4L80E - No Power?

[The_Captain]

Received my GP M1123 today. Went through it today, put in glass/oilchange/CDR Valve, and then took it for a spin.

Drove good, but noticed the transmission wasn't shifting. Did a bit of research and it seems it's in limp mode so I tried the jumper diagnostics...but notice my trans light doesn't come on. Even a normal start, I get no trans light.

Is there a way to confirm power to the trans? I went underneath looking for loose connectors or something unplugged but all seemed correct.

 

[Mogman]

Yes you get the docs and troubleshoot it, sound like you have no power to the TCM

 

[NDT]

Look behind the battery box, there is the circuit breaker for the trans ECU. Wires from it should be going to the battery.

 

[Hummer Guy]

Received my GP M1123 today. Went through it today, put in glass/oilchange/CDR Valve, and then took it for a spin.

Drove good, but noticed the transmission wasn't shifting. Did a bit of research and it seems it's in limp mode so I tried the jumper diagnostics...but notice my trans light doesn't come on. Even a normal start, I get no trans light.

Is there a way to confirm power to the trans? I went underneath looking for loose connectors or something unplugged but all seemed correct.

One thing to check is the internal wiring harness as well. Mines done the same thing and it was because of the internal wiring harness being jacked up

 

[TOBASH]

After confirming fluid levels….. First thing to check is was the TCM removed? Next thing to check is are the electrical connections to TCM functional.

 

[The_Captain]

Thanks guys. Didn't realize about the relay box. I'll be back with the truck tomorrow and will start with fluid level and will test around with a multimeter.

TCM is certainly still in place, however one thing I did notice is a bit of electrical tape wrapped around the harness coming into the TCM.

If power is coming into the relay but not out, any concerns with jumping the relay to get power to the TCM?

 

[Mogman]

Do not "jump" anything, if the relay is bad replace it, there is a mixture of 12V and 24V at that relay (coil is 24V) one mistake and it will get much more expensive, if you can't afford to do it right the first time you certainly cannot afford to do it twice!
Allot of relays get replaced just because folks do not want to take the time to correctly troubleshoot the issue and it does not solve anything that, is just wasted money, just like the grounding "kit"

 

[The_Captain]

So went through the troubleshooting and leads me to replace the relay so I'll be ordering that today.

Interesting though, Pin 3 was going to 290B and Pin 4 was going to 290C.

I flipped them back correct, but still nothing.

If Pin 4 was connected to 290C, would that just be bypassing the relay?

 

[Mogman]

No, 290B has to be connected to 290C to power the TCM it really does not matter which way it is connected to the relay contacts.
So you have 24V across 29 and 57 and the relay does not close?

 

[The_Captain]

No, 290B has to be connected to 290C to power the TCM it really does not matter which way it is connected to the relay contacts.
So you have 24V across 29 and 57 and the relay does not close?

Correct. You can hear something happening with the relay but possibly just not closing all of the way.

Flowed through the TM flowcharge right to replace relay.

Mac Motors is only a mile or so from the truck so no big deal to pick one up.

 

[Mogman]

And you have 12V feed to the relay from the transmission breaker on wire 290B? if so looks like you have a handle on at least one problem, one step at a time will get you there!!

 

[The_Captain]

And you have 12V feed to the relay from the transmission breaker on wire 290B? if so looks like you have a handle on at least one problem, one step at a time will get you there!!

Yes sir. 12 on B. 0 on C.

 

[Retiredwarhorses]

So, what does the trans lamp status while truck is in “start“ position and what is status for lamp in “running” state?
also…have you pressed the transmission circuit breaker button?

 

[The_Captain]

No lamp lighting at either state. Did try the circuit breaker as well.

 

[The_Captain]

Final update. Relay in, trans light is on now and truck starts in 1st and shifts through all gears. All fixed.

Thanks for the help.

 

[Bscap]

Final update. Relay in, trans light is on now and truck starts in 1st and shifts through all gears. All fixed.

Thanks for the help.

Relay behind passenger seat took care of it? I'm having the same issue with mine. I have checked all the wiring, grounds, etc. cannot pull codes manually and no lite on start up or when pulling codes.