M1123 4L80E Trouble

[Redneck92]

Hello Guys,

I´m currently working od a M1123 6.5 NA with the 4l80e transmission.

My problem is, that the trans seems to be stuck in limp mode.

When I turn the ignition switch to the run position, Glownox does it´s thing, Trans light is glowing.
When I start the car, the trans light starts blinking. But it seems that it blinks not in any pattern.

What I have done to this time:
- checked the trans fluid with the dipstick ( red, not smelly, enough in it)
- new batterys
- cleaned all battery connections
- cleaned the grounds on the back of the engine
- cleaned the connections on the TCM box
- TCM inside the box is clean and dry. No signs of corrosion or moisture
- measured the trans circuit breaker. There are going 12v trough it

When I bridge the A and E terminals on the tcm, it flashes only code 12 in a loop. No other codes stored.

Did someone had such a problem with his truck and could lead me in the right direction?

Thank you guys in advance

 

[Mogman]

Did you let the codes finish?
It will flash 12 three times then give the codes, if there are no codes it will flash 12 four times then stop.
EDIT, if there are codes it will flash 12 three times, then each code three times then after all the codes are flashed it will flash 12 one more time then stop.

 

[Redneck92]

Yes I did let it finish blinking. Besides Code 12 there are no other codes stored.
Very unusual I think.

 

[Mogman]

Is it possible you had disconnected the batteries or removed the harness connector from the TCM before reading the codes, this would erase the codes

 

[Redneck92]

When I cleaned all the connections I had the batterys unplugged.

After that I plugged them in again and made a 5 mile test drive.

Symptoms as before, trans light flashing when the engine is running or when I drive it.

After the test drive I bridged A and E on the TCM again and no codes besides code 12.

I cant get my head arround why the trans light would be flashing unregularily without throwing codes when the engine is running.

 

[Mogman]

Also make sure the continuous 12V is applied to the TCM, there are two 12V feeds one is switched and one is continuous.
That would be wire 537D and pin Z on the TCM connector.
Do you have the trans supplement?

 

[TOBASH]

Let’s start from the foundation and work upwards…

Is the filter new and clean or old and clogged? You only mention using the dipstick.

Is the tranny full and topped off? You addressed this.

Are you using Dextron III?

Is your shifter properly aligned?

Is your transfer case in H and your tranny in D?

Are your batteries and connections good and clean? You already addressed this.

Is your throttle position sensor hooked up and adjusted properly?

Not trying to be funny here. It’s sometimes about the simple things…

 

[Mogman]

Also make sure the continuous 12V is applied to the TCM, there are two 12V feeds one is switched and one is continuous.
That would be wire 537D and pin Z on the TCM connector.
Do you have the trans supplement?
View attachment 916916

I would say if that checks out then the next step would be to substitute the TCM, the flashing of the trans light when running might be a TCM error

 

[Coonass77]

When I cleaned all the connections I had the batterys unplugged.

After that I plugged them in again and made a 5 mile test drive.

Symptoms as before, trans light flashing when the engine is running or when I drive it.

After the test drive I bridged A and E on the TCM again and no codes besides code 12.

I cant get my head arround why the trans light would be flashing unregularily without throwing codes when the engine is running.

I had the exact same symptoms - irregular trans light flashing while running and no codes... Ended up being a loose ground wire on the shunt in the battery box. It was siliconed over so it wasn't readily apparent, but I eventually found it by wiggling wires. Might be something to check.

 

[Redneck92]

It ran perfectly before with no Problems at all.

Transfercase is in H gear. I also checked the linkages from the transfercase and the trans.

I did not Drop the pan nor checked the filter yet. Fluid on the dipstick looks perfekt.

When I Shift the trans to R or D it shifts firm in a split second To either gear and goes forward or reverse as soon as I release the brake.

TPS is hooked up and contacts are clean.
I dont know if it is alignt properly.

I will measure the two 12v sources in a few days and I will check the siliconed commections im the battery Box.

I will reply back. Thank you guys so far