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Off to a bad start

donkeylips

Well-known member
Joined
Jun 18, 2009
Messages
552
Location
Rochester, NY
Hey guys, I haven't posted much yet because I didn't actually get my gvr4 until tonight. Not new to DSMs though, and I've got plenty of wrench time in on my brother's car, 924/1000. You may know him...smooth customer. I had 497/2000 towed to my house today and I gotta vent.

The guy told me he was gonna use his friend's truck and trailer to make the 15 mile trip. First of all he was about 2 days later than he said he would be, even after bugging me all week saying that he needed it gone. He calls me and says he's leaving, and that they're going to flat tow the car. It's just a shell, but the tranny is under the hood. My first thought was "center diff" but I figured flat towing it would be fine.

When he gets to my house, I hear clanking...and I remember a horrible fact. He had taken off the transfer case for some reason. He had a tow strap supporting the drive shaft. I decide to not give him the $250 +gas for towing that I owed him until I checked to make sure that the front axles were not connected. Well, he gets it in my driveway, I pop the hood, look at the transmission WITH the front axles connected and no transfer case that has just rolled in neutral for 15 miles, and I guess I had some kind of brain fart. Absent mindedly I forget about the whole thing and give him $250 + $50 for gas. That's a total of $550 for a shell that has front end damage and a destroyed center diff. The only reason I paid $500 was because it was supposed to have a good tranny.

For $60 I could have had a flatbed, a good trans, and the car would have arrived on the day that it was supposed to. What a horrible experience. I'm too poor for a tranny rebuild, and I've already got 3 bad ones at my house. Is there anything I can do for this thing or is it complete junk? I have no experience with transmissions but I have one that is good besides a chunk taken out of the bellhousing, and another one that has no first gear. Should I try to make one good one or something?
 

Barnes

Well-known member
Joined
Feb 9, 2003
Messages
6,249
Location
Richland, WA
Are you sure the center differential is ruined? Someone else should chime in, but I'm trying to figure if it would be damaged by rolling in neutral. I know the VCU would be trying to spin the disconnected transfer case output shaft. I just wonder if it would actually be harmed since that is such a small load. This is a bit confusing because I'm so used to thinking of this stuff in terms of power being applied by the engine.
 

Muskrat

Well-known member
Joined
Jun 13, 2004
Messages
2,107
Location
Lexington, KY
My understanding is the damage to the VCU comes from a high speed differential. Tradiationally: the rear wheels running at however many mph, while the front are held stationary. However any combination would cause damage.

In your case the front wheels turned, causing the output shaft to the t-case to turn. However, it wasn't being held = no speed differential. The center dif is a viscous coupling. As the input shaft to the dif turns, the viscosity of the fluid in the dif causes the output shaft to turn. When there's a high load on one end (aka a speed differential) the energy goes into moving/heating the fluid instead of the shaft. Since there was no load on the output shaft, it would just be turning at the same rate as the input shaft.

Short answer: your center dif should be fine. The clunking was most likey the driveshaft moving around.
 
Last edited:

donkeylips

Well-known member
Joined
Jun 18, 2009
Messages
552
Location
Rochester, NY
Wow that's great to hear, I hope it is fine, and the clunking was definitely the driveshaft. How do I go about checking the viscous coupling? I was kind of wondering if it might be ok, but I figured there couldn't be much difference between the "usual" wrong way to tow an awd car, what with one end spinning, and the other end not. Still not sure if I understand why it would be different then say, towing the car with only the front wheels on the ground. Is it just the torque required to turn the driveshaft and rear wheels while in the air that would ruin the coupling? Or you're saying that the load required to turn the rear differential(or front, if it was the other way around) is what ruins them, and since that load wasn't there, the output shaft should have spun fine without causing any damage? Forgive me for not having much time to research the subject yet.
 
Last edited:

Barnes

Well-known member
Joined
Feb 9, 2003
Messages
6,249
Location
Richland, WA
The big difference in what you describe and your situation, is that when being with two wheels on the ground, the rear wheels are moving, and the fronts are fixed. This is what melts VCUs and center diffs. Driving around with broken rear axles REALLY destroys center diffs. In your situation the fronts where moving, but the rear wheels (effectively) were not fixed. That's why we think it is fine.

I would drain the fluid and see if it has any metal contamination. AFter that, just get it together and drive. See if there are any unusual symptoms.
 
Last edited by a moderator:

Muskrat

Well-known member
Joined
Jun 13, 2004
Messages
2,107
Location
Lexington, KY
The VCU fails because one 'end' is fixed, and the other is spinning very fast. Look up how a viscous coupler works. All the energy from the spinning shaft is transfered to the fluid in the VCU, which is a friction material. Working that fluid causes it to get really really hot, and "burns up" the VCU.

In your case the fluid wasn't absorbing any signifiacnt amount of the energy put into it, it was all transfered to the free spinning output shaft. Hence no significant increase in temp, and no failure.
 
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