Torque converter/brake light switch

[Thumper580]

My torque converter/brake light switch is messed up. Did Ohm checks on both and got erratic results.... While I'm waiting for a new one is there any bad, damaging effects if the torque converter lockup doesn't unlock when you hit the brakes? Wondering if I can still drive it or just let it sit till the new one arrives. Thanks.

 

[Mogman]

If the TC is in lock when you come to a stop it will kill the engine just like stopping a standard without pushing in the clutch, other than that no harm will come, most transmissions will not go into lockup in 1st or 2nd so even without the brake switch it should operate normally (to say the transmission should automatically go out of lock as it downshifts with or without the brake switch) unless you lock up the brakes at highway speeds, that may be interesting....

 

[Thumper580]

If the TC is in lock when you come to a stop it will kill the engine just like stopping a standard without pushing in the clutch, other than that no harm will come, most transmissions will not go into lockup in 1st or 2nd so even without the brake switch it should operate normally unless you lock up the brakes at highway speeds, that may be interesting....

Then my switch must be stuck where it's telling the TC not to go into lock because I drove it yesterday and it didn't stall at at a stop.... Time for a new switch assembly anyway.

 

[Mogman]

Not necessarily, as I said it should go out of lock with or without the switch as it downshifts.
That was an edit that came in as you were writing the response.
Also most transmissions go out of lock when the accelerator is returned to idle, so the brake switch is just a redundant safety.
If page 15 is correct you should be able to hold your throttle steady ( EDIT, at highway speeds) and TOUCH the brakes and you should get the TC to unlock, if you have a tach you will see the raise in RPM if not you should be able to detect it by sound.

 

[Thumper580]

Not necessarily, as I said it should go out of lock with or without the switch as it downshifts.
That was an edit that came in as you were writing the response.
Also most transmissions go out of lock when the accelerator is returned to idle, so the brake switch is just a redundant safety.
If page 15 is correct you should be able to hold your throttle steady and TOUCH the brakes and you should get the TC to unlock, if you have a tach you will see the raise in RPM if not you should be able to detect it by sound.

Mogman, different question. On the transmission controller is there a cable that connects to the TCM and has a regular OBD scan plug on the other end. I went through the 4L80E troubleshooting guide but didn't see one, maybe I just missed it. The owner of my local transmission shop asked me about it. Thanks.

 

[Mogman]

If the 4L80 is set up to go out of lock at idle and you have a tach you should be able to remove your foot from the accelerator and then when you press it again you will see the RPM drop after a second or two as it goes back into lockup.
The only trucks I work on with 4L80s are a couple ranchers around here and they are not street legal, might have to play with one on the road anyway.

 

[Mogman]

I don't think I have ever seen any documentation on an OBD port, maybe someone else can address that, meanwhile I would see if that shop can read the TCM just for kicks
The one on my 6L80 is really handy, I can see temps, etc.
You can get a reader for doodly squat on fleabay, I have one in every vehicle that has an OBD port

 

[Mogman]

Please post pics of the cable.
Here is a cable someone designed for the military 4L80 but it is for the tech 1 tool, not sure if that is a standard OBD, seems the reg OBD uses pins in the center of the plug..
EDIT, I may be thinking of the OBDII connector

 

[Thumper580]

I don't think I have ever seen any documentation on an OBD port, maybe someone else can address that, meanwhile I would see if that shop can read the TCM just for kicks
The one on my 6L80 is really handy, I can see temps, etc.
You can get a reader for doodly squat on fleabay, I have one in every vehicle that has an OBD port

They looked everywhere on the truck and couldn't find one(obd) plug in...me either. Can't believe someone on here hasn't tried to figure this out. It would be much easier using the regular handheld code scanners.

 

[Mogman]

That cable above should work, you will need a OBD1 scan tool, I thought you said it had one, here is a pic of the OBD 1 connector, much different than the OBD II used today.

 

[Mogman]

Probably all you will get on that old of a port is codes, which is easy enough using the trans light.