Hi,

My issues surrounding this are as follows:*
New truck (to me), 248000 on it, 95 4x4 5.0. Problem started about a month ago. ABS came on and speedo was all over the place, but within about 10mph of actual. Then it all stopped when I replaced the ABS fuse (blown). Yay, I'm all good, right? Speedo is intermittent and problem is on/off again.

Just took a trip about 500mi each way. On the way down, I noticed that it was hard to move from a stop at times. Like the parking brake was on (which I now notice the indicator light is not working). Also, a couple of times on the way down the truck felt like it slipped or the rear brakes went on just for a moment. But only when the speedo was going nuts, which fortunately only happened for about 10 mins or until I turned the truck off.

So, needed a clutch anyway while visiting family, so I got a good deal on all that stuff, they checked the rear diff and it was low and nasty but nothing too hardcore. Truck was just fine until about 350mi into the return trip, and speedo goes nuts again. BUT, I could stop, turn the truck off, back on, prob gone. Now the speedo problem happens constantly. And it's sometimes tough to get going from a stop.

Replaced the VSS a couple of weeks back, same problem. No shavings, just some blackish oil on it. Checked wires, all good. Unplugged the ABS behind the glove box as I saw in another thread, no change. All fuses good. Odometer works fine. Again no parking brake indicator light. Also, truck seems to want to stall about 2ft into rolling forward or reverse and I have to give it about 150% of the gas I normally would to overcome that. Which can't be good.*

I had a shop check the diff as I said, and they said it looked good. Then again, they also forgot to connect my 4wd lever when done (I have nothing good to say about that). No metal on the old VSS, but the top 1/2 of the sensor was yellowish (like overheating or something) so I figured a new one wouldn't hurt too much. I don't like autozone, but it's all I have here as I am a solid hour from the dealer and after hearing about the tranny pressure issue which might be happening, I'm a bit freaked out about my last trip and don't want to go too far to figure this darn thing out.

I just tried to get some codes from the computer in the engine compartment, all it says is 556 (fuel pump issue). I don’t think that would cause the speedo problem, would it? I’m open to just about any option. I checked the RABS and got a 16, no codes stored. Now I’m going nuts as the truck just doesn’t run right at all, shifting is still difficult at times, and the speedo is just as nutty as I am at this point.

Well I hope this helps a bit. Thanks to all that have helped me so far without knowing it.

 

Well it is either a bad rear end or PSOM.
Unplug the RABS and PSOM.
Put jack stands under the rear end.
Hook up a DVM on the RABS connector under the hood on the drives fender.
Run the truck up to about 30 MPH and see what ACV you get.
Does the voltage increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to approximately 48 km/h (30 mph)?





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Thanks for getting back to me again on this subford. I think I know where to unplug the RABS, behind the glove box, right? And do I unplug the whole thing or just the test wire like last time?

The for the PSOM, I tried to find pictures but most were missing from the previous articles. Could you repost the pic of that connection please? I want to make sure we stay on the same page.

 

Reading comprehension fail.

 

That's really not what line 2 says.

Unplug the RABS and PSOM.

So I'm just trying to clarify there ace.

 

Yes you unplug the RABS module behind the glove box.
Unplug the PSOM behind the gauge cluster.
Read the AC voltage from under the hood.

PSOM:


PSOM Plug:


RABS Module Plug at E1 in diagram below:


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Cool I just unplugged it while waiting, found the right one, and am lifting it now.

 

I don't get any AC voltage off of the plug by the fender. I cleaned the contacts, still nothing. Is the speedo supposed to be off when the PSOM is disconnected? I would think so, but never know.
I did drive it around the block with the PSOM and RABS disconnected. Truck seemed to run a little better.

 

If the meter was set to AC Voltage scale and you got zero Voltage at 30 MPH (The voltage will be higher if you drive faster) then you have problems in the rear end, VSS sensor is not in right or bad wiring (maybe grounded).

No the speedometer will not work with the PSOM unplugged.

By the way the VSS (ABS) sensor is a small AC generator so to speak and needs no outside voltage to work. It is a variable reluctance sensor (VRS).

 

I did replace the VSS sensor a couple of weeks back, and had consistent results with the changeout. I did switch them out at the time after driving and am still using the new one since I couldn't return it. Stupid return policy on electrical parts.

How and where do I test for the short?

 

Also, I tested for voltage between the two poles on the RABS fender connection. Was that the right procedure?

 

Leave the RABS Module and the PSOM disconnected and unplug the VSS (ABS) sensor.
Using the ohm scale check each wire to ground. If you have a complete circuit to ground on either wire you have short to ground and you need to fix the wiring.
Also there should not be a complete circuit between the two wires.

Yes it was.

 

Well this might spell trouble then. No complete circuits, no shorts to ground or between the two wires. Also checked the diff fluid back there. I think I see very fine pieces of metal.

 

Then short the two wires to one another at one end an check at the other end with a ohm meter and see if you have a complete circuit. If not then you have an open circuit.

If the bearings are bad in the rear end and the tone ring is not square on to the VSS sensor then you may have a problem there.



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Checked today for the open circuit. Negative on that one, got a nice beep from the tester. Plugged the VSS and everything else back in. Tried to test it for the heck of it , power on and off, and got nothing (meaning no closed circuit). Does that make sense considering I had to unplug everything to try this? It seems like that defeated the point.