Hey everyone, this is my first post here . I'm not a mechanic, but I normally switch out my own clutches and do my own head jobs (not the milling of course) as a general representation of my skill. This is only my second automatic transmission vehicle though. I also feel pretty comfortable on the electrical side of things.

I bought my first Ford, an '91 F250 with 130k mi, off a widow of a friend. I knew he had barely used it, and was original owner. It's had the usual problems of a car that did a lot of sitting, but I had thought I had everything running really well for about two months.

Then after work, I got in it to drive home and the speedometer, odometer, and radio are all dead. Rest of the panels and gauges are working, and the transmission started shifting hard. I assumed it was fuze and pulled off the road and checked both fuze boxes (if there's a third let me know, I'm guessing there is, I just couldn't find it/don't have a manual). I couldn't find a blown fuze.

At this point I crank it up and drive home in 2nd as this is a good gear for most of the drive, and immediately starting thinking about VSS and grounds. Get home and the VSS looks clean, and the wires seem to be carrying fine. I was going to grab a new VSS on the way home, but I was thinking with the radio out, there's a problem somewhere else. I'm really hoping it's not the ECU.

Any ideas where to start hunting? I've looked around the forum and seen a couple of similar things, but not quite this problem. Sorry if this is in fact a double post.

Side note: If anyone has a triangle window for one of these, I'll buy it off you, along with a service manual if it doesn't look like it's been washed in used motor oil or transmission fluid

 

I thought 1991 model year trucks used a mechanical speedometer/odometer?

 

The odometer at the very least has a digital read out. And I'm pretty sure it was a VSS Though I could be way off.

Combined with the radio going dead, I really feel like this is an electrical problem. I will dig more on the details. Anyone know for sure?

 

Wow, I feel like an ***. It's a 1992. Sorry slip of my mind.

 

Did you pull all the fuses and check each fuse individually or did you just look at the tops while they were installed?

 

I'm pretty sure the 91 has a mechanical gear driven speedometer. But if the radio quit, there's a good chance that you've got worse problems. Perhaps a mouse chewed through some wires when it was sitting and they came apart now. Perhaps you lost a ground somewhere? Or perhaps you do have a bad computer. Crazier things have happened

 

He's talking about a digital odometer and a non-functional speedometer so obviously he made a mistake in the OP and his truck is a 92 (or later).

Sounds to me like the PSOM failed. Do a search in this forum it's not an uncommon thing.

 

Yes, sorry everyone as I said just before this volley, it's a 92. Last truck I owned was 91, just got used to saying that.

I actually pulled each individual fuse, and they seemed fine. I didn't hit them with a multimeter, so I suppose I could check more thoroughly, but I feel like visual check is normally good for a vehicle fuse.

I like the PSOM failure idea, that does seem to line up well. Except the radio going dead? Does PSOM failure often cause that? I suppose it could be a coincidental failure, but it seems more likely that it would be related.

Double checking all the grounds is on my to-do list for tomorrow morning. And I just found the wiring diagram through here so thanks for that.

 

Do your dome and cargo lights still work or are they off as well?

 

The radio is on a different fuse, 11. The PSOM is fed constant power by Fuse 8. It is fed Key On power by Fuse 18.

 

Or at this age, the radio may simply just be dead. Check to see if the dome light still works or not (the PSOM/Speedometer share one of two fuses with the dome lights) then check to see if your brake/shift interlock is working (the other fuse).

 

I didn't notice so had to go out the parking lot and check. Both the dome lights and the shift interlock seem to be working fine (would have noticed interlock failure earlier).

Radio could be dead. It is after market (I didn't do the install), and it seems to be from 2004 (in a wonderful twist I have the owners manual for the radio and not the truck).

I couldn't find any varmint tail tail signs when I was first tuning up the truck, so I don't feel like that's the problem. That said I do park it outside. Are there any common wiring failures that might be directly to the power distribution box (or anywhere high up on the power circuits)? I feel like I have to have a power problem pretty early.

I don't have any codes/MIL/ABS coming up, which brings me back to the ECU. Yet the truck does seem to be running fine if I throw it in 1st or 2nd....

I'm not going to have time to get really down and dirty on the wires until Saturday morning, but any guess on a starting locations would be nice.

Again I'm not eliminating PSOM + Radio combo failure, though that seems pretty unlucky if it's true.

 

If the dome lights and the interlock work (won't let you shift out of park unless you press the brake pedal - make sure you test to make sure it's still working that way as a lot of people remove them) then the problem isn't power but the instrument cluster itself - the PSOM, most likely.

The dome lights working show that fuse 8 (which feeds the speedometer as well) is working. The interlock working shows that fuse 18 is working (which feeds the speedometer as well), so that means something downstream of the fuses has died and it's not an upstream power supply issue.

 

My 1992 F350 did not have a shift interlock, it did have the E4OD transmission like the OPs. My 1992 EVTM has no mention of a shift interlock either.

Perhaps it was used on lighter duty trucks of that era? It would not be the first time Ford's service documentation omitted a little tidbit like this.

 

Sorry for the lag. I took some photos and I'm working on the write up for the fix. It was in the wiring. I don't have a pay account so it seems I can't post images or a marked up diagram. Is it kosher to post links to the images?