Is there supposed to be 5 v. to orange/white wire on the vss on a 91 w250?
When I check the wire it has 2.4 volts and if I pull the speedo head, I have "0" volts on the o/w wire.

 

There should be no voltage on that wire when the truck is not moving, it is the signal wire. A small AC voltage of varying frequency should be present when the truck is moving.

 

Thank you very much for responding,even though I thought I checked everything,I must have missed something since the pcm is eliminated.

 

The VSS is grounded through the PCM. If there is no ground the speedo will not work.

 

I saw this post and remembered I needed to go back and do this one last thing to complete my PCM removal. I was working on that last year and got to the point where the speedometer was the last thing I needed to get working without the PCM. Sure 'nuff, ran the VSS blue wire to ground, unplugged the PCM and voila! Everything works now including the speedometer - with no PCM.

Thanks Wanna!

 

You should do a short little write up on how you bypassed the ecm for each thing it controls.

Although if you have cruise control I don't see how you could get around that?

 

No cruise, no grids, no shutdown solenoid, KSB switched, VSS grounded. 91.5 was the last of the external voltage regulators. That's about all there is to it on a stick truck up to that last half year. Putting the A518 OD on a manual control of some sort should be rather trivial as well, but I never had to deal with that.

 

The only thing stopping me would be the cruise, but if the computer crapped out someday I sure wouldn't spend $500 to have it.

 

I finally have a speedo now! Everything I checked said it should work, but didn't.
I had 12 volts and ground to speedo head, if you checked the w/o wire to the head and spun vss with your hand you got pulse's, speedo still wouldnt work!
So I cut the w/o wire at the pcm and left everything else as is , and it works great.I'm using push button switch for overdrive for now, will change that later,just excited to see needle move off "0".

 

My speedometer started to act up it would drop to zero but the odometer would still work.
So I crawled under the truck to look for something loose but all looked ok so I removed the VSS and took it out for a better look.



And I looked at this side.



Not sure if this was a pulse generator or a voltage generator (I have worked on both AC and DC Tach generators.) So I was curious to find out so I chucked it into my drill press to simulate the truck moving, connected it to my Fluke DMM, set it to turn at 400 RPM and turned it on.



For reference @ 400 RPM I was only reading 13 millivolts AC although I didn’t think at the time to check the frequency.

Ok someone how many drive shaft RPM with 3.07 gears for 30 MPH



Just thought you would like to know the kind of things that go through my head.

Oh yea after I put it all back together now my speedometer works fine.
Jim

 

I've seen used ones going for $300 on the big auction site. I don't know if they are even available new anymore or who would be the source for that - dealer maybe? That is of course assuming your local dealer is still in business. My old PCM will be going on the big auction site if I ever get laid off and become strapped for cash....

 

Even $300 is a lot for something that a $20 locking cable would accomplish just as well....