Truth or Fiction

[spazz]

"Case in point, my '93
325is with an automatic suddenly issued a "trans program" warning and began
running in default 3rd gear. When I restarted, all was well for a few miles
than the warning would reappear and the transmission would again drop to
3rd. Although the symptoms seemed to be signaling eminent transmission
mechanical failure, my mechanic found that the alternator had failed after
90,000 miles and simply stopped sending power to the transmission control
module. The transmission is programmed to go into survival mode (3rd gear)
whenever electrical power is disrupted."

My question is my 92 325i is in Limp Mode, check a hell of alot of possibilities (Service manual) my last possibility is this,

Belt seem loose,for me there seems too much play.
New Battery (bench tested 12.6v)
New alternator (bench tested) 14.39v
Installed in car, 13.9v (no load)
Installed in car,13.72 (with load)
Sort of noisy the belts at start up,

Now the manual say self tensioning (no adjustment) so in short I have to buy new belts and try again to see if this fixes the problem?

Or ???????????????

Again your advice greatly appreciated.

PS: No codes till maybe tomorrow
Also MAF is working properly ( the wire glows and there is power)

[saltshaker185]

i know this sounds outlandish but is it possible for you to hook up an external electrical source just to your tranny module to see if it even is your power supply to it? maybe run a lead from a spare battery inside your car down to the module and see if with that power it is able to work normally? if so, you know that you are on the right track to fixing it, if not then you know that it could be your module that isnt right...

[sweetkandii]

try fixing belt issue see if it helps if not def an electrical issue
i know many bimmers including mines which had electrical issues

[CJH]

Check the AC voltage at the alternator output terminal with a voltmeter. Anything more than 1.0 Volts AC is excessive. Defective diodes in alternator allow alternating voltage "leakage". Control modules are very sensitive to supply voltage.

[spazz]

Originally Posted by CJH

Check the AC voltage at the alternator output terminal with a voltmeter. Anything more than 1.0 Volts AC is excessive. Defective diodes in alternator allow alternating voltage "leakage". Control modules are very sensitive to supply voltage.

ok i will check , stupid question, is that with the engine on and running or off

[CJH]

Engine running. Here's a explanation of the system, "ripple voltage" is discussed toward the bottom:

How Generators Work