Cadillac Owners Forum banner

1 - 9 of 9 Posts

·
Registered
Joined
·
8 Posts
Discussion Starter #1
I am having trouble diagnosing my system charging trouble. I just replaced the alternator with an expensive new oem replacment. The car was driving fine with a nice charging voltage of about 14 v for about 3 weeks. The other day I'm heading out and the system charging trouble message comes on. I switch the diplay to look at the volts and sure enough, 12.5 volts... so I turn around and watch the volts as I return home. At one point in the trip the volts went back up to 14, but then back down to 11.7. when I got home I checked for codes and saw that a pcm 1637 code was showing current. pcm p1637 is Generator L-terminal Circuit. Seems to make sense. so I look in my haynes book and look up the charging circuit schematic. There is supposedly a red and grey wire coming off the alternator and going to the pcm. That is all fine and dandy except one thing. I just switched this thing out (4 hour job), and I do not remember any other wires to the alternator so I check the procedure in the haynes manual and sure enough there it only speaks of disconnecting the battery cable, and the ground strap. So my question is, where is this generator L-Terminal circuit wire? or - Did I just get a bad alternator? or- may there be something wrong elsewhere in the car that I am ruining alternators? Another weird anomally is that if the car sits a while and I start it up, the voltage comes on at 14 volts for about 30 seconds then back down to 11.something. Anyone have any knoledge of this pcm code? I did not find much on it here although the corvette forum had a lot to say about it, but their alternator seems to have the said red and grey wires on it. Thanks guys.
-rob
 

·
Registered
Joined
·
8 Posts
Discussion Starter #2
ok I am an idiot, I do have the three wire connector shown in the schematic. Although I think I am still stumped. after cleaning the contacts I still am getting the pcm p1637 code, generator L terminal circuit. Do I need a new alternator? What could make this circuit fail?

-rob
 

·
Registered
Joined
·
8 Posts
Discussion Starter #4
I checked that wire back to the pcm. No open, no short to ground. Plugged the wire back into the alternator and checked- no short to ground. Alternator ground strap is good connection. Batt connection is also good. Any other ideas? There is a grey wire connected to a thermistor and back to batt, should I see voltage on that wire? Any other thoughts?

-rob
 

·
Registered
Joined
·
8 Posts
Discussion Starter #5
I still am trying to figure out why I would be getting 14 volts for at least 15 seconds after starting the car. Seemed like it drops down after the load of the fuel pump kicks in. Well, I'm not sure if it's the fuel pump, but some motor sounding thing whirs in the back seat and the voltage seems to drop just then.

-rob
 

·
Registered
2008 SRX
Joined
·
403 Posts
I was hoping someone with more direct experience would chime in. I had one bad thermistor several years ago, I don't remember the symptom, and haven't seen one since. I think the part was unavailable and I bypassed it.
Here's the FSM description.
Mounted on the battery is a battery thermistor. Power is provided from the DIM fuse through CKT 3340 (ORN) to terminal A of the battery thermistor. Terminal B of the thermistor is wired through CKT 725 (LT BLU) to the S terminal of the generator. The thermistor provides a temperature-compensated battery voltage to the regulator S terminal in the generator. This circuit allows the generator to adjust its output voltage according to the temperature.

Not real descriptive as usual. Try unplugging the thermistor and monitor system. The other option is run a bypass jumper wire.
 

·
Registered
90 Eldo -SOLD-, 98 ETC -SOLD-, 86 XR4Ti Track Car, 07 Ranger
Joined
·
963 Posts
I had a similar experience symptom wise but not code wise. My problem was that when I had my alternator replaced the wrong option code was used (the shop I got it from didn't have the full book) and put the wrong pulley (there are a few depending on the year/make/model/options) on so the alternator provided less power for my abundant load of accessories. The issue was that over a short span of time 1-3 months the undue burden placed on the alternator would simply start damaging them (The shop replaced it 2 times before they figured it out) and the voltage outputs would slowly drop until after a few months they'd simply give out. I would maybe look into this...
 

·
Registered
Joined
·
8 Posts
Discussion Starter #8
Thanks for the ideas guys. I jumpered across the thermister and started the car but the alternator still did not charge. Raze- I didn't experience a slow degradation but an abrupt (and inconsistant) one. I guess I will just pull the alternator again and bring it down to the parts store again and have it tested. Perhaps I can get it replaced under warrenty.
 

·
Registered
Joined
·
8 Posts
Discussion Starter #9
Well, yesterday I pulled the alternator and brought it in and it tested bad. I guess I will try another alt. and see how long this one works for.
 
1 - 9 of 9 Posts
Top