Ronny Bailey

The speedo on my 92 quit working, so I've been checking out the system.
I learned several interesting things from reading here and checking the parts.

The VSS receives 5v from the wiring harness on the white wire going to the connector. As the shaft on the VSS turns, it acts like an on/off switch. This converts the 5v to a series of pulses on the black wire going to the speedo.
Since it doesn't generate any voltage by itself, you won't get any reading trying to measure volts.
To test it, connect a meter to the terminals and set it to measure resistance.
As you turn the shaft, the meter will alternate between a few ohms and infinity. It is the same as if you are checking an ordinary on/off switch.
If you have a digital meter with a beeping diode tester, use that and you can just listen to the series of beeps.
Mine is part #56027014 and I noticed that it has eight on/off cycles for each turn of the shaft. It seems to spend more time in the open part of the cycle, so the output to the speedo would be a pulsating 5v signal instead of a square wave.

Before this, I thought the VSS actually generates a signal on its own and mine was testing bad. Fortunately, I figured out it's good before buying a new one.

rockmike

Thanks! thats good to know Im trying to fix my speedo now,Ill try that test on my sensor.Unfortunatly the gear in the trans is toast...Ive got a new one just got get in there and get dirty,be a good time to change the trans fluid and do the overfill thing.

fatt

If the square rod on the VSS is worn the speedo won't work either. Mine tested good but the rod was rounded enough on the corners that it wasn't catching.

wannadiesel

Only the 2wd auto trans VSS gets a 5v supply.

Don't ask me why.

Ronny Bailey

Mine is a 2wd with the Getrag and it has 5v on the white wire...

befitz

Thanks Ronny, I have thought about using an old distributor, with
condensor removed, to make a tester for these electric speedometers. A distributor from an 8 cylinder engine would provide the 8 grounding events per revolution. What do you think?

thrashingcows

I'm so glad my crew has the 89 motor/wiring dash cluster etc....it still runs the old cable driven speedo.

I see this is a fairly common problem with the electronic driven speedo's. Especially since most of these trucks are pushing 20 years old. Way beyond the manufacturer projected service life.

Trebor

I thought that in a 4 cycle engine the number of times the ignition points close per crankshaft revolution is equal to the number of cylinders divided by two. Or are you talking about distributor revolutions by themselves?

Ronny Bailey

Sounds like you should give it a try! Let us know how if it works.
I've probably thought about a hundred times about how much better it would be if this one was the 'normal' mechanical type.

befitz

Trebor, I'm talking about connecting an electric motor directly to a distributor and using the points to ground the signal wire coming from the speedometer. That's all the vss does, like Ronny said, is to close and open a circuit 8 times per revolution.
Ronny, I'm changing my '90 to a cable drive. Peace of mind is priceless!

brobrad

To test it, connect a meter to the terminals and set it to measure resistance.
As you turn the shaft, the meter will alternate between a few ohms and infinity. It is the same as if you are checking an ordinary on/off switch.

This is the line from above. I tried my old one with a drill and got nothing. There are three prongs so I tried sever different combinations. SO, I went to town, bought a new one and same results with it as I had with my old one. I must be doing something wrong. Any Ideas?