This evening the speedo started jumping around wildly on my '97 F-250HD (460, E4OD). Mostly it works fine, but every once in a while it drops to zero, jumps to full scale, or bounces back and forth between zero and full rapidly. So far it's always settled down after a couple seconds. The speed control gets all wonky when this happens, so it's affecting the inputs to that. One time the ABS light came on and stayed on (it went out and stayed out when I shut the engine off and restarted it). At least so far the trans is shifting fine.

I haven't had a chance to check for any codes yet. Hopefully I can do that tomorrow.

My question is how worried do I need to be about using the truck now? My only concern is if the trans decides that it's not getting the signals it needs and won't shift well. I'm planning on taking my family up to the cabin in this truck this weekend, and won't have time to do much before leaving on Friday. Should I be making alternate plans? Or am I unlikely to have a hard failure?

 

You can fix it yourself by Friday no problem if you can solder.

PSOM REPLACEMENT F150 1995 photo series, step by step

 

I can probably do that tomorrow night if I need to, but i do have other stuff I was hoping to get done too. What if I don't fix it right away? Is it likely to start causing more serious problems? I can easily live with a wonky speedo, no speed control and no ABS for the weekend. But my wife will not be happy if we get stranded.

 

An ounce of prevention is worth a pound of cure...

Think what it will be like to be stranded now versus later if it does.

 

Good advice, but it doesn't answer my question.

Are loose solder joints in a PSOM likely to contribute to a "hard" failure that will strand the truck?

So far I'm getting no symptoms that would indicate that this is affecting either the engine or the trans. For all I know, the PCM (that controls the engine and trans) doesn't know or care what the PSOM thinks. But that's just it, it's "for all I know," which in this case isn't much.

If the only thinks the PSOM talks to are the speedo/odo, speed control and RABS, then I'll drive the truck this weekend and probably try to make time to fix it next week. But if the PSOM could keep the trans from shifting, or make the engine not run, then I probably need to make other plans.

The post FORDF250HDXLT linked to talked about this solving rough running. That might be true, but it doesn't make a lot of sense to me that the engine would need to know the vehicle speed. So I'm thinking that the engine is pretty safe.

But the trans clearly needs to know the vehicle speed, so it's less safe. But does it get vehicle speed from the PSOM, or does it take the signal direct from the VSS, or does it use engine speed and knowing what gear it's in to calculate what it needs to know about vehicle speed? I don't know. So I'm asking if loose connections in a PSOM are likely to strand the truck.

 

If the PSOM fails altogether or bad enough to send trash signals to the ECM,then it can't send a clean signal to the trans either.
Trash in = trash out.It's my understanding the vehicle speed signal works as such; The VSS feeds the PSOM,the PSOM feeds the ECM,the ECM feeds the solenoids in the trans.
There is no doubt about it,without a working PSOM the trans will not shift correctly.To the point of stranding the truck? I would say no because you can always get away with shifting into manual 2 and just crawl home.Making the worse case,the trans can't be used in the D position because it could get lost and shift late,hard,and perhaps not even find 3rd on it's own without a clean vehicle speed signal.

In short,your wondering if the PSOM were to fail altogether,would it ruin your time away.The answer is 100% yes.Depending on how far you get from home,it would result in a tow,or I suppose at least a cab ride home for the wife,while you took hours limping a truck home on side roads where you could get away with going 25mph to avoid a tow charge if you don't have roadside assistance/tow.Maybe she'll ride home with ya,I just hope you pack some hear muffs.

 

Perfect, thanks. Sounds like if tonight doesn't give me enough time (and it looks like it won't) that we should change plans slightly and drive her car (2005 Escape) this weekend. Typically that's not a good option because we usually have 3-4 people and 2 dogs (which don't all fit in the Escape). But this weekend it's just the 2 of us (and the dogs). A little tight and it doesn't let us pick up the load of gravel I had hoped to put in the trailer, but better than risking ruining a weekend.

The crappy timing is that my Bronco is down right now as I replace a broken motor mount, so I can't just take my time with it next week either. Oh well, that's old trucks.

 

My wife said I needed to reevaluate my priorities, so it turns out I did have time to work on the PSOM tonight! The only catch I had getting it out was the gear indicator. The instructions in the picture link FORDF250HDXLT posted apparently were for a manual trans. With an auto I needed to get the gear indicator out of the cluster, which hung me up for a few minutes, but not a big deal.

The pictures also said to disconnect the fuel tank switch. I found it easier to remove the two screws that hold the switch to the dash panel. But no bigee there either

None of the solder joints looked bad, but I resoldered them all anyway. Boy, that was a pain! I've always hated soldering PC boards, but it's got a lot harder to see what I'm doing now with 53 year old eyes. And my finest tipped soldering iron bit the dust. The next smallest one is barely small enough to do PC boards. But I got it all together and it works. Of course the flaky speedo was intermittent yesterday and I never noticed it happening today, so I can't be sure I fixed it. But at least I didn't make it a lot worse!

I'll drive the truck to work tomorrow, and if I don't have any issues we'll take it to the cabin tomorrow evening.

 

So far so good! A successful 10 mile drive doesn't prove much, but at least it wasn't an unsuccessful one (which would have proved a lot). I'll come back and update this thread if things go downhill (I hate threads that end without being completed). But otherwise assume that it's fixed.

 

You said:
" One time the ABS light came on and stayed on"
The PSOM will not cause the ABS light to come on unless the VSS signal is shorted.
So you PSOM is more than likely OK.
Sounds like you have problems in the rear end or the wiring under the truck.

 

^Agreed. Pull the VSS and look for shavings - check wiring for chafing and damage.

 

Well then I'm REALLY glad I didn't make it worse by resoldering the PSOM!

Is pulling the VSS something I can do pretty easily without any major disassembly? I'm thinking it probably is, but I haven't looked there recently.