Speedo off, -91 RWD

Speedo off, -91 RWD

Postby rynik [OP] » July 29th 2015, 9:37am

Hi!
I have a customer with a -91 Astro. I don't have the car here yet but as I recall it has the digital cluster. Speedo was working fine until recently. Now it's off, showing a lot slower then expected. Anyone kind enough to educate me on what's going on. Is there a cable? Where how is the speed signal picked up? What can go wrong?
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Re: Speedo off, -91 RWD

Postby Big_kid » July 29th 2015, 1:45pm

I saw the title "Speedo off" and thought this thread was about going skinny dipping. :lol:

I'd look at the VSS- Vehicle Speed Sensor.
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Re: Speedo off, -91 RWD

Postby AstrofarianAstrofarian is online! » July 29th 2015, 2:09pm

Like Big_kid said; VSS. Look at wiring, also, fuses, dash grounds, the cluster itself.....what can go wrong is a long list. Check back in and let us know what you find.
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Re: Speedo off, -91 RWD

Postby Lumpy » July 29th 2015, 2:38pm

IF your 91 is like my 89 -

VSS (Veh Speed Sensor) mounted on driver side of the transmission. Mounts with 1 bolt, has an electrical connector plugged into it. That's an inductive coil pickup, sort of like a magnetic guitar pickup. It senses the revolutions of the tail shaft. That pulsating signal goes "north" toward the dash. It's AC voltage pulse is 40 pulses per 1 shaft rotation.

Somewhere near/in the dash the signal goes to the DRAC (Digital Ratio Adaptor Converter). That box converts the pulsating AC voltage from the VSS into 4000 pulses per mile for the digital displays.

The DRAC is a plastic, maybe white, maybe black, box, about the size of a pack of Maher's Marlboros. It's in the dash behind the radio/HVAC controls and toward the driver's side. It has an electrical connection (two if you have cruise control), and some kind of clip on mount.

The logicalist (... :shock: ...) place to look for a problem, and the easiest to access, is the VSS on the transmission. If you remove it, expect to get a small dripping of ATF. No real loss of fluid, just be ready for a little drip. You may need to put the trans in neutral to get the VSS out.

The way to TEST the VSS is to raise the driven wheels off the ground, start and run the van in DRIVE, disconnect the VSS electrical connector and test for AC voltage at the output terminals of the VSS.

The rest of the testing process for the speedo is essentially the same, testing for AC voltage at all ends of the wiring.

At some point, simply replacing the VSS seems simpler and more cost effective than tearing the dash apart, finding contacts and testing for voltage while the wheels are turning. New sensor, about 35 dollars -
http://www.amazon.com/gp/product/B0016I8P6Q

I recommend AGAINST buying a cheaper BWD brand sensor from LAPS.

IN YOUR CASE - I might first pull and clean the VSS. Examine the electrical connections, clean, dielectric grease etc. Clean off the part of the sensor that lives inside the transmission tail shaft. It will likely have a coating of metal velvet, just like the magnet in the drain pan always does. That right there might resolve your problem. While it's out, examine it for obvious wear damage. Theoretically nothing physically touches it. But every one I've ever removed has obvious wear marks, I could tell which way the tail shaft was turning.

If troubleshooting or replacing the VSS doesn't resolve your problem, then I'd start looking elsewhere up the chain. But the VSS is the most likely gremlin.


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