Diagnosing OD Flashing when shifting to 3rd. Brake light circuit?

JeffKPDX

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In the last few days my 92 idi NA started going into limp mode when shifting into 3rd. It doesn't happen every time but it seems to eventually happen if I drive long enough. It also happened when shifting form 1st to 2nd and from 3rd to 4th but the vast majority has been when going into 3rd. If I shut it down and turn the key back on the OD flashing stops and it'll crank fine and run fine again until it shifts into 3rd. Usually I'd pull a code with the key on and engine off but since it reverts back I haven't been able to.

Previously and unrelated to this issue I have replaced my tone ring, speed sensor, and PCM so I'm hoping those are excluded from what's wrong. I've also had my trans rebuilt with a HD torque convertor so I'm hoping it's electrical and not actually anything wrong with my trans. I replaced the FIPL today and it did drive better and seem fine for awhile but then when shifting into 4th it happened again. The RPMs are displaying properly so I don't think it's my tach sensor.

While looking for a solution I saw a comment by aggiediesel01 mentioning that the brake light circuit and TCC solenoid circuit are connected and that can cause a similar issue. I do have a trailer brake controller. Would that affect the brake light circuit? I have had issues with the ABS light going on in the past and it does feel like my brakes lock up quicker than they should when I brake hard. Like they will lock if I brake hard at 35. Is there a way to determine if this is the issue?

Any thoughts on other routes I should explore?

Much thanks!
 

Noiseydiesel

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Fords are reputed for range selector switches. You haven't done that one?
You mentioned you replaced the TCM.
Scan tool won't do a KOER test?
Generally if a PCM won't remember a code, it is a faulty PCM.
I suspect your TCM is of the same nature.
One of the ways to test a PCM is to have key KOER and disconnect a sensor.
This should cause the CEL to illuminate and the fault code should stay in the PCM memory.
If the PCM does not store the code in memory for KOEO diagnosis, then the PCM is at fault.
TCM should fall into the same category.
As to which trans sensor to disconnect with the engine running, I would tend to go with the range selector switch due to relative ease of access.
An internet search or you~tube may also provide an answer.
 

JeffKPDX

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I haven't replaced the range selector but I'll add that to the list. I replaced the tach sensor today since I had one laying around and topped off the trans fluid as I noticed it was sitting a little low. Took it out for spin and it did great for like 15-20 minutes then went back into limp mode after a shift uphill. This time I did try to do a KOER test and as soon as it started to pull code it reverted back to normal and OD stopped flashing.

I have replaced the PCM before so I'm hoping that it isn't that though I'd rather have an answer than replace a part twice. I'll try that out and see what I get. Thanks for the input.
 

aggiediesel01

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For the brake controller, it would depend on how someone wired it in. If your truck came with the factory tow pkg then the chassis wiring was setup to have a controller added without affecting any circuit in the truck. If it was added without that package then it could possibly be contributing to the issue depending on how it was done. You'll know if the tow package was factory if you follow the wiring from the controller under the dash. If it goes to a factory style 3 or 4 pin round plug and there is a trailer tow relay for the running lights under the hood near the fuse box then you have it, if you don't then isolating the added wiring from the factory harness would be a good place to start to see if it helps fix your issue. Be sure to check all grounds in the circuits of the harness between the TCM and the trans and make sure you've got a good ground and continuity from the trans case to the chassis of the truck. You'll find all the grounds in the link to the EVTM manual I PM'd you.
 

testlight

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The engine rpm (tach) sensor is also a common failure point. The sensor usually keeps working but misses so the trans gets an incorrect RPM signal. They're cheap to replace.

Something to consider.
 

JeffKPDX

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I do believe the brake controller was wired from the factory but I'll double check once I get back home. I installed the controller myself and the plug was already there. It's also an XLT so I believe that was included as part of the tow package. I could be wrong though.

I'll check the grounds next. That's a great idea and something I probably should have already done.

I did swap in a new tach sensor and unfortunately that did not stop the problem though it did seem to delay the onset of it though. Usually if I started it from cold (Portland cold not actually below freezing) it would go into limp mode at the first shift into third. After I changed the tach sensor it ran fine for the first 10-15 minutes and then went into limp on a shift from 3 to 4.

Anyone know the part number and programming call out for PCM with 4.10 gears? I forgot to mention that I have done a gear swap to a 4.10 and I'm not sure if that would contribute since the shift points are programmed for the OEM gear ratio. I believe it was the 3.73 originally.
 

testlight

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So I'm not super familiar with the system is there a speed sensor of sorts on the rear end? And would get confused by the gear swap?

I know there's a sensor back there but I thought that was just for the anti-lock brake stuff. I don't know.

Does the cruise control still work before and after limp mode? Does the brake light come on when it goes into limp mode?

I had a brake light circuit problem (aftermarket LED tail light bulbs) and it would make the cruise control stop working whenever I went above a 35 ish.

Sorry I don't have much of anything to contribute and I see that you mentioned the tach sensor before I brought it up...

Maybe there's a Ford transmission forum it would be better suited for this?
 
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