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Electricduane 11-16-2015 01:12 AM

89 Suburban - leaking TH400 transmission
So my TH400 is dumping fluid. I "fixed" it once. It lasted a few days. Now dumping fluid again. 1989 burb with the 6.2 diesel.

I dropped the pan, checked it carefully, put a nice thick gasket on there, sealed it with Permatex gasket sealer. Torqued to specs (144 inch pounds). The problem is not the pan gasket.

The problem (well, at least one known problem)is the kickdown switch/solenoid, located on the driver side of the trans just above the pan gasket. It was loose and wobbly because it's O-ring was worn and flat. I tried to fix it by getting a set of O-rings from Autozone. "HELP!" brand. I got 2 O-rings to fit where the old, flat one had been. I had a hell of a time getting the switch stuck back into the trans case, but I did it, made it click into place. It was very snug with the 2 new O-rings.

Now, after driving it a bit, the switch is loose in the socket (side of the transmission case) again, and is dumping fluid. I think the hot transmission fluid somehow got the thing to loosen up. Just a guess.

So, what now? I can drop the pan again (we all know how much fun that is), get this stupid kickdown switch loose, and do what to it to make it stop leaking? Slather black RTV on it?

Also: the kickdown switch does not work. It possibly was never designed/intended to work with the 6.2 diesel.

If I am fine with it not working (which I am), why not just leave the kickdown switch out of the transmission case hole and put some sort of a rubber stopper in the hole instead?

All advice or comments are appreciated.

Front Range 11-16-2015 12:10 PM

Re: 89 Suburban - leaking TH400 transmission
You could dry the outside and wipe it with grease/wax remover then slather on RTV but it might still leak. Could be the old connector has wear on the plastic body, weak retaining clips, or a crack. You may have the wrong o-ring on it, could have damaged the o-ring going in.....etc,etc

The correct way is always a time consuming PITA initially - but it usually fixes the problem. If it was my transmission I would drop the pan and replace that connector and o-ring with a brand new unit making sure the inside radius where the o-ring sits was very clean - before sticking the new piece back in. Then cross my fingers. A new connector will run you anywhere from 3 to 10 bucks online.

For what it's worth I had a similar issue with the shift rod seal on my TH-400. Pain in the damn ass to get under there again after dropping the pan trying to figure it out - but when all done the leak free garage floor was worth the frustration.

Electricduane 11-16-2015 01:55 PM

Re: 89 Suburban - leaking TH400 transmission
Thank you kindly. . . looks like I will be dropping the pan (again) and replacing that damned part. . .

Front Range 11-17-2015 11:24 AM

Re: 89 Suburban - leaking TH400 transmission
Post up again when your done, I'd be interested to know how it works out for you. Make sure to grease the o-ring before attempting to shove it in the hole.

Electricduane 11-22-2015 09:12 PM

Re: 89 Suburban - leaking TH400 transmission
I installed a brand new plastic kickdown switch with the correct O-ring. Put some white grease on the O-ring, then clicked it in.

I reinstalled the pan just finger-tight today, will wait for the gasket sealer to cure before torquing it to specs.

My conclusion at this point: the new plastic kickdown switch clicked in very snugly, unlike the old, worn out kickdown switch. I think that over the last 25 plus years, that plastic piece actually shrunk, thus causing the leak. It is an unbelievably poor design . .. over time, that plastic piece is guaranteed to cause a leak. Maybe mine will be good for the next 10 years . ..

I also loosened the screw in the vacuum modulator on the right/pass side of the trans. I understand this should allow the trans to shift a bit earlier.

Tomorrow I will tighten the pan to specs, add fluid, and test drive. Wish me luck, and thanks for the help!

Front Range 11-22-2015 11:50 PM

Re: 89 Suburban - leaking TH400 transmission
Good luck, hope it works out well.

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