Fail Safe Trans Mode

[AJe31]

Hi, my 1997 840ci has been going into trans fail safe mode intermittently. I read codes with a Peake code reader and come up with 6B which is control unit supply voltage. After starting the car 2 or 3 times the car runs properly.
Both batteries are about a year old and read 12.5V at rest. At idle voltage at sig lighter is 15.2-15.5V. At RPM voltage is 15.5 with spikes to 16.1.

1) Should I replace the alternator, the voltage seems high to me, it is a cheap aftermarket replacement unit that is installed. If so what is recommended, a bosch 120A-140A

2) Do you think the high voltage is related to the control module code and fail safe event, if not any suggestions for solutions to that problem. Is it likely the transmission or another control module.

Thanks for your help,
Alec

[CDO5OO1A]

Hi, my 1997 840ci has been going into trans fail safe mode intermittently. I read codes with a Peake code reader and come up with 6B which is control unit supply voltage. After starting the car 2 or 3 times the car runs properly.
Both batteries are about a year old and read 12.5V at rest. At idle voltage at sig lighter is 15.2-15.5V. At RPM voltage is 15.5 with spikes to 16.1.

1) Should I replace the alternator, the voltage seems high to me, it is a cheap aftermarket replacement unit that is installed. If so what is recommended, a bosch 120A-140A

2) Do you think the high voltage is related to the control module code and fail safe event, if not any suggestions for solutions to that problem. Is it likely the transmission or another control module.

Thanks for your help,
Alec

Voltage at that range at the cig lighter would first lead me to check the condition of the ground strap between the engine and engine support and the ground strap's two connections. If the ground strap's connections are corroded/rusted that could cause any electrical device on the engine to float above the body when current is flowing. The transmission computer (EGS) is in the rear of the car on the right side above the battery. In many places on the car you will find brown wires going to ring terminals on threaded posts. Those threaded posts are body grounds.

First let's get the engine and transmission case connected, electrically speaking, with ample conductivity to the body with the ground strap.
(remember they are vibration isolated from the body with non-coductive motor and transmission mounts)

Ground strap is right side of car at motor mount.
1) Not yet.
2) Possible. Certainly, at this point, I would not rule that out.

[Timm]

Yes, that sounds like a dodgy regulator on the alternator - and that part can be changed without replacing the complete alternator. I would take it easy, voltages that high may damage a few bits and bobs!

[AJe31]

Thank you CDO and Timm, i appreciate your thoughts. I will do some checking when I get back, i am currently traveling.

[AJe31]

120A or 140A alternator? which is the best choice?

[dragon850]

140a

[AJe31]

I bought a new ground cable but it was too long so I used the old one and its fine.
I bought a bosch 140A reman alternator and my output voltage is now 14.45 volts.
I havent had the trans alarm or voltage control code since we put the alternator in...
so looks like that was the problem.
my flashers are coming on now by themselves...we just had the caps replaced on the GC2 board...not sure if they are damaged. I guess we will see what happens.

Thanks for your help.

[TxGR8White]

Check the crash relay...

[AJe31]

Thanks Tx
I re-soldered the crash relay joints (after reading some posts) ..it looked perfect - no change resulted (FTP & park lights/rear lights flashing).
Replaced crash relay...the FTP lights no longer flash with front/rear lights but problem persists (an improvement!).
Seems like hot days trigger it...turning key to left stops the flashing & then back to lock...sometimes they come back on after an hour or so.
If car left unlocked there are no issues.

Any ideas for next step(s)?

Had a couple of trans failure incidents after replacing alternator..but not in several months...I had 2 very large batteries (donors from retired e38 & e46)- went to one new H8 as outlined in a separate thread. No more issues- Perhaps difficulty for "small" alternator to manage two oversized batteries.... Of course maybe its the trans selector switch or something else and will re-occur!
(AJ's Dad)

[llcarlos]

My car died from sitting too long so I charged the batteries and while the batteries are showing full charge with a Ctek tester cord thingie the car would not start with only the passenger side battery connected. It would start with both batteries connected. This was puzzling so I switched the batteries around and now it would start with only the passenger battery. As soon as I connected the passenger battery negative onto the left side negative post the flashers went off and would not stop. The flashers were all the lights except for the head lights. This may have been the alarm but there was no sound as that battery operated part is dead. I started the car, used the key fob, stopped it, locked it, and unlocked it, started it again, opened and closed the door and I could not get the flashing to stop so I disconnected the battery and then reconnected the battery and it's fine now. The window on the driver's door would not respond to the door opening micro-switch as well even though the window worked and that also was cured. It's possible the flashers are going off right now with the car in the garage but since the alarm has turned silent the management company never phones me about it anymore. Yes it was the FTP and park lights rear lights that were activated yesterday. So maybe try disconnecting the batteries for a few seconds at the negative post on the back left corner.

I don't know why the one battery is not working the same as the other battery.

[Diskovod]

This looks to be a battery terminal/cable problem. See if everything is tight, incl wires under insulation. If they had blackened over time, it means the copper basically corroded, having high resistance.

[AJe31]

Is the K16 relay (thanks Timm for the e31 electrical/relay diagram) a suspect for rogue flasher issues?...or are caps on the lamp control module (board above General Module under dash from memory?) the next place to go?

[Timm]

Yes, it can be - but it is usually the crash-control relay. A test can be made by just removing the crash-control relay - the hazard lights and indicators should work without it fitted.

[AJe31]

Yes, it can be - but it is usually the crash-control relay. A test can be made by just removing the crash-control relay - the hazard lights and indicators should work without it fitted.

Hmmm...interesting idea! I just replaced with a new one..

So if relay removal does not clear issue then problem is elsewhere- which one would assume is the higher probability with a new part installed. And next step the lamp control module?..appears that circuitry drives the flasher relay (not looking forward to getting under there again)?

Thanks!

[Timm]

If you have a new crash-control relay fitted and the problem still exists then the next on the list is K16 and then hazard switch itself - the LKM is not known to cause these issues.

[AJe31]

Ordered a Wherle 61361388533 for $22 from RM Euro...worth a try for 20 bucks...sent a pint thru the Ether over to meeknet- thanks for the help!

Lane (Aj's Dad)

[AJe31]

Installed new flasher relay- problem persists!
Clears after unlock doors...or simply leaving the car unlocked. Not sure there is an easy way to clean the flasher push button...it appears to work normally when depressed/exersized.
A friend indicates he had success with cap replacement on LKM board....he was treating a droopy headlight- after which flashers no longer operated on their own...pulling new crash relay the only other solution I can think of as suggested above.

[AJe31]

Update on flashers coming on un-controlled - replaced capacitors on LKM board- problem solved! (courtesy of Scott D)

[EEDegreeToDrive]

Update on flashers coming on un-controlled - replaced capacitors on LKM board- problem solved! (courtesy of Scott D)

Awesome, thanks for the followup; great to know. For years, capacitor problems only really mattered to general module operations; It's got a CPU and when it's past its prime, has pretty impactful effects (locks, wipers, trunk, battery drain, etc..).
Several folks have inquired about LKM and EKM repairs in the last year or two, and I usually suggest an "if-it-ain't-broke---" as they rarely have issues.
Thanks for contributing to the E31 electronics quirks-and-gotchas knowledge-base.