TH400 with transbrake problem
 

Anybody ever have any issues with a Coan transbrake rolling forward when applied? It has been in the car for four years with no issues but last night in the third round once I pressed the transbrake button it rolled forward? I unloaded the car tonight and it does it every time now! Going to call my trans guy tomorrow just interested if somebody has seen the problem before.

Thanks Vince

When I had that problem with mine it was due to the ground connection for the TB solenoid deteriorating. I improved the grounding, and no more problems.

Thanks for the advice I will check that tomorrow!

There can be a couple of other things.


The band adjustment can be off a little bit.
The pump pressure can be too low.

Coan could definitely point you in the right direction.

ditto, move your ground wire to one of the bell housing to block bolts if you have it on any trans case bolts.

if memory serves there is no bad adj on a th400 -- is the valve body one that needs the t/b button pushed to engage reverse ???

No the button does not need to be engaged for reverse.

Changed the ground installed a new wire right to the back of the head where the main ground is connected no change.

I talked to Coan today they had a few suggestions but the main issue seems to be low line pressure. I measured it tonight at 170psi. Coan suggested raising the rpm before engaging the brake that did not help my situation pressure stayed at 170psi. They also asked what my staging routine was and suggest after going to neutral to clean out the engine to press the transbrake button a few times to ensure the pressure has built up after going into neutral and before I pre-stage also no help for me. Thinking the pump needs rebuild?

The pump is where I would start. I'm not familiar with Coan brakes but I'm assuming they supply you with a stiffer pump spring? Check that as well.

Possibly the reverse clutch pack is worn and has too much clearance.
I don't know about a th400 but when the pump is weak in a glide it rocks backwards.
Sounds like a pump pressure problem though. If there isn't enough pull out of the converter from the transmission when you bolt it to the flexplate it will eventually eat out the back half of the pump and cause a pressure drop.
I'm assuming it just rolls a little bit forward? Or is the transbrake just not holding at all?

t400 reverse works when direct clutches are applied AND low/reverse band is applied. If a direct clutch problem 3rd gear could be at fault.

An issue with the reverse band,the pins that hold it or its servo perhaps?

Worn clutches/seals or cracks in drum(s) or case can bleed pressure as can servo issues.

Yes, it moves forward about an inch.

After 4 years of use it's probably time it was looked at anyway,just as a maintenance item. It's better to catch the fault before it does any real damage. Pull the oilpan to see how much clutch material etc is in the bottom,if it's clean,try a new solenoid first.

That's what I did tonight except the new solenoid(took it apart and cleaned it). No change fluid looked good a little metal on the magnetic plug but figure after two seasons on the fluid I would think that is normal. I may just footbrake it the rest of the year it's only two more races what the heck!

Make sure the spool valve under the solenoid is moving freely - they can chew into the case over time.

The th400 is slower to engage the t/brake than a P/G -- if your routine is to get off the foot brake to fast then it may roll forward just a tad - I experienced this when going from my p/g supergas car to my th400 turbo car -- the fix was to wire the line lock to the delay box so the front brakes stay engaged when the t/b button is depressed, it releases apon release of the t/brake - that car went from rocking back and forth to dead solid

Sounds like a good idea but I have a question? You would not be able to use the line lock in the burn out box without engaging the trans brake if wired together. Did you add multiple switches or use a diode to isolate the line lock?

Vince (take delay box out of picture)if you have 2 switches...

power in to line lock switch 1st...turn on line lock... power out of line lock wire also wired to transbrake power in side of transbrake switch...

the trans brake wont turn on until its switch is turned on.

When you turn off line lock switch you simultaneously drop power to both line lock and transbrake switch.

when used thru a delay box-- mine is, Dedenbear - it is diode protected and only goes one way