Advice on intermittent no reverse

**Cjarvis** · 12-11-2020, 09:12 PM

Hey all,
I am getting tired of the intermittent no reverse, its been over 10 years, way past time to deal with it. Car is a May 85 745i, 98K miles. Failure only occurs after the car is driven, equally likely to fail after 5 minute drive or 5 hour drive. Cant remember it ever failing on a cold start, but it might have, Recently it did fail after sitting and hour or so after a drive. Failures are just frequently enough to annoy, maybe every 5th time or so the car is driven. I swapped in a new (literally new) transmission 10 years/35K miles back. No luck. Recently cleaned again the elec control connector on the trans. No luck. Next step I am thinking is to swap in a used control unit. But thought I'd check here first for advice. So there it is. Any ideas?
THanks!

**Hit Man X** · 12-12-2020, 01:41 PM

Great seeing you around! I was just talking about you to someone... in positive light about the previous owner of your car. He found the other which I THINK is your same color combination. Very neat too.

I can assume the trans is a correct -091 or -286 case code depending on your build date? I am not sure of specific technical difference, yet I believe it has to do with increased fluid volume to the overdrive clutch or sprag back there. Application of reverse are the B, D, and E clutches.

Have you tried to clean the gear indicator in the shifter assembly? The electrical guy, its name slips my mind. It is basically a position sensor for the TCU to command reverse to apply. Mine will at times blank out and not upshift from 3rd to 4th, unless I jiggle it a hair.

All I have to help!

**Cjarvis** · 12-13-2020, 02:08 PM

Originally Posted by Hit Man X

Great seeing you around! I was just talking about you to someone... in positive light about the previous owner of your car. He found the other which I THINK is your same color combination. Very neat too.

Yea, I'm still around, haven't had much time for cars lately. Hopefully that is changing. I still lurk here and really, really glad you are still here helping people out! I'd love to hear more about this other 745. I wasn't aware of it.

Originally Posted by Hit Man X

I can assume the trans is a correct -091 or -286 case code depending on your build date? I am not sure of specific technical difference, yet I believe it has to do with increased fluid volume to the overdrive clutch or sprag back there. Application of reverse are the B, D, and E clutches.

Have you tried to clean the gear indicator in the shifter assembly? The electrical guy, its name slips my mind. It is basically a position sensor for the TCU to command reverse to apply. Mine will at times blank out and not upshift from 3rd to 4th, unless I jiggle it a hair.

Yes, both trans are 091, the original and the one in the car now. I did look at the gear selector a while back. But kinda ruled it out as the "R" on the dash would still light up when positioned on R and the trans failing to go into gear. Since I cant find a wiring diagram that includes the TCU, I cant confirm that would eliminate the gear selector as the fault. I have tried jiggling it around when it wont go into R, no luck. But turning the car off, sit for 10 seconds, restart does frequently work. Also, I dont know exactly when this problem started, it was before the new trans went in. Reason I say that is the SI board and the gear indicator board were replaced 6 months before the transmission. The SI board to fix a temp gauge issue, the gear indicator board to fix a burned out 'D'. Hard to say if the problem started after that, my gut says it didnt. I'll look at the wiring diagram for those and see if there is any info related to the TCU connectivity. Would really be nice to know all the inputs to the TCU. A wild guess on that is one of the temp sensors. My after cooler sometimes does not run and other times seems to run a long time after shutdown. Maybe a failing temp sensor/temp switch is causing issue with the TCU. Could explain why a shutdown and restart sometime clears the problem. Anyway, thanks for the help! Plus any more ideas you have.

Best...

**Cjarvis** · 12-13-2020, 08:25 PM

Thought I'd post an update. I may have learned a few things.

The 86 US E23 ETM includes diagrams for electronic transmission control. The only inputs to the TCU from the gear selector are for neutral and forward gears: N, D, 3,2, 1. There are also inputs for the program selector: Economy, 321. Nothing for reverse. This implies reverse gear engagement is hydraulic and not electronic. There are also several motronic inputs, RPM, TPS, etc. Looking at the TCU outputs to the transmission, there are five solenoids it controls. The reverse lockout solenoid is likely my problem. The others deal with hydraulic pressure and forward gear shift points. A search on reverse lockout solenoid landed on this post from shogun: https://www.bimmerforums.com/forum/s...4#post28509044 which seems to confirm.

The next question is why would the reverse lockout solenoid be energized when it should not. It would be nice to know the conditions that cause the TCU to energize that solenoid under normal operation, but that is likely guesswork. So it seems to boil down to faulty wiring between TCU and the trans, or the TCU is bad. My gut says wiring is the more likely culprit, but the TCU is easier to verify. I'll post what I find.

**shogun** · 12-14-2020, 06:10 PM

you can test the solenoids of the valve body on the connector at the transmission, details are here http://www.bmwe34.net/E34main/Troubl...ch%20Guide.pdf
when I do a valve body reman, I often find the valve body is dirty and some valve are stuck and sometimes do not move, I disasemble each valve, clean it with brake cleaner and polish the solenoid pistons that they move smooth again.

**whiskychaser** · 12-15-2020, 04:16 AM

If reverse is selected at above 3mph, the TCM will lock it out. It would be like selecting neutral (electran1.pdf)

I don't have a diagram for an 85 745i but would expect that lockout is activated when pin 20 (grey wire) of the TCM is grounded. So if that is the case in reverse, there would appear to be an issue with the TCM or a random short to ground in the wiring. If that is not the case, then as Shogun suggests, the solenoid my be operating but a valve may be stuck

**Cjarvis** · 12-15-2020, 11:35 AM

Originally Posted by shogun

you can test the solenoids of the valve body on the connector at the transmission, details are here http://www.bmwe34.net/E34main/Troubl...ch%20Guide.pdf
when I do a valve body reman, I often find the valve body is dirty and some valve are stuck and sometimes do not move, I disasemble each valve, clean it with brake cleaner and polish the solenoid pistons that they move smooth again.

Thank you shogun, great information! Saved the document. I looked again at the 86 US E23 ETM (guessing it's mostly the same as 85 745) and found 7 pages of specs and diagnostics after the transmission control wiring diagrams. Wish I had paged forward and seen it the first time.
Thanks again.

Originally Posted by whiskychaser

If reverse is selected at above 3mph, the TCM will lock it out. It would be like selecting neutral (electran1.pdf)

I don't have a diagram for an 85 745i but would expect that lockout is activated when pin 20 (grey wire) of the TCM is grounded. So if that is the case in reverse, there would appear to be an issue with the TCM or a random short to ground in the wiring. If that is not the case, then as Shogun suggests, the solenoid my be operating but a valve may be stuck

Thank you @whiskychaser! I swapped in a spare 013 ECU/TCU yesterday, and the problem is still there. So it seems it is down to a wiring fault or valve body/solenoid. I doubt a wiring fault would be easy to find (obvious damage somewhere), so it's likely soon time to drop the pan and the valve body. Thanks for the help!

**Hit Man X** · 12-16-2020, 01:27 PM

I have the ETM you require! PM me your email and I can send it over. Also, if you do drop that valve body, you may as well pop in the shift improver spring.

- - - Updated - - -

Another idea. Remove power to trans for flaccid mode the next time this occurs, this leaves you with 3rd and R only.

Easiest would be to pull the relay in passenger kick panel.

**Cjarvis** · 01-28-2021, 11:15 AM

Thought I'd post an update on this intermittent no reverse issue. I dropped valve body to inspect internal wiring harness and test solenoids. Solenoids all looked great and tested fine. No faults to ground found in the internal harness (or the body harness) but would not expect to see one with an intermittent problem. The speed sensor however tested at about 1060 ohms, spec is around 250. I'm a little skeptical this is the issue, but its the closest thing to a smoking gun so far. Local BMW parts manager said they replaced a lot of those sensors back in the day. Anyway, its back together now with a new sensor and filter, waiting for clean and dry roads to test. Gonna be a while.

Thanks for the help and pointers to documents, etc!

**Cjarvis** · 01-28-2021, 01:09 PM

For future reference, below is the TCU pin out for the combined 013 TCU/ECU. I found it to be different than the 85 US E23 ETM. Don't know if that's due to it being Euro or if its unique to the 745 013 ECE/TCU. The diagram also attached I've seen around before. I got this one from the German language E23 repair manual, and the TCU pinout on it matches my 013 TCU.
Also adding the EH connector pinout.

TCU Pins 1-18:

TCU Pins 19-35:

EH Connector Pinout:

Diagram: