Coan XLT Turbo 350 Issues
 

In my wife's Cutlass we have always run a PG trans with no issues. A couple of years ago we purchased a Coan XLT lightweight turbo 350 w/clean neutral used but freshened from Coan with zero runs. We finally had a chance to try it recently and at the top of the wheelstand in low gear it felt like it almost like a rev limiter for a moment. I still ran it thru and went I clicked it into clean neutral and shut the motor off the engine continued to turn over. Pulled the trans to find the direct clutches had welded themselves together. After talking to Coan they wanted to see the valve body and pump and supplied a baffle for the pan to eliminated cavitation during a wheel stand. After $1200 later with all the upgrades the trans was redone with new clutches etc. and clearances provided by Coan. Fast forward to yesterday and we ran it again. Result? A complete repeat of our last runs. Car falls flat on its face at top of wheelstand and the clean neutral doesn't work. I've checked the cable adjustment several times. I haven't pulled the trans apart but I'm assuming the direct clutches are burnt up again. Any trans gurus out there with a solution?

Re: Coan XLT Turbo 350 Issues
 

I would guess a cross leak between 1st and high gear.

I've seen this with a well known builder on a 350 reverse pattern. Aluminum separator plate and no gaskets. 1st and 3rd we're right next to each other on the plate. Top of 1st it would start applying 3rd and give a running out of fuel feel to it.

Just my guess

Re: Coan XLT Turbo 350 Issues
 

When the car is stopped and in neutral, does the car try to move as if it's in gear?

Re: Coan XLT Turbo 350 Issues
 

Coan probably wanted to see the valve body and pump to check for crossleaks.

Re: Coan XLT Turbo 350 Issues
 

In the clean neutral position it does. The regular neutral it doesn't

Re: Coan XLT Turbo 350 Issues
 

If you get on the Coan website, they have a break down of the tricks in the XLT. It seem to me if you are in the clean neutral position, and the direct clutch is welded, then to get the car to move there must be a cross leak into the forward clutch . Could be leaking at the manual valve, or could be mis alignment of the valve in the clean neutral position.

The XLT 350thm doesn't have an intermediate sprag, so I think it is critical how the second clutch comes off as the direct clutch comes on. If second clutch is slow to come off, or has a restricted exhaust, then this might be the reason the direct clutch burns up, due to a tie up on the 2 to 3 shift. This could be a problem with how the manual valve is positioned in the valve body during the shift.

Re: Coan XLT Turbo 350 Issues
 

After the first time this trans smoked the clutches-1 pass down the track (Coan had just freshened this trans) , Coan has since updated the valve body, supplied a new pump and an oil pan with baffles in it for a super stock application( it was not free) . According to them, this is everything I needed. they also supplied every spec on clearances etc. What happened?? We hooked up a pressure gauge and are at a constant 200 psi at all forward gears, reverse, and with the transbrake engaged at 2000 + rpm. :mad:

I am having trouble finding that this could happen twice in 2 freshens after one hit on the track (one freshen by Coan and one by a rep trans builder) .

Coan's site talks about wheel standing cars and fluid cavitation. I eliminated this-tied down that front end. Minimal wheel stand action-same problem.

very frustrating!

Re: Coan XLT Turbo 350 Issues
 

There are two other pressure test ports on the TH350 case. One next to the accumulator is the intermediate clutch and the one under the cooler fittings is the direct clutch. Some may have one near the line pressure tap for reverse, some may have the boss cast in but not drilled and tapped. I would test those in every gear to make sure they have the correct pressure when they are supposed to and none when they're not, like direct clutch pressure in clean neutral. I wonder if the manual valve is not positioned correctly and not fully exhausting 3rd gear. Are you using the shifter that's required for the clean neutral trans?

Re: Coan XLT Turbo 350 Issues
 

Mystery solved! Behind the governor cap Coan has installed a Holley jet that controls pressure and bleed off for the direct clutches. I guess this is sensitive to your application and it can burn the clutch pack in a heartbeat if you do not have the correct jet.

Re: Coan XLT Turbo 350 Issues
 

Cool, glad you found it.