'01 Dmax, Allison, 2500HD, 150,000 miles, Old model Juice Level 4, B99.
SES Light came on for a while, then self cleared. AutoZone code reader said he found code P1871 Transmission. He asked if it's been shifting funny, I say no and he shrugs his shoulders. Truck still running good. Is there a problem? I assume he knows what he's reading and the code he found was stored in my ECM and not a leftover from somebody else stuck in their OBD II.

History:
Let the wife borrow the truck for a few days, she casually mentions after 3 days that the Service Engine Soon light came on one morning and is still on now. It was probably on for only that one day, 50 mile round trip, outside temps here have been 99 to 105F. She may have gotten stuck in city traffic and left it in gear while idling and it may have gotten a little hotter than normal. Saturday, she said the SES light was gone. Sunday, my turn, since I wasn't able to get to a code-reader, I disconnected the Juice, changed the engine oil, oil filter, fuel filter, trans spin on filter. Fuel filter wasn't real tight (O-ring might have seated better after a while) and I had a hard time repriming the new filter. Thought THAT was the problem, but the SES light had already cleared itself so I was just guessing. It's had approximately 20k miles on new regular fluid and spin-on filter. Fluid level still normal, no strange smells or colors. I have towed heavy for thousands of miles and run higher than normal freeway speeds since the truck was new, this is the first problem.
As luck would have it, I'm towing an 11,000 pound RV to Florida 1500 miles this weekend and naturally I'm a little concerned.
Should I be?
I did a search on the entire forum for this code and only got one hit, totally unrelated. Read thru most of the posts in this category and didn't see anything to check. Looked at the Allison FAQ page and got nothing.
Is this even a valid code for the Allison?

Can anybdy offer any help or advice?

I can't find that code. I would not tow unless I knew what the code was.

How about P1781? I got one this weekend. It's a communication/voltage drop between the ECM and TCM (per the search feature here) during startup as I have read, but mine didn't act up on start up. I did a sudden u-turn, braked, threw it in Reverse, then accelerated, but it revved like it was in Neutral. Threw it back in Drive, then Reverse again and it kicked in.

I was resonably gentle doing this, but I can tell it is shifting funny now. I read mine with my Power Pup. Gonna check in a few days if it has come back since I cleared it yesterday. Never got the SES either.

P1871...transmission mechanical transfercase 4x4 switch circuit,short to battery.

P1871...transmission mechanical transfercase 4x4 switch circuit,short to battery.

OK, I'll check the 4x4 switching and report back.
Wonder if this is the start of the old 'encoder motor' problems, where it would get stuck between 4hi and transfer case neutral?

Thanks, more to follow...

OK, I'll check the 4x4 switching and report back.
Wonder if this is the start of the old 'encoder motor' problems, where it would get stuck between 4hi and transfer case neutral?

Thanks, more to follow...


GOOD LUCK!!!! hope its a easy fix,for ya!

Troubleshooting continues...

Drove the truck this morning, everything seemed fine but I was in a hurry and didn't observe anything unusual. Ran a few errands, shut it down for lunch. Afterwards, fired it back up, seemed to be turning over kinda slow, but it started. Wasn't off long enough to get cool so the glow plugs should not have come on, but the volt meter acted like they were on. After several seconds, it jumped up, but only to around 13 volts instead the usual 14+. Strapped on a digital volt meter, sure enough, 12.7 volts. Tried the 4x4 switch several times (forgot to try it in the morning) it does NOT seem to be engaging, though the light changes from 2HI to 4HI. Volts still low, but climbing, 13.3 after running for a half hour or so.

SO, based on the description for that code, I pulled the 15amp fuse for the 4X4 system and the indicator light goes out. Fuse tested good. Voltage picks up, after a few minutes it's up to 13.7, and hits 14 after 10 minutes.

What the heck is going on? I don't need 4x4 at the moment so driving without the light won't bother me, but should I start looking for a new encoder motor or switch (or whatever that motor is really called)???

SERVICE 4 WHEEL DRIVE

That's in the message center now - most likely because I have the 4WD fuse pulled. I didn't get that light as soon as I pulled the fuse yesterday, probably because it was still running when I pulled it?
I let the truck cool overnite, started this morning like normal - turned over about the same speed, battery meter read low for a while, then jumped up to 14.4 volts.
I'll swing by a dealer today to see if there's an easy fix, but i GOTTA leave town Thursday morning. I'll leave the fuse out until I get to a new home, find a new dealer and get it fixed correctly.
Wonder if I could unplug the harness at the encoder motor to disable 4WD and eliminate the ground but not get the Service light?
Other than the light, am I going to cause any more problems?

Thanks for all the help. I'll post again when I find out the real problem.

The volts will stay low,foe a while,Cuz of the glowplug module.It draws alot of amps