usally under or over voltage situations) these clusters can go wacky -as another alternative you may try disconnecting the car's battery for several minutes and then reconnecting it to see if that makes any difference.

Again it make a lot of sense to go back to the last thing you changed before the "problem(s)" started...the plugs.

Any chance you did some distributor work you didn't mention like changing the cap or the hei coil out?

 

i have this problem.. mostly when the clutch is in and i rev it up. the SVC always scares the life out of me when it turns the radio to full blast.