91 7.3 idi from charging problem to transmission issues

[FineLineIDI]

Hey guys, I've been helped a lot just reading and researching on here maybe you could help me out with this.

Basic question: Transmission is not shifting, ODO light stays on, RPM gauge and tachometer not working all happened at the same time after mechanic worked on electrical system (battery, alternator, voltage regulator, and starter relay)

Back story:

I've got a 91 f250 7.3L IDI. A few weeks back it failed to start after work due to the glow plug relay just clicking and not sending power to heat the cylinders. The next morning the truck's batteries were completely dead. Towed it to a shop where they replaced the alternator and one of the batteries and said they fixed it. A week later the batteries were completely dead again. the guys replaced the external voltage regulator and the ignition solenoid (or starter relay mounted on the passenger side fender) and the charging issue went away.

When I picked up the truck I noticed the RPM gauge was not moving at all, the ODO light was stuck on whether i pressed the button or not, and the transmission seems to be in some kind of fail safe mode where if it is in drive it doesn't shift at all as you accelerate. like it is in constant overdrive or 4th gear or something. It shifts into reverse park and neutral fine. In low one and two it has more power at low speeds but in drive it the engine revs but accelerates like a snail.
The mechanic who worked on it wouldn't even acknowledge there was an issue with the trans.

Took it to an aamco. they said it had something to do with the powertrain module and it looked like the wires of the harness going into the cab were corroded but on my inspection they

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were a little dirty but i didn't see any corrosion.

The Glow plug relay at this point began to keep clicking and not work at all so I checked it out and the terminals and connections were all burnt up so I replaced the solenoid and all the glow plugs but it is still doing the same thing.

I know somehow all this electrical stuff is related and I was wondering if anyone has experienced a similar problem or has the knowledge of this truck's electrical system to help me out.
Thank you!

 

[IDIoit]

you have electrical gremlins for sure!
i would start with checking all your ground wires.
battery to block, block to body, block to chassis and chassis to body.
redundant, i know, but they need it!
then, pull every fuse, disconnecting batteries, and with an ohm meter, checking for what circuit is grounding out.
this is more than a corrosion issue, you have some melted wires, somewhere.

 

[trackspeeder]

Check the TCM relay under the hood. Check for a blown fuse too. No power to the TCM will illuminate the OD off light. No codes will be stored because it is dead.

 

[The Bus]

On a side note, when replacing batteries, it is always best replace them in pairs. If one battery is weak or bad it will shorten the life of the other battery (parasitic). My friend learned this one the hard way.

Just take your time and follow the advice you'll find in these forums. They'll get you to a solution.

 

[FineLineIDI]

Hey guys. Thankyou for taking the time to reply.

I just went under the hood and checked the power terminal on the glow plug relay solenoid with the ignition turned and there was zero power so I traced it back to the starter relay and realized those dudes had wired it to the terminal meant to trigger the starter motor solenoid. I moved the connectors to the constant battery power terminal and what to you know, everything works fine.

just have to see if that charging issue comes back now that that solenoid is getting battery power.

The Bus, I'll have to save up for another battery soon... these guys aren't cheap.

 

[The Bus]

Hey guys. Thankyou for taking the time to reply.

I just went under the hood and checked the power terminal on the glow plug relay solenoid with the ignition turned and there was zero power so I traced it back to the starter relay and realized those dudes had wired it to the terminal meant to trigger the starter motor solenoid. I moved the connectors to the constant battery power terminal and what to you know, everything works fine.

just have to see if that charging issue comes back now that that solenoid is getting battery power.

The Bus, I'll have to save up for another battery soon... these guys aren't cheap.

Good to hear. As far as the batteries, I have three of those "little jewels" to replace. I schooled myself after the first ones did not last as long (less than two years ). The second ones lasted over 7 years.

I feel your pain.

 

[OLDBULL8]

The mechanic who worked on it wouldn't even acknowledge there was an issue with the trans.

He sure as hell is not a mechanic, I'd say not even a halfassed shadetree one. A parts changer, and not a very good one at that.

Come to this OB forum for the next time you have trouble.

 

[madpogue]

I traced it back to the starter relay and realized those dudes had wired it to the terminal meant to trigger the starter motor solenoid. I moved the connectors to the constant battery power terminal and what to you know, everything works fine.

There must be some kind of infection going around; this is at least the third time I've seen a post to this effect. What on earth would compel a previous owner (or their "mechanic") to wire the GP circuit this way? Something in the water supply? :

 

[IDIoit]

theres only 1 cure for stupidity!

one of the reason i never take any of my cars to a mechanic.
some of those guys have more screws loose than i do!