1996 F250, Ext cab, 4x4, 460, E4OD?, 71k miles.
Just in case it's relevant, for about the last 5 years the truck has been used to haul a 450 gallon water tank 2-3 miles up a decent hill, the past 2 years only used in the summer. Prior to 5 years ago it was only used to haul water in the winter (very seldom), and used to tow a 22' Bayliner 7 hours to saltwater in the spring, launch the boat once in a while during the summer, and tow the boat back in the fall. The truck has sat a good bit, I know there are some corrosion issues. Last October I pulled the battery and took it into the house to keep it warm so it wouldn't discharge and freeze. The truck sat without a battery in it until May.
The truck belongs to my landlady, she hauls the water. She said that last time she got water, the OD light flashed, and it hicupped sometimes. I believe that means it shifted hard. Her late husband taught her to engage the overdrive off system when hauling water, which she said she does and did this last time.
I am a retired aircraft mechanic and I've always done most of my own vehicle maintenance, so I have some mechanic skills and tools, but a lot of this stuff is new to me.
I managed to find a code reader (OBD1) and got the 452 code, no other codes. I was reluctant to test drive it. I saw somewhere that it should not be driven when shifting hard because the trans could be damaged. From my internet research, I get the impression it's not that critical. At this point I am thinking test driving is okay, hauling 3200 lbs of water up a hill is probably not advisable.

I would like to try to fix this myself. Based on what I have learned (a lot of it from this forum), I have a plan. I would like some input on whether or not my plan is good, or suggestions on what to do and not to do.

My plan:
Pull the VSS and check it and clean it, and check the tone ring.
Check resistance of VSS
Check / clean the plug on the trans
Check for a delay in shifting into reverse
Test drive to verify speedometer works (she said she's pretty sure it did), check if speed control works
Test drive with RABS controller disconnected
Test drive with speed control box disconnected
Perhaps check VSS output depending on what I find with other checks

Any problems with that plan?
Any thoughts on damaging the transmission by test driving or hauling water?

 

Looks like a good plan. I suspect disconnecting the RABS computer will fix the problem.

 

Thanks for the reply. I think I've done enough research to get a good start on understanding how the system works and how to proceed, but not sure. Some reassurance helps.

 

Speedometer needle moves at low speed, just took it up to about 10 mph in the driveway. I was going to drive it and check to see if the speed control (cruise control) works, but didn't see an indicator, so I thought it wasn't turning on. I found that it does not have an indicator light, so I will try speed control tomorow.

 

Cruise control won't engage under ~30mph. To answer your original question, you won't hurt the transmission by driving it around with the 452 code. But I wouldn't haul water til you get it fixed since the transmission may select the wrong gear at the wrong time.

 

I'm no Ford expert but if the code is for the speed signal I would recheck your diagnostic process.

I would stick with testing the speed signal starting at the diff and ending at the EEC-IV. I tend not to remove parts till I find what is wrong. Too many times, parts look good but test bad. So, testing would resolve that.

If I recall, the speed sensor makes a sine wave. That pattern is sent to the cluster. The PSOM takes that sine wave pattern and changes it to a square wave pattern for the EEC-IV [and cruise]. I believe the ABS uses the sine wave pattern. Until you determine if that system is working properly, you will probably be wasting a lot of time or money on parts.

I also believe there is a plug in the engine compartment that you can check the speed signal. Not certain if that is the square wave or sine wave pattern. You can also test the sine wave signal at the ABS module.

If the ABS module is corrupting the speed signal, then unplugging the module might make the problem go away. That is another simple task that needs no test equipment.

Anything with a capacitor is probably compromised since capacitor will leak over time. That leads to EEC-IV and PSOM's that might be bad.

Trust the information the EEC-IV is telling you. You got a speed sensor issue. Fix that and the odds are better than your problem will go away.

 

Thanks for the replies, good to get confirmation that driving the truck empty will likely not hurt the transmission. Also good to know that the cruise won't engage below
about 30 mph. I could see me trying to engage it at low speed and thinking it wasn't working.

I'm no Ford expert either, but here's what I think I know based on my research. Please correct me if I am wrong. BTW, I have no plans to replace parts yet, I am just trying to diagnose at this point.

Code 452 doesn't necessarily mean that the VSS is not putting out a good signal, it means the PCM doesn't like the signal that it's getting from the PSOM.

Continuous Memory DTC 452 indicates the PCM detected an error in the PSOM output signal during the last 40 warm-up cycles.
Possible Causes:
-- Damaged Rear Anti-Lock Brake Sensor (VSS).
-- Damaged PSOM.
-- Damaged harness circuits.
-- Damaged Powertrain Control Module (PCM).

The VSS signal drives the speedometer and the RABS, the output of the PSOM goes to inputs for the speed control module and the PCM.
The RABS module /computer can weaken or corrupt the signal going to the PSOM, disconnecting the module could restore the signal to the PSOM.
The speed control module / computer can corrupt the signal coming from the PSOM and going to the PCM, disconnecting the speed control module could restore that signal.
IIRC, I have seen where subford (member of this group who seems to be pretty knowledgeable) has said that if the speedometer works correctly, that is an indication that it is getting a good signal from the VSS which makes sense to me based on what I think I know about the system. Of course it would be good to verify that, but I dont have an o-scope. The best I could do is measure the signal with a voltmeter.
At this point I wouldn't eliminate the VSS signal being bad, but it doesn't seem likely to me. I need to test drive and verify that the speedometer works correctly, the RABS works, and the speed control works. The only problem I can think of with my plan is that if the fault is intermittent, my tests could prove nothing, but no testing will prove anything if the fault doesn't happen during testing.

 

Correct, but if it is hard shifting, probably in LIMP mode. The EEC does this to prevent further damage to the transmission, it uses really firm pressure to shift gears, and possibly even not shift gears.
That is why it feels like hard shifting.

If there is a mechanical failure, the damage is already done.
If it is an electrical failure, no new damage will be done to the transmission with firm/hard shifts.

I say go drive it, see if the speedometer and odometer work, or bounce.

 

Took the truck for a short test drive.
No hard shift, no flashing OD light. My understanding is that means that the PCM has not seen the fault since the truck was stopped and re-started.
Speed control works.
No annomolies noted in speedometer function.
No change with OD off engaged.
No noticeable delay when shifting.
Removed and cleaned VSS, cleaned corrosion from the pumpkin surface under the VSS flange. A light coating of very fine dark material on sides of VSS, a few very small pieces of something on thew face.
Saw one little defect in the tone ring, I would guess a casting defect that wouldn't affect VSS operation.
Thinking about re-seating the PCM plug, I assume that would clear codes and I'm not sure I want to do that yet. Also thinking about going to a couple of shops tomorrow and asking if they suggest making an appointment for them to look at it (they're 4 to 5 weeks out). The only thing I can think of that they would probably be able to check without a hard fail are the signals going into and coming out of the PSOM (on an o-scope). Not sure that's warranted for one fail that's not there now. I might put some miles on it for a few days to a week to see if I get another fail.

 

Good luck. Let us know what you find.

 

I'd clear the code, and go from there.
Maybe the code was set over a year ago, and never did it again.

Zero point in taking it to the shop, you've already diagnosed it. I don't think they can tell you any different.
And, if they aren't an old school shop, they probably don't even know what an oscilloscope is.

Also, here is a post where the 452 code was set, and zero problems with electrical or components.
Interesting read and possibly a one of, but if it only happens once there probably isn't a problem.
Troubleshooting Code 452 - Lots of detailed troubleshooting (SOLUTION IN COMMENTS) | Bronco Forum - Full Size Ford Bronco Forum

 

Thanks for the reply. I would guess you are probably right with the exception that the hard shifting would be a result of the PCM seeing a 452 code. So I believe that the code is recent.
I'll read through the post you linked.
I'll also guess that if I go to a shop and ask what they could do to troubleshoot that I haven't already done, I won't get to talk to anyone who knows anything but how to write up a work order.

 

I went to one shop on Friday and told them about the problem, and what I had done. They said there's no point in bringing it in until it's a hard fault, which will probably happen. On the way home there was some strange feelings, especially on corners. I pulled into a parking lot and found that the front hubs were hot. Last night I pulled the front wheels, apparently the calipers have been dragging for some time. It needs new rotors and calipers, or caliper rebuilds.I've never seen rotors that bad.

 

In that process, disassemble and check/adjust your rear drum brakes. The rear and front need to work together.
Every 3 or 4 months, I remove my front calipers, clean and grease the slides. Worth the effort to me. My '88 caliper slides are different than yours, but the principle applies.

 

Every 3-4 months seems way more than necessary, but I'll bet you calipers work great, and you don't have to worry about sticking.