Last couple of times I have data logged I have noticed I am getting some surging car speed numbers.

Car is converted to MAP from MAF and everything was fine for the last 2 years. Just the last couple of data logs the speed on the laptop shows from 27mph to 41mph in the next cell. Obviously this is incorrect as the car cannot accellerate 16mph in a tenth of a second. There are quite a few instances of this happening. Going through the data logs it may look like something like this

Vehicle speed

19
21
42
25
25
40


etc....

Then it might be fine for a while and it happens again. All the other data in the data logs looks fine nothing wrong there.

I am using the moates software and cables.

Any ideas what could be causing the problem?? MAG VSS is unchecked in the flags so that should be fine, I am also getting speed readings on the laptop so it should be fine.

 

If you have an older IROC, you have a VSS buffer module on the back of the instrument cluster. It'a may have problems. I believe those were installed through the '89 models.

 

Yes you are right I do have the VSS on the back of the cluster. I am thinking 2 things either the cable into the speedo is not seated properly that comes out of the transmission. Or the VSS is faulty.

I have a 145mph speedo gauge that is broken but the VSS is in working condition. I think I might be able to swap the VSS off the 145mph gauge and put it onto my 85mph gauge. Or are they calibrated differently? The VSS just unplugs and that would be all I would be changing so I think it should be a straight swap?