Reduced Engine Power (REP) popped on and now dead battery

[BanditGTP]

The other day (Tuesday) while working at night I was on my way home and the Check Engine Light and Reduced Engine Power (REP) came on. It was around 35F outside and it was raining lightly. I was driving around 40mph when it happened and pulled to the side of the rode. I turned the truck off and on again but it didn't go away and I still had a 9 mile trip to get home. By the time I got home in limp mode it was after midnight so I had no time to look into the problem. The next day my family and I were flying to Chicago so I still didn't get to check it out until today (Sunday). The REP and CEL lights are both off but the battery was nearly dead. I needed to jump start the truck to get it going and everything seemed to be working fine.

I scanned for codes before leaving for Chicago and the battery was still OK at this point. This is what I found:
P1221
P1271
P1635

I started to look into the dead battery as my battery is less that 1yr old. I disconnected the negative battery from the battery and put my multimeter between the cable and the battery to see if there was a drain. I was getting 12.4v so I started to pull each fuse one at a time and the voltage never dropped. To save some time with pulling each individual fuse from the rear seat fuse block, I just pulled the entire Mega Fuse and the voltage only dropped to 10.8v. Am I missing something as to how else to first find the drain before I can start looking into the REP and CEL lights? I've read many people mention the fan clutch and being problematic and was hoping that might be shorting out but now I'm not sure since pulling the fuses didn't show point me in that direction.

 

[BanditGTP]

Ugh...I haven’t had to chase battery drains before and just realized I need to check for amps and not volts. Now I need to go get a different meter since mine won’t work for this. Please excuse my stupidity. SMH

 

[freddyboy61]

The P1635 code is related to the 5V reference voltage that the accelerator pedal and throttle body sensors use, as well as the fan clutch. Sometimes the fan clutch wire harness gets abraded on the radiator support bracket. Check this out.
Not sure if this is related to the battery drain, but see if you can get an ammeter on the battery and check for excess drain.

 

[Mooseman]

Or sometimes the fan clutch itself fails and shorts the 5V reference. Try unplugging the fan. You'll get a code for it but maybe will eliminate the others.

 

[BanditGTP]

Hey guys. I spent some time over the weekend trying to look into my issue. After having the battery unplugged for a while the errors have seemed to clear themselves and I'm not having the REP or CEL lights anymore. The battery has also been fine and hasn't needed to be jumped again. I've driven the truck about 120 miles over the last 3 days and everything appears to be working like usual. I guess for now I'll see if the problem returns but I'll also try to look into the wiring by the fan clutch to see if there are any damaged wires. I have had the fan clutch out of the truck twice in the past while replacing the water pump so maybe the man-handling of the clutch might have also damaged the wiring. I'll post again if I find anything or the problem returns.

Thanks

 

[mrrsm]

Hopefully your 5-Volt Gremlins will stay away... but if the frustration rears its ugly head again... one of our clever members, @Matt , penned the quintessential "How To Repair the Water Pump"...which of course includes the necessity of first removing the Fan Clutch apparatus. This Sticky Worthy and brilliant way of doing the Fan Clutch Part will interest you:

https://gmtnation.com/forums/threads/how-to-replace-the-water-pump.15639/#post-505126

If you want to see What Wires are Fraying and Where that short out the 5 Volt Clutch mechanism, I have the diagram posted here... along with my Rube Goldberg attempt at doing what @Matt did... but in a much less elegant manner. ;>)

http://s557.photobucket.com/user/60...GINEREPAIR/GM42LFANCLUTCHREPAIR?sort=3&page=1