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Well this one has me stumped. I got a check engine light and the transmission light in yesterday on my 1995 XJ12 with the corresponding P1775(Tranny MIL fault) and P1739(3rd clutch pressure switch issue?) codes. The thing that’s confusing the hell out of me is the car did NOT go into limp home mode. I’d figure if the transmission oil pressure was lost or low or otherwise not being electronically told to the ECM by the TCM it would be in limp home immediately but this car is still a mystery to me so here I am taking to the forums lol, anybody have experience with this or know what minor or major things could be here? Have not checked the EGR for carbon yet, and the only real symptoms are harsh up-shifts(only up) but again not in limp home mode and the car still shifts. NO symptoms at all until the lights came on mid drive.
Looks like P1739 "Transmission Component Slipping" is a transmission internal monitoring function. It could come up because of a bad connection in the wiring harness. The most likely spot for a bad connection would be at the connector at the rear left of the transmission. This bundle could get strained if the transmission mount spring is worn or worn out (not uncommon), causing the transmission to settle lower than it should. Hard shifting would be a result of the error, but it doesn't set the Limp Home mode.
Another possible cause and an easy check is fuse F14 (10 amp) in the left-hand heelboard fusebox. That is the transmission ignition power supply fuse. I believe it powers up the shift solenoids (A and B) and the torque converter clutch solenoid. This could be the bad connection as per the above.
Of course the fluid level is very important. Problems will occur if the level is not correct. That could result in Limp-Home mode.
Lastly, you won't find EGR on this V12. One less worry!
Were you able to reset the codes, and if so, did the problem reappear?
I am able to reset them however I haven’t yet. The fuse looked great, and the lack of an EGR helps narrow it down. Fluid level is great and is as clean as can be, however I’m clearly very new to all of this so how exactly do I get to the transmission connections just to check them? Where do I go to see those and potentially fix it? I’m an electrician so if the wiring is the issue that’s fine by be, I just don’t know where to look lol thank you for all of your help truly, I wouldn’t have her running if it weren’t for the forum...I was also tipped off to a voltage relay, basically a voltage regulator is what he said that’s a creamy brownish color that essentially regulates the battery voltage to the TCM. The guy his 6 liter XJS has a connectivity problem there that caused this issue but he wouldn’t know where it is on the XJ-12. Any thoughts?
I don't know of (and I can't find) any additional relay or regulator for the TCM. But the TCM also gets power through fuse F5 (5A) in the Right-Hand heelboard fuse box. Brown relays were used on some models like XJS and some X300s, but the XJ12 will have blue ones.
To check the wiring harness at the transmission, you have to at least get the front of the car up on ramps, and squeeze under there to take a look. It's under the center of the car so it is tight even with the car on ramps. Much preferred to get the car on a lift. You can check the condition of the transmission mount too. The plug is near the mount, above it. The picture below is the transmission's internal harness and plug at the rear of the case. It's a large and round connector, pretty easy to spot once you are down there. A quarter-turn release if I recall correctly.
03-21-2019, 08:02 AM
P1775 and P1739
