Hopefully someone can help me with this because I am about ready to set this thing on fire. This would be bad, because I need it for work. It's a great vehicle overall. 1991 E350 Ambulance w/ 7.3IDI NA and E4OD. Generally speaking there's a lot to love about a drivable tool shed, especially in my line of work. However, it has had a problem that has been getting worse the whole time I've owned it and it is getting so bad it's making me crazy.

When I bought the vehicle, the torque converter lockup only worked reliably at or above about 39mph in 4th. It also had issues in 2nd and 3rd, and seems to be dependent on engine revs, nothing else. I'll use 4th as the reference gear for the purpose of this discussion because it provides good comparison. When below the engine speed required, it tried to lock, starting at about 35, but wouldn't hold, so it's rocking in and out of lockup, and the revs bounced, and the vehicle lurched, until it got to 39, whatever revs that is, and could hold it.

Not too long after I bought it, the flywheel broke, and while I had the trans out, I had a local shop rebuild it. No sense not, while you have it out, right? Except that apparently these guys are morons, because it was basically exactly the same when I put it back in as it was when I took it out. Same stupid torque converter lockup issue. The guy drove it, and said my torque converter was junk, and sold me a fairly pricey replacement one, which gave me no different results at all. Whatever the issue is, it is either outside the transmission, or they missed it. I replaced the vehicle speed sensor as part of the R&R since the old one went to pieces. Supposedly the rebuild included a replacement solenoid body.

But it gets weirder. After about a month of driving on the rebuilt one, something changed, and the required revs to lock the torque converter reliably raised, suddenly and for about 50 miles intermittently, then consistently. At that point, suddenly 39mph wasn't fast enough to stay locked; now it needed 47mph. That means it was rocking from 35 to 47 unless I kept my foot into the loud pedal far enough that it didn't try to lock, or stayed in 3rd. A minor nuisance became a serious one. But I was busy, and had no time to pursue it farther.

About 2 days ago, something changed again. Having previously replaced both the transmission shift lever sensor and the accelerator position sensor, trying desperately to resolve this issue but again seeing no change at all, I was startled when suddenly it stopped even attempting to lock the TCC below about 45, and rocking as it locked and released from about 45 all the way to 57! 57! I wish I could capitalize a number! FIFTY SEVEN! There, I did. On top of that, I noted that some of my shifts are becoming a little slippy sloppy, which is unnerving to say the least. This is a large, heavy vehicle and will be a very expensive tow if I have to pay someone to move it for me.

Always the OD OFF light has blinked at low RPMs. It doesn't necessarily always blink when the TCC is rocking, but sometimes it does. It is pretty much always on now unless the engine speed is above a certain RPM. As this issue gets worse, it blinks at higher and higher RPMs.

I have suspected all along that there is a problem in the pump that is causing low pressure. The bozo who "rebuilt" it as much as told me he didn't do anything with the pump. To me it seems like the pressure is enough to lock the TCC, then it locks, and the revs drop, and it isn't enough anymore, and it unlocks. Where I'm baffled is that I can't see how it could suddenly change, unless perhaps there's a seal, maybe in the pressure regulator piston, that is literally losing chunks, and becoming instantaneously more leaky.

I am comfortable replacing this pump myself, but hate to remove this unreasonably heavy transmission AGAIN without knowing that I'm looking in the right place. I assume there's a test port, perhaps several, which could be connected to a gauge to see for myself what the trans pressures are doing. Can anyone recommend a specific procedure, or is it pretty straightforward?

 

Given the age of this van along with your described symptoms have you checked the PCM for faults etc? I don't know I'd call your transmission shop morons if this issue turns out being something NOT related to the physical workings rather than an electronic component.

 

Check your wiring. Every time the brake light comes on the torque unlocks. A crossed wire, a bad brake light switch, or even a broken filament in a turn/brake bulb can cause the torque to unlock.

 

The flashing OD OFF light means codes have been stored. You need these codes to fix this. Here is how to read them: www.troublecodes.net/ford

 

Wow, I'm honored that Mark took the time to respond. It's rare that a real expert can be bothered about stuff like this. I'm sorry to have seemed like I didn't care anymore but I've been very busy with multiple serious vehicle issues this week and I didn't realize that any replies had come beyond the first one suggesting a more conciliatory tone. Having a vane housing explode in the pump on a 4L60E in the only vehicle you own that can tow and needing to make a tow in a few days will do that to you! That issue is now resolved.

Unfortunately, I have been down all these roads and reached a no-resolution dead-end. I have a friend with a Snap-On Zeus and ALL the adapters, for which he paid dearly, and he has allowed me to use it on a couple of occasions. The first time I was checking things I got told the throttle position sensor (not what it's called on a diesel but that function) was giving junk values. I replaced that prior to my original post, but after the nudge from @JWA I ran codes again and got code 11, all good. Unfortunately whatever is causing the light to flash is intermittent, though worsening, and not getting stored.

I am aware of the way the brake light interacts with the TCC, and don't believe that is the issue. At sufficient engine speed it will lock and stay locked forever. If I ride the brake to keep the lights on it unlocks and stays that way forever, and this is how I would expect that system to work. Also I don't see why an erroneous brake light signal would cause the OD light to flash; as far as the computer knows everything is behaving as expected.

Further, when the engine speed is below certain RPMs, it no longer even tries to lock now. Between that RPM and a higher one, it will cycle the TCC rhythmically, on, off, on, off, on, off, with the off being longer than the on as though the instant it tries to lock the pressure plummets and it can't hold it. As the diesel is very loud, it would be easy for me to record it if that would be helpful in understanding what the behavior is.

A local transmission builder whom I've recently befriended has suggested that it might be the TCC valve in the pump failing and not giving the TCC what it needs to lock properly. This is plausible to me. He had said that he was going to contact the person who built the transmission since the builder has been avoiding me and just lives 6 houses down from him lol. I don't expect I'll hear back from him before next week though, holiday and all. Again, the original builder pretty much told me he did nothing with the pump, which is a disappointment to say the least. I do wonder if perhaps it's as simple as worn out gears and faces, because at higher revs it gives the characteristic unhappy hydraulic pump scream, and it has the entire time I've owned it.

At any rate, it definitely seems like that behemoth trans is going to have to come back out of the vehicle. Anyone who has an idea that might save me from that fate is a potential hero.

 

I think it is more likely that you are losing KAM when powering off. Codes ARE stored when the OD light flashes.

When the light flashes try running key on engine on self test without shutting the engine off.

 

As in, my computer is just not storing them properly? I guess it's possible.

I'd love to do KOER without shutting down, but unfortunately this thing has a weird old computer that simply will not talk codes except immediately on wakeup. If you try to get it to talk after that, it just doesn't. The only way to get it to talk to the diagnostic box is get the box ready, tell the box go, then power on the Ford computer. Any other procedure and it just sits there forever because the scanner missed the handshake from Ford and it only does it once.

There's a reason OBD2 changed this behavior. Unfortunately mine is older and not as clever.

I did just have a thought though; I wonder if I could trigger handshake with the one-pin connector by physically connecting it only after the box was ready to talk without shutting off the engine. Do you think this might work? I could probably try it in a few days. I have always been nervous about hot plugging things, but I'm getting a little desperate lol.