Cosmo Kramer

FINALLY got this issue sorted out, I even called Steve Cattaneo for advice as I was at the end of my rope! My issue was a very hard shift at WOT from 2-3 gear due to the modulator valve seeing boost pressure and upping the line pressure in the transmission. Steve C confirmed that the modulator should not be seeing boost as the Mercedes 300D turbo diesel cars have a check valve on the modulator just for this reason (called Mercedes and the valve is NLA ). Tried an inline check valve but all of them hold vacuum and that didn't work. Tried lowering the modulator pressure and that caused flared shifts at light throttle.

Ended up taking the check valve and putting a T fitting in and attaching it the other way, so it bleeds off the pressure and closes under vacuum. This works really well, but most boosted cars have the modulator valve hooked up to the 7 way splitter. If your FMU is also hooked to the 7 way it needs to find another source for vacuum and boost reference as it will have the boost bled off at the transmission valve and that would be a very bad thing.

I ended up hooking my FMU and boost gauge up to the brake booster line T'd in before the check valve for the accumulator tank and it works great.

I haven't put my heat shield on yet so if anyone wants pics please let me know.

killav

Ted,

Please send me a picture. I noticed that my car down shifted really hard under boost the other day. Does having this pressure released at the transmission not affect boost pressure in the engine/manifold? Is the amount to small to notice?

I will PM you with my e-mail address.

karl ruiter

Could you make a sketch? I am not quite following the description, and I think I will need to do this at some point.

Cosmo Kramer

I will take a picture and post it here tonight so you can see what I did.

Mike: The amount of pressure bled off is not significant enough. The opening of the valve is only about 1.5 mm in diameter.

Cosmo Kramer

Here are the pics. I put the vacuum T right at the transmission and used some small vacuum hose to make the connections. The valve can be found in the "Help" section of your local auto parts store for about 5 bucks. Take the valve and blow through it, whichever end you can successfully blow through should be connected to the T, the other end just vents outside.

If your FMU is T'd into the fuel pressure regulator or fuel damper vacuum circuits you will have to move the line over to the brake booster to ensure there is no boost pressure loss to the FMU. Make sure it is connected before the accumulator valve.

I went for a few drives tonight and adjusted the modulator until it shifted perfect. I actually killed two birds with one stone, my car used to hit the rev limiter on the 2-3 shift at WOT in drive all the time, even with my newly set 6700 rev limiter. Now it shifts at about 6100 just like it is supposed to. I guess the increase in line pressure from the modulator seeing boost was causing my kickdown solenoid to go wonky and not shift the next gear.

karl ruiter

Cool. I get it now.