Encoder repair on Wife's '04 Blazer.... pictures at: www.rlfarms.com/blazer/

Blazer's 4x4 has been flakey for several years, but on the ranch in central California it never snows so, 2-hi and 4-low worked fine.... But we moved to Reno, Nv. and winter is comming (it snowed a few days ago at Lake Tahoe, so it's comming here in a couple of months). Reading through the GM shop manual, only 2-hi and 4-low IS a FAILURE mode the TCCM (transfer case control module) goes into when it can't properly "read" the encoder signals. To see DTCs (diagnostic trouble codes) for the 4x4 system, ground pin 13 of the ALDL (assembly line data link) connector (under the dash near the steering column), turn ignition key to "on", observe the DTCs on the 4x4 push button's LEDs. Hers had a DTC of 002 (bad encoder or connections). I've cleared the codes before (by unplugging the TCCM for a while so the capacitors could discharge) and it would shift fine for several shifts, then revert to only 2-hi and 4-low again. So, time to fix it (before it snows). I unplugged the connector from the encoder/motor and checked the "channels" with a voltmeter (turn ign key to "on" and measure the channel pins to ground, should be 5.0 volts +/- 0.4) OK. I removed the Encoder / Motor assembly (after removing the front driveshaft), and took the unit into the operating room (aka the kitchen table). I removed the three security torx screws and seperated the encoder from the motor / gear assembly. SEALED.....DANG.... so I tested the Encoder "switch" for channel "A"... WOW, while SLOWLY rotating the encoder, the ohmmeter jumped from 0.9 ohms to 40.0 ohms to 300.0 ohms (very flakey), then open, then back to ground at the other end of travel. I figured if one channel is bad, odds are the other channels are bad too. Well, it's only core value now, so nothing ventured....... I took my pin vise with a SMALL drill bit and drilled two SMALL holes just on the inside of the "O" ring (see web pics). Introduced some contact cleaner into the switch assembly with a syringe (the wife is a nurse, she gives me some very handy tools!!!). I rotated the encoder "switch" through its range of travel several times (like 30) and retested the "channel" pins to the ground pin. Very low resistance and steady, either ground or open for A, B and C, depending upon position, no more high resistance. Channel P is a "pulse" to ground while the encoder is rotating. Reattached the encoder to the motor assembly and reinstalled it into the truck. Cleared the DTC by unplugging the TCCM for several minutes. Tested the truck on and offroad, shifted just fine!!!! many many times... FIXED, at least for now... Later I thought that I should have put a drop of silicone seal or black tape over the holes....... oh well.....

 

nice...I should do this, mine has trouble getting to 4 low

 

Don't know if this would fix yours..... I suggest trying to display the DTC codes (if a 2000 will???). Try grounding pin 13 in the ALDL (or connect pin 13 to pin 4), see my web pic for pinout of connector. When the TCCM has trouble decoding the encoder position, it's supposed to fail to a mode where it runs the shift motor from end to end of the range (2-hi to 4-low), per the GM shop manual for a 2004. But when it comes to electronics and bad connections, "Your Results May Vary"..... Good luck....

 

2hi and 4hi work fine, just doesnt like 4lo

 

Suggestion: Try removing the encoder / motor assembly and put a wrench on the transfer case shift shaft, try to shift the case by hand. if it doesn't shift smoothly, it could be a case problem.. BUT a weak shift motor (gummed up or dammaged gears in the motor / encoder assembly) or a poor / corroded electrical connection (causing a voltage drop under load) to the shift motor isn't out of the question..... also, possible bad contacts on the relays in the TCCM..... Could also be the "channel" that makes to ground when the encoder is in the 4-low position doesn't make it, but under that condition the TCCM should detect a fault and set a DTC of 002. Try to get trouble codes to display on the 4x4 pushbutton's LEDs (I don't know if a 2000 works the same way as the 2004).....

 

This is all well and good but, what if there is no wire in pin 13! I have taken the ALDL connector thinking it just slid out but no...its not there. pulled back the harness thinking they forgot to install it...no orange wire so I cant get the codes out. I have been a ASE tech for 25 years and I have never seen this. Is a 1999 blazer LS special.