Hi guys (and gals)

Created this account because anytime I search google this forum usually proves to give good answers.

Recently purchased a 2nd owner 1996 F-150: 4 wheel drive, 5 speed manual, manual locking hubs, 5.0

Here's the situation. The speedometer will snap back to zero and or jump anytime I'm in gear and on the gas. Let off the gas and it goes back to reading correctly. Take it out of gear while coasting and it will read correctly even if you rev it a little. I replaced the rear ABS/VSS. Is this a PSOM issue, tone ring issue or something entirely different? I wouldn't think PSOM because with it out of gear or off the gas in gear it reads correctly. I would think the same on the tone ring since again, it reads correctly off the gas or out of gear.


Am I wrong in my thinking? Really want to get it working so I can accurately calculate gas mileage, oil change intervals etc.

I do want to add that after going through all the receipts the PO had in the glove compartment this truck was taken care of. Nothing has been monkeyed with. Everything is factory original barring wearable parts and an aftermarket cd player.

 

First thing I'd do would be run the self test (KOEO) retrieving stored codes (CM) that follow that test.

If happen to get a "system pass" (111) on those two, doubt it based on what you've stated, I'd run the KOER test to see what it came up with for a return.

Many sensors share the same reference voltage circuit, I haven't reviewed the diagram for any possible interference that fit your symptoms caused by a failed sensor or chafed wires but sounds like you may have something along those lines going on.

I wouldn't be at all surprised if you got a "failure mode" (998) in return due to what you are experiencing if make it to the KOER test.

Might not be as bad as all that but regardless running the self test to see what if anything the computer can tell you about it is the first best step.

How to run a self test (if don't own a code reader, it can be done manually see link below)

Ford Fuel Injection » How To Run a Self-Test

Keep in mind the separator code (10) between KOEO and CM 10 is signified by a single flash if doing the test manually. That aspect tends to throw people a little bit when counting flashes.

KOEO = Key On Engine Off, CM = Continuous Memory. KOER = Key On Engine Run

 

My uncle owns a trans shop and he hooked it up the day after I got it for an ABS light that was on. It didn't throw anything on their computer. Will the KOER test pick something up that a high end reader won't?

 

Using the information provided in the link will probably bring up everything his high end reader wouldn't if your truck is OBD I that is, I'm all to quick to automatically assume OBD I in this section at times.

A '96 half ton could go either way I believe.

If its OBD II his modern reader should have worked and some of them include OBD I support but even then do not when put into practice on these trucks.

 

A 1996 half ton is OBD-2, you can't pull the codes with a paperclip in this truck.

I'd find out what ABS code you got, because the ABS and speedo get their speed information from the same place.

 

Interested in this, my speedo only works in the rain. New VSS, but haven't checked the PSOM yet. All else is good. Same specs as the OP. Doubtful my 10 1/2 31 15's have anything to do with it.

 

The ABS is taken care of. The new VSS took care of it.

 

If this is not the VSS then it is most likely the PSOM itself. I had an issue with mine a while back - https://www.ford-trucks.com/forums/1...l#post15990636

 

Same tire size that's on mine. BFG 10.5x31x15

 

Tire size has nothing to do with the problem. Different tire size will just change the speed and mileage reported. The PSOM can be calibrated to compensate for this.

 

I should add the odometer doesn't read accurately either. Don't know if that's useful information, but thought I would add that to the symptoms list in case anyone else has this issue in the future and an answer is found to my problem. I know it's linked to the speedometer, but just trying to keep a record of what all is going on.

 

I agree. Just trying to get as detailed as I can so that anyone in the future can see and compare if they have the same issue.

 

Take it to someplace such as autozone and have them try retrieving any codes it might have stored, see if they can establish a connection with the computer.

Might know nothing about it but give you a starting point, what system/s to look at to correct the problem if it will respond.

One thing I do like over OBD II the KOEO and KOER self tests OBD I offers. OBD II has its strong points yea but those couple tests do come in handy.

OBD I (EEC-IV) computers this vintage are suffering board level failures, small but real chance your problems are computer related.

If all attempts fail to connect with the computer? I'd suspect it might be suffering like issues the EEC-IV before it can and do suffer from these days.

Might be as simple as a failed sensor or chafed wires someplace in the harness.

 

Sounds like a wiring issue going to the PSOM on the VSS line. Check the bulkhead connections on the firewall, something is loose when engine is torquing up.

 

Will do. That does make sense, but what harness feeds through to the cluster? I thought it was funny tonight on my way to work that under light throttle it would attempt to read, but still be way off and bounce. Under load, notta. Needle snaps (literally) back to the peg on zero. You can actually hear the needle hit the pin on the speedo.

I don't know how the circuit works, but it almost sounds like the signal is getting grounded maybe?