7.3 IDI Tachometer Woes

[scooterrr]

I recently acquired a 1993 F250 4x4 with the 7.3 IDI for $2300. Came with a dash saying 150,000 miles, but who knows if that's accurate. It seems this truck started life off as a V8 gasser and one of the previous owners must've swapped in a 7.3 IDI and E40D auto. All things considered, this thing runs great and the interior is quite nice. The only problem I'm having is wrapping my head around this darn tachometer not working. When I bought it and started driving it back (boy was that fun, it had a big hole in the front fuel tank I didn't notice) that tachometer started working after about an hour driving. Just randomly, out of nowhere. And after that it worked maybe 1 or two times. Normally the tach just slightly lifts (it's getting power), but reads 0rpm. Here's the whole deal I've gone through trying to resolve this

I've replaced the tach sensor just to get that out of the equation, but that didn't resolve it. I tested to make sure the tach sensor was sending signal to the PCM harness/plug at the driver firewall and it was. I tested to make sure the tach is getting power at the dash and it was. I've checked all fuses/relays on this entire darned truck and not a one is bad. And in my other fiddling around with this thing, I found someone on a forum post mention some folks jumping pin #4 and pin #36? (circuit #11 and circuit #648) to resolve this issue. And wow that fixed it... for a short while. I must've drove it 20 times with it working and now it's just stopped (same symptoms)?

I'd like to clarify, when my tachometer is not working my truck IS stuck in limp mode (i.e. only has 4th/OD, and when column shifting only has 2nd), HOWEVER my OD light is not blinking. I can't get in/out of OD, but it is NOT blinking. That seems to be a symptom everyone but me is having.

Any thoughts on this would be much appreciated. I wanna start using this truck haha....

 

[IDIBOBS]

Your trans computer is not getting good power. It’s either a bad shorted wire or the trans relay. Mine used to do this exact thing when it got cold. I replaced the trans relay and I have not had the problem since.

 

[scooterrr]

Your trans computer is not getting good power. It’s either a bad shorted wire or the trans relay. Mine used to do this exact thing when it got cold. I replaced the trans relay and I have not had the problem since.

I assume you're referring to Relay 1 in the fuse box in the engine bay? I've checked this relay and so far as I can tell it is working just fine, so I don't think it's that. I also swapped it out with one of the unused relays (same part number) from the unused slot a and symptoms were exactly the same. Any specific wires/grounds you could think of to check here?

 

[IDIBOBS]

I assume you're referring to Relay 1 in the fuse box in the engine bay? I've checked this relay and so far as I can tell it is working just fine, so I don't think it's that. I also swapped it out with one of the unused relays (same part number) from the unused slot an and symptoms were exactly the same. Any specific wires/grounds you could think of to check here?

You’re gonna have to check the wires in and out of that relay and I to the trans controller. This seams like a classic symptom of the trans controller not getting power to it. Time for some diag

 

[scooterrr]

You’re gonna have to check the wires in and out of that relay and I to the trans controller. This seams like a classic symptom of the trans controller not getting power to it. Time for some diag

I'll take another look at things tomorrow, and write down my results so I can share them here. Fingers crossed it's just a bad ground or something.

 

[gandalf]

The transmission receives input from 3 areas, if I recall correctly, the tach sensor, the FIPL, and the speed sensor on the rear axle. If one of those is not working properly, the transmission may/will go into limp mode. These signals go though the TECA (I think thats Transmission electronic control a...). That itself map be a bad unit. Also, I'd strongly suggest get the error codes for the transmission. You need a xxxx1 code scanner for that.

 

[scooterrr]

The transmission receives input from 3 areas, if I recall correctly, the tach sensor, the FIPL, and the speed sensor on the rear axle. If one of those is not working properly, the transmission may/will go into limp mode. These signals go though the TECA (I think thats Transmission electronic control a...). That itself map be a bad unit. Also, I'd strongly suggest get the error codes for the transmission. You need a xxxx1 code scanner for that.

I think I've got a code reader hanging around somewhere, I'll take a look for that and see what I can find.

As for those inputs, I'll take a look at those tomorrow and see what I can figure out. Although, I'm kinda at a loss as to why my jumping pin #4 with pin #36 seemed to be the resolution for quite a while, but just didn't work when I went to start it up this morning. It's not cold here, actually if anything it's warmer than it's been recently. Maybe a grounding issue with the PCM (TECA)? My understanding of what I was doing when jumping pin #4 to pin #36, was I robbed the ground from pin #4, so maybe my ground for circuit 11 is having issues.

Also, speedo and all that stuff works just fine. The only thing that I'm noticing being new with it not working this morning is the wait to start light doesn't work. I presume this isn't linked to the tach in any way, but I'll check the wire for it tomorrow on the relay.

 

[scooterrr]

The transmission receives input from 3 areas, if I recall correctly, the tach sensor, the FIPL, and the speed sensor on the rear axle. If one of those is not working properly, the transmission may/will go into limp mode. These signals go though the TECA (I think thats Transmission electronic control a...). That itself map be a bad unit. Also, I'd strongly suggest get the error codes for the transmission. You need a xxxx1 code scanner for that.

So I was just able to work on it for a bit before the rain started pelting me. Here's what I found.

PCM is definitely getting power, and the grounds seem to be working just fine. I'm getting ac signal from the tach sensor as I should, and there doesn't seem to be a grounding issue on circuit #11. However, now I'm more confused. My tach works when the PCM harness is unplugged??? AND circuit 11 and 648 are jumped. It does not work with PCM harness plugged in with the same configuration, even if I add another ground to the circuit. What in the world is going on here. Why would my tach work when the PCM harness is unplugged, and why would it only work when I jump those two circuits? This doesn't make sense to me. I feel like I'm missing something major here. Also, still no blinking overdrive light or any of that still.

 

[scooterrr]

Interestingly the tachometer does work with the PCM harness plugged in and the PCM relay pulled. However, it reads really low RPM's (like 150-200rpm), but reads normal RPM's when PCM harness is unplugged. Also all of this was done while, and contingent upon pin #4 and pin #36 being jumped. Is this an issue with the PCM itself? Do I need a replacement PCM or is there something else going on here. Thanks!

 

[gandalf]

Okay, you're into this way beyond my knowledge level. For this, and perhaps future problems, you might consider buying a rather detailed ELECTRICAL and vacuum (that's a link) manual for your truck.

I have that same manual, in printed format, for my '92 truck. The information is very good, as are the diagrams, though sometimes a bit difficult to find.

According to my manual, pin 4 is the RPM sensor (positive), circuit 11. Pin 36, circuit 648, is the tachometer feed to the instrument cluster. My best guess is that by jumping those pins you're simply bypassing the TECA and feeding the RPM information to the tach rather than running it through the TECA.

 

[scooterrr]

According to my manual, pin 4 is the RPM sensor (positive), circuit 11. Pin 36, circuit 648, is the tachometer feed to the instrument cluster. My best guess is that by jumping those pins you're simply bypassing the TECA and feeding the RPM information to the tach rather than running it through the TECA.

This seemed to be kinda the idea from what I gathered from the other post I found. It just seems strange to me that by jumping those pins I was able to fully resolve the issue for such a long while. The OD button worked (turned on and off), and it shifted through all gears as it should (so it wasn't bypassing the TECA?).

My assumption is that Pin 36 is given the signal from Pin 4 in that magical electronic box and then sent to the tachometer. I feel like it's gotta be a TECA issue since directly feeding tach signal to the tachometer allows it to work (when TECA harness is unplugged, and those pins are jumped) as it should, but when that signal has to go through the TECA it's lost. IDK, I'm starting to be at the end of the road for anything else I can think of testing.

Also, thanks for the links. I'd like to get a physical copy, but for the time being I've been using charm.li. Maybe some other folks might find that helpful, although it's somewhat annoying to navigate.

 

[scooterrr]

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Well I popped the PCM out and it seems like this blue square component is burnt up. Does anyone know what that component is, I can't for the life of me find what this is online. Any help would be appreciated

 

[Nero]

That capacitor next to it also doesnt look too happy....

 

[gandalf]

Nero is right. That capacitor with the bulging top is very suspect.

 

[rvitko]

Generally on old auto boards, just replace every cap on the board and reflow the solder on everything else and they are like new. Electrolytic caps are polarity dependent so keep track of that when you replace them. They generally have a 12-20 year lifespan so definitely due and it’s a fairly easy job. In some cases the boards are lacquer coated and need a soak in lacquer thinner before you work on them. I redid the cruise control, climate control and temperature sensor interface board on my 300D Mercedes and it went pretty smooth and if you shop around you can find better NOS caps on eBay, like Phillips etc instead of ****** junk