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**pjmoreland** · 09-15-2021, 12:52 PM

If your truck is wired like the second schematic you posted, then I believe it is designed to lock the TCC in all gears, and the vacuum switch unlocks the TCC during acceleration in gears 1-3, but the TCC will just be locked all the time in 4th gear. This arrangement would need a normally closed 3-4 switch, as shown on the schematic. The internal wiring in your 88/89 transmission won't support this type of functionality because one side of the 3-4 switch is connected to ground instead of being connected to the TCC solenoid. If this is correct, then I see two options:

1) Drop the transmission pan and rewire the transmission to be like an 85 transmission. You will probably need to get a new 3-4 switch that has two terminals as part of this change so it will no longer have one side of the switch connected to the transmission case.

or...

2) Make some modifications to your truck's harness. This option would result in different behavior of the TCC. You could make the following changes:
-Remove the wire from the B terminal and tape it off
-Install a jumper between the B and D terminals
This assumes you have a normally open 3-4 switch, which you would need to confirm. This arrangement will make your TCC lock up in 4th gear only, and only with high vacuum. You will need to have a vacuum delay valve in the vacuum line that feeds the vacuum switch to reduce the amount of on/off cycling that can happen on hills. You probably already have one of these there if everything is stock.

09-15-2021, 12:52 PM	#4
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,333	Re: 700R4 LockUp Troubleshooting If your truck is wired like the second schematic you posted, then I believe it is designed to lock the TCC in all gears, and the vacuum switch unlocks the TCC during acceleration in gears 1-3, but the TCC will just be locked all the time in 4th gear. This arrangement would need a normally closed 3-4 switch, as shown on the schematic. The internal wiring in your 88/89 transmission won't support this type of functionality because one side of the 3-4 switch is connected to ground instead of being connected to the TCC solenoid. If this is correct, then I see two options: 1) Drop the transmission pan and rewire the transmission to be like an 85 transmission. You will probably need to get a new 3-4 switch that has two terminals as part of this change so it will no longer have one side of the switch connected to the transmission case. or... 2) Make some modifications to your truck's harness. This option would result in different behavior of the TCC. You could make the following changes: -Remove the wire from the B terminal and tape it off -Install a jumper between the B and D terminals This assumes you have a normally open 3-4 switch, which you would need to confirm. This arrangement will make your TCC lock up in 4th gear only, and only with high vacuum. You will need to have a vacuum delay valve in the vacuum line that feeds the vacuum switch to reduce the amount of on/off cycling that can happen on hills. You probably already have one of these there if everything is stock.