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> Bravada Awd - Front Wheels Not Engaging

Grant
post Dec 29 2005, 12:03 PM
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Hope a GM expert can help me. Have a 1999 Olds Bravada AWD bought this past June. Very clean vehicle with only 46K miles...all paperwork.

I noticed front wheels not getting any traction during snowstorm last month. Took vehicle to empty parking lot and was able to fishtail and do “doughnuts” to the point of the vehicle being out of control.

Took vehicle to recommended transmission shop that does work for all other local shops and dealerships. They replaced the slightly worn clutch pack and added correct Auto Trak II fluid. Said rest of transfer case internals are in perfect condition.

When up on lift, front wheels turn when vehicle is in gear. When you put pressure on front wheels, they stop turning. As front wheels still weren’t getting any traction in snow tests, technician checked all related fuses, and driveshaft sensors and all check out fine. He then tried linking his Snap-On MODIS computer to the AWD computer. Couldn’t establish link to computer or transfer case motor.

Tech then took vehicle to service center with a GM Tech One computer. This computer also failed to establish any link with AWD computer or transfer case motor.

During computer links, tech did notice that an ABS code has been set, yet the check ABS system light hasn’t illuminated. During very hard braking, the wheels don’t lock up but you also can’t hear the ABS motors kicking in or the pedal pulsing.

I've heard all sorts of suggestions...ABS computer controls AWD and if code is set, AWD won't engage; there's some sort of 4X4 module that goes "bad" on GM AWD's; even if sensors check out fine electrically, then can keep AWD system from engaging if one tire is 1/4" out of round; vehicle has three computers: one for engine, one for AWD, and one for ABS.

Anyone have any direct experience with this problem?

Thanks MUCH in advance...and best wishes for 2006!

Grant
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Crew Chief
post Dec 29 2005, 07:53 PM
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AWD will not work properly on a lift there is no resistance also there are 3 computers but the ABS & AWD work together go right to the 2 computers and check for power and ground do a voltage drop test plug and unplug all connectors do not twist when you shift it can you hear the motor run with no comm this could be a tough one to repair did you hjook up to the ABS computer if so did it have codes if not check all the wheel sensors wires usually when you cannt contact the computer is defective
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n-gin
post Dec 30 2005, 05:31 AM
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Some of the Bravadas were under braked. When you said that the brakes would not lock up, were you on dry pavement or loose dirt. You need to establish the the brakes lock, and the abs cycles because the computers do work together. You my have a burnt out light bulb in the dash for the abs.
Also those transfer cases have a aluminum shift fork for the abs. They bend slightly and do not engage the front wheels. Also there are wear tabs on the shift fork if those tabs are gone the front wheels will not engage. When it does not engage it grinds away at the syncronizers for the clutch pack.
Some AWD transfercases have an option of 4 high engagement and 4 low. Do you have this option with this tranfercase. This is important information and we need this to help diagnose your problem.
If you do have a switch does the lights flash when you engage the transfercase by hitting one of the buttons?
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Grant
post Jan 3 2006, 08:58 AM
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QUOTE (n-gin @ Dec 30 2005, 05:39 AM) *
Some of the Bravadas were under braked. When you said that the brakes would not lock up, were you on dry pavement or loose dirt. You need to establish the the brakes lock, and the abs cycles because the computers do work together. You my have a burnt out light bulb in the dash for the abs.
Also those transfer cases have a aluminum shift fork for the abs. They bend slightly and do not engage the front wheels. Also there are wear tabs on the shift fork if those tabs are gone the front wheels will not engage. When it does not engage it grinds away at the syncronizers for the clutch pack.
Some AWD transfercases have an option of 4 high engagement and 4 low. Do you have this option with this tranfercase. This is important information and we need this to help diagnose your problem.
If you do have a switch does the lights flash when you engage the transfercase by hitting one of the buttons?


Thanks for replying Chief and N-Gin. To answer some of your questions and give more info for you to consider:

- I pulled ABS fuse...light on dash comes on. No matter what I do (key on, pull fuse, etc.), the SERVICE AWD light does NOT come on in the dash.

- Had a light snow over the weekend so I tried some hard braking. We could hear ABS motors coming on and vehicle stopped straight and in control.

- Front wheels still didn't engage in this light snow.

- There is no dash switch for 4WD nor is there any light that indicates when AWD is operating. Also, no 4 high or low...just AWD controlled by computer.

From other threads I've found after ALOT of searching, I might start with the Transfer Case Control Module. Not sure if this is the "computer", but if the Tech 2 can't communicate with the AWD system via this Module, GM recommends (according to the threads) changing the TCCM first.

Thoughts?
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Crew Chief
post Jan 3 2006, 09:25 AM
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I agree replace the module
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Grant
post Jan 3 2006, 01:13 PM
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QUOTE (Crew Cheif @ Dec 29 2005, 08:01 PM) *
AWD will not work properly on a lift there is no resistance also there are 3 computers but the ABS & AWD work together go right to the 2 computers and check for power and ground do a voltage drop test plug and unplug all connectors do not twist when you shift it can you hear the motor run with no comm this could be a tough one to repair did you hjook up to the ABS computer if so did it have codes if not check all the wheel sensors wires usually when you cannt contact the computer is defective


Dang...forgot to address this in my last reply. Yeah, the first trans technician (highly regarding by many fyi) said after he couldn't get his MODIS computer to connect to the AWD computer, he took it to a dealer friend and hooked it up to their GM Tech 2. That still didn't connect to the AWD computer but it DID connect to the ABS one and it had a stored code. He didn't say what the code was...

I'm taking it back to the Ford dealership where I bought it this past June. They're giving me 50% off diagnostic with two techs that used to work for GM. They have the Tech 2 as well so I'm hoping between all my background research on what to look for and their experience, they can figure out what exactly is wrong after a few hours.

Since the ABS system is working and he found a code, could the the AWD system still be affected by the ABS system? Can the ABS computer disable the AWD if the ABS system has a stored code?

Thanks again for your help.
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Crew Chief
post Jan 3 2006, 05:49 PM
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depending upon the code yes it can disable the AWD mainly if it is a wheel sensor code I still think you are looking at a module if nothing Else it wont communicate see if you can find out the code and get back to me also be sure to let us know what you find
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Grant
post Jan 4 2006, 11:19 AM
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QUOTE (Crew Cheif @ Jan 3 2006, 05:57 PM) *
depending upon the code yes it can disable the AWD mainly if it is a wheel sensor code I still think you are looking at a module if nothing Else it wont communicate see if you can find out the code and get back to me also be sure to let us know what you find


I sure will...I appreciate your time and if you're like me, you can't rest until you know all the facts. Besides, I want to make sure the next mechanic or vehicle owner with this problem can find our thread and be helped by it.

Wife takes it in tomorrow...I wrote her a reminder note to get the ABS code.

Again...thanks for your help. You're the only source I've found either on-line or in-person that knows anything about this!

Grant
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Grant
post Jan 4 2006, 01:33 PM
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Cheif...I was doing some poking around on the web today and came across this thread:

http://www.partsmike.com/forum/showthread....p=5470#post5470

Basically, he had the same problem on a 1998 and just unplugged the PCM fuse under the dash and this reset the TCCM (I've learned this is also referred to as the "encoder")

As my Service AWD light is NOT on, do you think the AWD system is in a sleep mode and doing this fuse reset might reactivate the AWD?

Perhaps this is why the Tech 2 can't establish a link to the AWD system...???

Grant
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Crew Chief
post Jan 5 2006, 10:44 AM
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wont hurt to try I would also disconnect battery and reset everything
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Grant
post Jan 12 2006, 02:28 PM
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Took this thread and other info I dug up on issues related to the AWD not working to the dealer. They've been pretty good about this as I bought this vehicle used from them 4 months ago and they're a Ford dealership.

They put a guy they call McGyver (remember that TV show?) on it and he was able to "jumper" the TCCM and get the module to work. The dash indicator light now works and they took the olds out onto some grass by the dealership and the front wheels grabbed in!

They couldn't keep this as a permanent fix because the connector was corroded as were the internals of the TCCM.

They guestimated the part to be around $450 and then labor to install it. The split the diagnostic fee so that was $125. They've had it for 3 days and have let us use a courtesy car so wife can get the kids to school.

I found the part at gmpartsdirect.com for $220 and another dealership said retail was $367 so it will be interesting to see what they charge me for the part.

But, they've been decent so far and really a pleasant surprise when considering dealerships.

As most discussions on this topic suggest, start with the TCCM (located in the passenger footwell panel) and eliminate that as the cause. Seems like the Jimmy's and such with the 4hi/low selector switch see a lot of problems with the switch itself.

And you'll find more information on line searching for TCCM as a term than spelling out the words.

Thanks to all for your help!
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Crew Chief
post Jan 12 2006, 02:41 PM
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did you ever do a voltage drop test on the ground for the TCCM I assume this corroded should connector would have shown up I have seen several of these that look corroded internally but continue to work fine but the connector must be free of corrosion
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Grant
post Jan 13 2006, 10:57 AM
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QUOTE (Crew Cheif @ Jan 12 2006, 02:49 PM) *
did you ever do a voltage drop test on the ground for the TCCM I assume this corroded should connector would have shown up I have seen several of these that look corroded internally but continue to work fine but the connector must be free of corrosion


I personally didn't do this. Unfortunately, with a new home and associated projects, a full-time job, two kids, etc. I just don't have the time to do extensive troubleshooting. What I try to do is find any tips, ideas, etc. and apply them. If they don't work, I'll take my findings to a pro. Additionally, as my wife needs this car daily, she doesn't get too happy when I start "poking around" and tie the vehicle up.

So, I'm not sure if the tech did this. I was told he had to jumper several connections to get the module to work. At this point, after a highly recommended mechanical trans guy had the vehicle for four days and couldn't locate the electrical problem with his MODIS computer, I was just happy to have someone say with certainty that the problem was fixed and I would have the AWD back.

I don't feel taken advantage of...esp. when you consider they gave us a courtesy car to use for 4 days now.

Your question is certainly valid and with more and more SUV's being controlled with TCCM's (Envoy, Escalade, etc) manufacturer's should really do some training on this component (and or the transaxle) industry-wide so there's not so much mystery and confusion. And it should be given to the aftermarket shops as well so drivers of older vehicles can get them fixed at a more affordable rate than what dealerships charge. I'll be honest, this is the first dealership repair I've taken a vehicle to in nearly 20 years. Unless it was covered by factory or extended warranty, I take it to a local discount shop where I've got a relationship. I source my own parts and the techs get the understanding that I'm pretty informed about the issue. This works well for all parties (including wife!) involved.

Bottom line to conclude this thread is that if AWD without a dash selector switch is acting funny, start with the TCCM and make sure it's working properly and communicating with a Tech 2. Everyone will save a lot of time and aggrivation with this process.

Thanks again to all that helped me...esp. Crew Cheif!!!
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