legacycentral bbs

How well does the AWD work in a 4EAT Legacy? Does it work at high speed, say 40 MPH?

I'm trying to use a Legacy TCU in my 88 GL-10. I have it all hooked up, and it works fine except for the AWD; there are no codes, no blinking power light.

With the GL-10 TCU it didn't lock up the transfer clutch until the car was moving. This meant that the front wheels would spin until the car moved a bit and then the clutch would engage the rear wheels - not ideal. The 1990 AWD Legacy TCU engages the clutch with a heavy application of throttle, and that part works great! Now from a standstill the clutch is engaged and no wheelspin. This was my motivation for 'upgrading' the TCU.

However, once underway the transfer clutch only briefly engages when the throttle is depressed. Basically I have AWD for a few seconds and then it opens up and the front wheels just spin and spin. The GL-10 TCU didn't do this, I could feel the car pulsate slightly as the transfer clutch is modulated and no wheelspin.

I'm guessing this is not normal behavior but I would like to confirm this before diving deeper into figuring out the problem. I think it has something to do with my different R&P ratio than a Legacy.

It sounds like you may have some issues with the rear duty c solenoid or possibly clutch packs.

It definitely doesn't sound normal.

Thanks! I hooked up a select monitor and the VSS1 and VSS2 are about 6% off. This is the same as what I calculated due to the difference in tire size (really the VSS2 gear) and the R&P.

I think the TCU is detecting this difference and disabling the AWD to keep from "burning" up the clutch if the tires are different sizes.

I've downloaded the TCU ROM and I've found the VSS1 constant. I've ordered up some EEPROMs so I should be able to try my fix soon. Then it's on to implementing my paddle shifters using the stock TCU and a modded ROM!

not sure if this applies to you, but there is some info about speed sensors, it says that if the readings are the same there is a problem with one of the sensors.

http://endwrench.com/images/pdfs/4EATInfo.pdf

Cool, thanks for that. I wonder what sort of error the TCU allows? When the error is too large the TCU will think the wheels are slipping, right? That's what the code looks like it's doing.

A long shot, but does anyone know the number of gear teeth on the speedometer gear?

rpress wrote:Thanks! I hooked up a select monitor and the VSS1 and VSS2 are about 6% off. This is the same as what I calculated due to the difference in tire size (really the VSS2 gear) and the R&P.

I think the TCU is detecting this difference and disabling the AWD to keep from "burning" up the clutch if the tires are different sizes.

I've downloaded the TCU ROM and I've found the VSS1 constant. I've ordered up some EEPROMs so I should be able to try my fix soon. Then it's on to implementing my paddle shifters using the stock TCU and a modded ROM!

I guess I missed the part about different tire sizes.....as long as all your tires are the same size, the two VSS's should read the same.

Legacy777 wrote: I guess I missed the part about different tire sizes.....as long as all your tires are the same size, the two VSS's should read the same.

The VSS2 reads correctly as both the GL10 and the Legacy have 4000 pulses per mile. VSS1 reads incorrectly because I have a 3.7 R&P vs 4.11 in the Legacy and my tires are 899 revs/mile and the Legacy has 862 revs/mile.

Alright, I got it working. At this point I'm not sure if it is the Legacy TCU that is just different or if my transfer clutch system is bad somehow. I was able to modify the lookup table for the slip (the same for 1st gear selector position) to fully engage the transfer clutch. This worked! It's a little harsh on engagement but it seems okay. I also corrected the VSS1 error, no biggie just a const in the code, but that didn't fix the AWD problem.

I previously took apart my transfer clutch and everything looked fine. Perhaps the Legacy has a different duty C solenoid or a different transfer valve body/orifice plate. If anyone can compare the part numbers for these three items I would appreciate it!

I also have the paddle shifter code working. It's sweet. Overrev protection for downshift, and automatic redline upshift. Now I need to make up the actual switches to mount to the steering column.