700R4 TCC Lockup Issues

I have an 85 GMC C1500 with a 700R4. I recently got it up and running, and I had TCC lock up for a while. Trouble started when the brake light switch started to fail, but if you pulled up on the pedal it would work. I replaced the switch and it never came back. I replaced the solenoid and did a filter change. I took it out for a test drive and no TCC lock up. So I made a jumper for the Vacuum switch, and I took it out for a spin and I had lock up in 3rd and 4th for over half the trip, but it stopped working. I stopped and check my jumper and it was still connected. My next step in to create a temp direct hot test wire that I can switch on while driving. I don't have a lot of experience with the 700R4 so help would be greatly appreciated.

1. What is the likelihood that one of the pressure switches is intermittent?
2. Would a failing TCC be intermittent like this?
3. What am I missing?

Thanks in advance for the help.

@NickTransmissions would possibly have an answer.

If it all started with the brake switch- I would personally start by looking there again. I've found that brake switches are inconsistent when they are bad &/or failing. I've even had a new switch that turned out to be bad. I'll mention this - sorry if you already know- be sure the brake switch you bought is correct. Brake switches typically close the circuit when you press down on the brake. You need one that will open the circuit when you press the brake. (brake switch with cruise control works well- has two connections- the one for cruise control will work for TCC)

Thanks for the response. The brake light switch is the correct one, and it still could be the culprit. The couple lock ups I had yesterday makes me wonder if it is something electrical/external to the transmission. I plan on running a hotwire from the battery to a switch and into the transmission. This should isolate it to internal or external to the trans...hopefully. The transmission doesn't act right past light throttle so I suspect it may be a bit weak.

I had a cracked TCC solenoid on my 1990 and it caused partial locking and unlocking. but on my 1990, I could use a scantool to check if the computer was commanding lockup and check to make sure the brake switch was working properly at the ALDL. guessing that your '86 might not have those options to help diagnose. have you checked the service manuals for your year on how to diagnose the TCC lockup?

farules said:

Thanks for the response. The brake light switch is the correct one, and it still could be the culprit. The couple lock ups I had yesterday makes me wonder if it is something electrical/external to the transmission. I plan on running a hotwire from the battery to a switch and into the transmission. This should isolate it to internal or external to the trans...hopefully. The transmission doesn't act right past light throttle so I suspect it may be a bit weak.

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Check for engine vacuum leaks, wiring concerns, etc. If no issues found, drop the pan and replace the transmission's internal wiring harness which will have a new TCC solenoid.

I'm currently driving a 700r4 with hot wire to lockup solenoid on a switch. it will lockup in 2nd 3rd and 4th. I usually just use it on overdrive once I'm up to speed and I will use it in whatever gear I want to hold. I plan to put in some pressure switches and wire it up to have it setup more automatically.

gmbellew said:

I had a cracked TCC solenoid on my 1990 and it caused partial locking and unlocking. but on my 1990, I could use a scantool to check if the computer was commanding lockup and check to make sure the brake switch was working properly at the ALDL. guessing that your '86 might not have those options to help diagnose. have you checked the service manuals for your year on how to diagnose the TCC lockup?

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I replaced the solenoid and checked the old one, and they are both in working order. I wish I could scan it, but it doesn't have any options like that. I have not looked in any service manuals. I will see if I can find some online.

NickTransmissions said:

Check for engine vacuum leaks, wiring concerns, etc. If no issues found, drop the pan and replace the transmission's internal wiring harness which will have a new TCC solenoid.

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This is pretty much my plan of attack. No vacuum leaks, and no wiring concerns as of yet. I plan to do some more troubleshooting this weekend to isolate the problem to either inside the transmission or outside the transmission. I realize that troubleshooting something on a computer screen with limited info is nearly impossible. I was looking to see if anyone has had similar issues and what fixes they may have employed.

farules said:

I replaced the solenoid and checked the old one, and they are both in working order. I wish I could scan it, but it doesn't have any options like that. I have not looked in any service manuals. I will see if I can find some online.

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The service manuals are in the Reference Library of the forum