1999 GMC Jimmy...HELP!

[Red97GTP]

I have a 99 GMC Jimmy in the shop right now. The customer said the vehicle is stuck in 4 wheel drive. We replace the 4 wheel drive motor, and it does the same thing. All of the lights for the 4 wheel drive system are on, but when it's hooked up to a scanner, it doesn't read any codes. Anyone have an idea about this POS?

Thanks!!
Brian

[Want_Notch]

Put in neutral
Shift out of 4WD

[Red97GTP]

Tried that...BTW, it has auto 4 wheel drive if that matters.

[Silver350]

Vaccum line, switch, or ground?

[Red97GTP]

Checked all that...

[Feature Pony]

I know those S-10 4x4 and S-10 blazers always had issues with the vaccum switch for the 4wd, even if its a shift on the fly it still has a vaccum switch which will either not let you get into 4wd or it will stay in 4wd. I think its a $30 part but a pain to install. My parents old 95 blazer had this same issue 3 times this happened, it was under warranty though so we never had to pay to get it fixed.

[johnny--2k]

short in the 4WD control module or shorted switch.

Otherwise, try this.... Similar prob on a 99 blazer with auto 4wd, solution posted below, but other things to check if you read through the first 10 posts.

http://www.automotiveforums.com/vbulletin/showthread.php?p=4521487


EUREKA!!!!!! Found the Problem with the Blazer. After testing all of the components involved with the 4 wheel system I couldn't find any problems...
I finally decided to pull the encoder motor to see if I could shift it manualy to make sure I didn't have a mechanical problem and found the problem. Looking at the encoder motor from underneath It looked almost new. I pulled it off and found a major build up of corrosion between the plastic gasket and the motor gearbox. So much so it distorted the gearbox and bound up the unit so it wouldn't complete the shift. I scraped all the corrosion off and it had a very pronounced bow to the unit. I pulled it all apart ( I'd like to kick the engineer that decided to put security torx bolts in the cover) and seperated the 2 halfs. I checked the machined surfaces with a straight edge and found that the housing was warped by at least 3/16th of an inch. I figured nothing ventured nothing gained so I pulled the unit completely apart and using a press straightned the 2 halfs the best I could. I then used a big mill file and flattned the two halfs so they were true. While I had the unit apart i checked the brake unit on the end of the motor and it was fine except that there was some gunk on the friction plate so I cleaned it off. I checked the motor and found one of the stabs for the power feed was almost burnt off. I cleared away some of the insulation and soldered the wires directly to the motor feed wires. No chance to loose continuity again.
I reassembled the unit and installed it back on the transfer case, installed the driveshaft and lowered the truck to check the shift modual connections.
After connecting the modual I replaced the fuses and started the truck. Bingo the truck snapped into 2HI and works like a champ now..
Sorry for the long post but thought you guys would like to have the info.
I checked with the local stealer and (IN New Brunswick) he gave me a price of 843.00 can for a new motor. I DON'T THINK SO!!!!
Thanks everyone for your ideas
Nick

[johnny--2k]

edit.... dont know if that's the same motor you replaced already.

[Anakonda69]

the vacuum switch in the transfer case is probably stuck. mine stuck except in the opposite position so it would never go into 4x4.

[Red97GTP]

I checked the transfer case and it doesn't have the vacuum switch on it. Johnny, the motor you were talking about was the motor we replaced. It did have pretty bad corrosion on it, but that obviously still didn't help.

[Anakonda69]

all 4x4 s-10's have the same system for push button 4x4. there is a small threaded plug like switch with 3 vacuum lines that run into it. have you checked the vacuum actuator under the battery?

[team beater]

all 4x4 s-10's have the same system for push button 4x4. there is a small threaded plug like switch with 3 vacuum lines that run into it. have you checked the vacuum actuator under the battery?

x2

[Russ Jerome]

Weatherpack connector 6" away from the motor, if it looks good use and inductive meter to measure amp draw, or a DVM at the motor while shifting. Seen the inside of the housing all crudded up like someone said already, should move freely.

[Red97GTP]

I looked at the transfer case and couldn't find anything that had 3 vacuum lines that go to it. In fact, I couldn't find any vacuum lines that are even near the transfer case.

[wrath]

There is a module underneath the dash that sometimes goes bad. The switch on the dash rarely does (unlike the 99-03 fullsizes) but once in a while it does fail. If the shift motor on the transfer case isn't telling the module underneath the dash what it is doing then it fails to shift. I don't remember if the failure mode is 2wd or 4wdhi. It's been a while. Usually the motor doesn't go bad, either the indicator fails or the power to the shift motor gets its contact burnt off.

There is a way to test the shift motor but I don't remember how anymore. You can figure it out if you tear the old one apart.

[Anakonda69]

vacuum switch is on the driver's side right on top of the transfer case.