Bad Alternator verses Automatic Transmission Shifting

[Ericvaughn26]

I need some advice with my 1994 Legacy, Front Wheel Drive only. It has a 2.2L with automatic transmission. Because I religiously performed the maintenance on it since I bought it new in 1994, it runs as good as it did then.

That being said, I began having shifting problems it was dropping into limp mode almost all the time. I did these tests/and or replaced the following:

 

Had a slight leak from one of the (passenger's side) CV axle slots where the axles go. I replaced the seals on both sides; and since they were out, I changed both CV axles and bearings and seals. This all took care of the leak, but didn't stop the limp mode problems. I didn't expect all this to help my limp mode problems, but, you know, you've gotta start somewhere.

 

So, at the same time, I changed every drop of transmission fluid. I took out the old fluid pickup filter. Realizing that it was just a screen (no filtering involved outside of screening), I changed it anyway. You never know.

I also changed the Diff fluid while I was at it.

Tested all four Solenoids (1) Transmission Control Module. Had a slightly high resistance, I replaced it ($110), but it changed nothing.

 

Aside from the normal engine maintenance, I made sure there was no blockage in the transmission lines and replaced the transmission cooler.

 

I did the whole disconnect-the-battery-terminals-and-put-the-terminals-together thing, to clear any bad codes, and I drove it all week (because I had to), and nothing changed. It went into limp mode every day.

 

Now, I searched for an inexpensive code reader; cheapest I found was $250 all the way up to $1000. I was out of luck. (OBD II starts in 1995 and won't work, my legacy is 1994). I tried four readers that said that would work with my old 1994 Legacy--2 OBD I and 2 OBD II). Money's tight and I couldn't afford any in the above price range. There is a method of getting codes by connecting and disconnection some plugs, but I got no response (no codes) from this method. The dealer said I'd either have to pay the price or get the dealer to do it for minimum: $225 for the part, $425 total (see next paragraph).

 

So, the dealer checked the codes twice for me. First time said the Transmission Control Module's resistance was slightly high (same reading I got with my multimeter) and I should replace it. $425 (see above). I did, and the second time I had the codes read nothing changed. They even tested the new module and it read exactly the same as my first module.

 

So, one day my battery died, so I bought a new one. As soon as I hooked it up, the car shifted perfectly. After I have my alternator checked--Boom! I'm done, right!

Wrong!

About a month later, limp mode came back, and my battery was almost dead again. This time I turned everything on that could be turned on when I had the alternator rechecked. It was barely putting out 12 volts. Charged the battery--perfect shifting.

 

So, the alternator needs replaced, right.

 

Just replaced it, and everything seems to be back to factory normal.

Soooooo, why would low voltage from the alternator make the transmission go into limp mode every single time? According to what I've read and experienced over the years, it shouldn't make any difference at all.

It's fixed (probably, at least it hasn't changed in over three weeks) but how could this happen? I rechecked everything I've already done, and all is well).

My apologies for the length of this--I've never written a longer post in my life--but I wanted to put down everything I did. I'm certain I did more, but I can't think of it right this second.

 

So, any ideas about how a low bat/bad alternator could make a transmission go into limp mode? I haven't brought my battery back for my core refund, so, just because I was curious, I charged it about 1/2 way and hooked it up.

Went into limp mode. Charged it all the way up, no limp mode.

 

Thanks for reading.

[whitexc]

I don't really have an answer on why but have had issues with low voltage. It can reck havoc on things that require and depend on a minimum or constant voltage. It sounds like you had a perfect storm of enough electrical power to run the car but not enough for it to run properly. I am kind of surprised that is the only issue you had.

 

Glad it worked out and thanks a bunch for posting this. I am sure someone will benefit from it.

 

Sent from my moto z3 using Tapatalk

[Brighton96]

I had literally the exact same problem with the same engine and trans combo. To summarize my situation, over the span of a week, I replaced the battery twice as well as the alternator twice and in the end it was just something stupid on my part. The car ran on just the battery for a day or so until it left me stranded, I got it jumped and it somehow ran but was very unhappy about it. I limped it home at night without lights and with a transmission that didn't want to shift at all. I assume because the battery was so dead, it was struggling to provide enough power to run, let alone shift. Even though the output on the alternator was fine, it turns out that with our cars (and also probably most cars) the power from the alternator goes ALL the way through the car and then whatever was left goes back to the battery. So it turned out I had a short in my dash cluster and since that circuit was now broken, the trans wasn't getting enough voltage to control the solenoids and thus wouldn't shift.

ALSO, it's possible the alternator was/is fine and there is something else in the electrical system that is drawing too much current and is suffocating the ECU, which can also make the ECU unhappy and trigger limp mode. But from the sounds of it, it seems as though it was on the solenoid end, not the ECU end.