Good call out, updated original post.

 

There is. However I fear, if the issue is your wiring between TECA (the trans control unit) and pack connector, this may be jumping ahead. However, here is how. Notice the second column of instructions.

 

To test your wires between the trans controller harness connector and your solenoid pack:

Pull the trans control unit's harness. It's held in by a 10mm bolt. I don't know where it is on your rig, but on the trucks, it's located down
by the drivers side inner fender. It's hooked right to the trans control unit. The 10mm bolts runs right through the middle of it. Holds load of wires. It's a large 60 pin connector and they're all numbers. Know the one? Though many pins are empty, there's still 40+ wires in it.

Unbolt it from the trans controller and notice all the pin #s on it.
Get under the truck and unhook the solenoid pack.
Now get a new wire (light gauge is fine.) long enough to run between the two connectors.
Set your meter to ohms and touch the leads together = make sure you get 0 ohm or replace batteries.
Now test your new wire and make sure this reads o ohms (don't even trust the new wire, just do it. Takes a second.)
Now your going to test the wires one by running down to the solenoid pack.
One pin at a time focusing on the EPC circuit!
Hook your new wire into one of the trans harness pins that lead to the sol pack, and run it down to you. And measure the resistance between your new wire and the solenoid pack pin. This way you see if you have a bad wire, see?

Now, ensuring your test wire doesn't move in the trans harness connector, shake and wiggle the wires as you read your meter. Watching for any resistance. If it spikes, you found a bad wire and it needs to be replaced.

See all the wires at the pack here in the diagram? Just above #21 at the bottom? Notice all the solenoids? That's the solenoid pack plug of course. This isn't as hard as it looks!!! See, there's only a few wires to go through? It's just one wire at a time.

If it's easier, you can unhook the whole trans harness, but you have to unhook the other sensors (4wd switch/MLPS (called range sensor now) and fight it up through so you can just pin directly into both sides of the connectors, see what I mean? Trans controller harness and pack connector? This is why I suggest just using the long wire to run it down to your pack, see? But I'm not there to see the harness. You might see it's just as easy to just hook it and bring the sol-pack up right by the trans unit connector.


This is the diagram for the 92-94 idi trucks. I don't know how much, if any, it differs from your truck. Ideally you'll find this for your truck. Maybe someone who knows where to find it, will join in and post it for you. If not, you can use this as a guide at least........




Do you understand these instructions? Just, don't forget to shake and wiggle the wires, especially if none of them are reading any resistance.



Why is this important?
1. You got the EPC code on a NEW pack.
2. You claimed in post #1, I just see, that there's a new TECA ('94) Installed. If that can be trusted as good info?
3. You can drive for a bit before bad shifting occurs. Did you know resistance in bad wires, increases as they heat up?
4. All roads now point right here. Aside from this, I know of nothing more that will cause code 99. Someone else might.
5. It's FREE (or cheap, if you need to buy a 10' of 14awg haha)

To fix any that read high, and or spike when you wiggle them (indicating the wire is corroded/broken) you'll need to replace the bad wire(s) of course. The odds are, you'll be able to snip the bad lines 6-12" away from the plugs and use heat shrink butt connectors to replace them. You can ohm the short sections again while wiggling them first to verify, see? If you get unlucky and get resistance in your 6-12" tails, you'll have to see about getting luck to snip back to 3" or re-pin in new wires.

Any questions? Think you got it, or did I might as well write this in French? You got this, right? If not, you can explain it to the trans shop and have them do this. It's easy really. All you're doing is checking for bad lines using an ohm meter. You can do this.

You're not getting any of the shift sol codes, so you don't have to test them all........But I would (you're right there, and it;s only like what? 8 or 9 in total) Focus on those two EPC wires though. Hopefully you find a bad wire right here, explaining this whole mess.

 

Instructions are perfect and understood. I’ve worked in the physical layer in telecom for 17+ years, and sat/radio coms for the Army before that so most of this make sense. I have a pretty solid foundation for troubleshooting electrical circuits, just need to be pointed where to look sometimes.

Also Mark mentioned that there’s a factory pinned wire to the PCM that tells it when 4x4 low is engaged and it would cause very early shifting. Even though that wire wouldn’t be connected to anything on my rig it may have the potential to have been grounded out over time. Going to look for that as well.

I have to say, thank you very very much. You’ve taken a lot of time and put a lot of thought into helping me with my issue. I’ll get to testing those individual wires tomorrow morning. I can pull the harness off the transmission through the doghouse in the cab to make my life easier working between the harness at the PCM.

I’ll report back with my findings tomorrow. Thank you again!

 

Oh awesome!!! So glad to hear that, after explaining it all. I might have been far less detailed, had I known your knowledge level first, but It may come in handy for someone less experienced with wiring someday anyway, so it's just as well.
Oh yeah, you got this no problem, unless of course my 92-94 schematic is worthless to you haha! I have a feeling, it's more than enough as a guide for you anyway. You know how to figure it out from here. I hope this is it.

 

Too easy! I can intercept those wires on the top of my transmission (pictured below) and use a simple test light with alligator clips. I have some nifty wire taps I can use to give me a test lead or can just expose a little copper and clip on that way. I’d really love to know if the transmission is getting a signal triggering a shift. Knowing if it’s before or inside the transmission would be huge as far as narrowing things down.

 

No details are great, when we eventually sort this issue out others can turn to it to help themselves. It’s perfect!

 

I’m finding that my wire colors are not matching up with the wire diagrams I have for the 90–91 year ford F series and Bronco. Anyone happen to have a E-series van specific diagram?

Also I’m 99% confident that the issue only happens after I turn the truck off and restart it for the day. The first drive no matter how long is completely problem free. I just did a 1hr + drive with zero problems. Turned the truck off, waiting about 5 min, restarted and bam, like clockwork….shifting early.

I was reading there were people experiencing very similar issues on 92 and later model years where the PSOM in the instrument cluster went bad. I’m pretty sure my 91 E-series does not have a PSOM. However are there any of the transmission circuits that interface the dash before routing to the PCM? I may pop the cluster out to inspect regardless.

 

Can you clean the connector up and see the pin #'s though? They're stamped on the plastic. Once it starts to shift too early, you should be getting a code....at least eventually (probably 99). You disconnecting the battery, probably isn't helping us.

Do you know what that wire is?

 

that was a dead wire I found in the loom coming from the PCM. I know, I got excited when I saw it first time also. There were no other wires in the loom matching it, I opened it up and peeled it back about 8 inches. The bare copper you see is from me exposing it and attempting to find continuity to anything else in the connector. It had none, not even on the matching pin opposite it on the connector. It’s also a smaller gauge wire compared to everything else.

 

If you don't see the pins #'s, you can count the placement. We know they're the same because all '94 trans controllers retrofit back. With both sides disconnected, you can jump power to one sol pack plug pin at a time, to send 12v to the trans pin harness while disconnected. Test for power to find it. Don't just trust the 12v as being good alone though, then do the resistance check on the line (while shaking and wiggling it). See what I mean? You should be able to find the corresponding pin placement using either method. If you find one really bad, maybe it will read like just a few volts while temp hunting for the line too. Start with sending power up (use a jumper with a inline fuse for saftey) the trans harness (again only with it disconnected of course; we don't know who else is reading) from pin 11 and then 12. These are the two main lines of interest.

sol pin #12 goes to trans harness pin #35 (color of wire doesn't matter)
sol pin #11 goes to trans harness pin #38 (color of wire doesn't matter)

Caution!
You wouldn't send "testing power' from the trans harness side, because you don't know where it might lead this way. This direction of course sends wires out to other sensors. You wouldn't want to "hunt" this direction, because if your wrong, you may fry a sensor.
You can "hunt" using power from the sol-pack plug (with the main trans control harness unhooked first of course) because all wires lead to the trans harness controller only......Don't send power up pin 8 at the sol-pack. At least not before unhooking all those sensors in the line first. You'll want to unhook them anyway so they don't frig up your resistance readings. So, just so long as they're unhooked first.
Even if you're sure of the pin #'s I still don't recommend sending testing voltage from this direction, because it's easy to make a mistake. Send power up from below, instead of down, it's far, far safer. Besides, if you know the pin # at that controller, then just test for resistance in the first place.

 

Please post back saying you found a corroded EPC wire today, @4x4E350Ambo Crossing my fingers for ya.


Well, if a line has corrosion in it, as it warms, the resistance increases. This can explain Code 99. There's no indication the trans controller is having any issues with a speed sensor. Hot or cold. Why would it continue to be fine, for as long as you drive it once warm and then suddenly be an issue, on next startup has me confused. Before you do shut it down, are you taking off from a stop again, to make sure it's actually still shifting correctly, until you restart the truck? If so, I don't know what the heck that would be, other than perhaps a failed trans controller? Hopefully you're performing a driving test today, and this will shed some light on whether the trans controller is working correctly at first start, then once that warms up, it's no longer working correctly. That would be strange, considering you claim the po just changed it, but maybe he found a used one and didn't get a reman? Maybe it was just never good. I have a good feeling, you'll find something today.

What should have been an easy failed solenoid swap, is giving us a run for it's money, that's for sure.

 

Well folks, we may have a winner today! While ohm testing, tearing apart every inch of my wire harness inspecting and rebuilding it I happened to notice that the harness 12v connection on the alternator wasn’t really connected properly. I couldn’t see it from the front of the van/engine hood side and had previously only been able to feel the nut on that post blindly. This time being able to see it from the back I saw that it was actually only snugged against another nut that had wandered almost all the way to the top of the post and I was able to move the connection with little effort. Snugged everything back up properly, put the truck mostly back together and took it for several test drives this afternoon.

well holy crap, no shifting issues! I’ll know without a doubt over the next couple days as it give it some more test drives but when I saw that post it all made sense to me.

What I can only assume was happening was the 12v current was varying as the truck vibrated from start or low RPM. Maybe also since RPM’s were lower the current wasn’t quite strong enough to hop whatever small gaps in the connection where being created. I think the amperage dips were either being interpreted by the PCM as acceptable shift points thinking in it was originating from a sensor (it can’t know) or significant enough that the solenoids were triggering as though the grounds were opening (this is probably most likely).

Anyway I will try my hardest not to get to excited just yet, but here’s a couple photos from my work today. I’ll update at the end of the weekend on how the trucks doing. Cross your fingers and toes!


Checking behind instrument cluster

Verifying no damaged wire coming from the cruise control, RABS and anything else I can see electrical

My harness is split it half for testing/repair between engine bay and transmission. This made testing much easier.

Removed all the protective loom for inspection

Then solenoid pack plug took the weather seal from the transmission with it on removal

No visually identified problems, ohm tests passed. I measured every wire since I was this far along

Putting it back together

Like new.

This is the post on the rear of the alternator. I had already snugged it down for this photo but both nuts and the ring connector had wandered almost all the way off.

 

I don't want to jinx anything haha but I'm not so sure that could have caused this. When you lose power to the trans controller, you'll get high gear start in D position and have 2nd gear in 1/2 positions. Besides this, your battery will still provide a constant 12v even if the alt isn't charging the batteries and you weren't having charging issues were you?

I wonder if moving the wires around might be more likely? Did you wiggle the wires really good while testing them for resistance?
Everything screams of a bad EPC wire.

Well, no matter what, it sounds great! Making progress nonetheless. Early shifts and code 99 (EPC) failure due to a loose connection on the alt...... I'm not sure about that........But I want to stay upbeat about it, so any progress is great, that's for sure.

 

I've been following silently.
Could blame a spike of some sort. BUT shifts good on the first start and malfunctions after restarting... Whatever it was hopefully it has been fixed now.
I rebuilt starter wires a while ago and all was good until one day starter won't crank. Turned out I never got starter solenoid wire tight enough. That's a different story of course.