VSS Problem?

[coop34]

I have a 70 Chevy with a 5.7L TBI and 700r4/4l60 from a 90 silverado. I hooked up a scan tool and the VSS quickly maxes out at 254 mph when I'm going an estimated 10-15 mph. Any ideas why it's doing this?

[60case]

Do you have a VSS buffer module installed in the truck. The VSS signal is 40 pulses per drive shaft rev The buffer turns it into a 2000 pulse per mile. The buffers are different for different gear ratio .

[coop34]

I don't have a buffer installed. Is that the problem? So the buffer would decrease the speed that the VSS is putting out?

[hugger6933]

Like the EGR solenoid, I'm wondering why you need a VSS. Your truck should have the mech speedo. You may need to have a chip burned for your app. That will make a TON of difference. I had one burned for my V-8 S-10 I had done I drove it to Little rock [bout a hundred miles] got 19 on the way over, and 26.6 on the way back. It had more power as well , it took about three tries but it was def worth it .

[coop34]

Thanks Hugger. I thought that the VSS controlled idle speed, air/fuel ratio, etc.?

[davepl]

VSS is useful for coast down and decel fuel cutoff, among other things.

In the ECU there is a setting for how many teeth you have on your speedo pickup. If you've got it set to 10 and you're running a 40-tooth reluctor, I'd expect your speed to be 4X actual... but hitting max 254 right away stumps me!

[coop34]

I recently changed the VSS due to a leak. Would a greasy VSS connector cause something like this?

[Acorn]

What type of scanner are you using?
Like davepl the vss actually contolls several things with the TBI/4l60 and affects the over all performance.
Does you wire harness have a wire on the A10 terminal of the Ecm?

[coop34]

Acorn: I'm using a Mac Tools MacLogic OTC scanner

Yes, the harness has a brown or grey wire (I'm color blind) on the A10 pin at the ecm.

[Acorn]

Coop, I'm so sorry I forgot to check back.
Double check the wiring diagram that is posted in your EGR thread. The brown wire on the A10 pin of the ECM should be from your VSS. I 'think' (key word, 'think'!) 1990 was the first year GM used and electric speedo on TRUCKS and they did not have a vss buffer. It was in the instrument cluster.
I am not familiar with that scanner plus it's been so long since I have used one I'm not sure why it is maxing out like that....
I will see if I can find my old analog (paper back) manual and see if I can find anything that might help.....

[coop34]

No problem man. Thanks for the help!

[ChopsawSurgeon]

I did a little research and it looks like you'll need to add in a DRAC or VSS Buffer module as this controls the output for all the modules that use the vss input. Which style VSS sensor do you have? Cable through or electronic?

[coop34]

Electronic

[ChopsawSurgeon]

If I'm reading correctly, it looks like you'll need a module from a 90-95 style truck as they had the electronic vss sensor