Ist tow problems

[konacat]

My GMC 2500HD CC/Long bed w/stock trans oil pan is 1 month old and has 3700 miles. No problems up to now. This was my 1st tow of 5th wheel 11,000-12,000 lbs. Towed w/o T/H mode until the Grapevine in CA. Switched to T/H mode for steep climb. Part way up a truck pulled out in front of me and had to slow quickly. Then on the pedal hard and trans downshifted. All going well, then it sounded like the trans was slipping. Almost got run over trying to figure it out. Then it seemed to grab a gear and was fine. Did this one other time for a few moments. Trans temp was 150 and engine temp a click below 200. Downhill no problem, T/H mode worked great. No other problems. Anyone have any ideas? Thanks

[Desert Diesel]

konacat,

It may have been your engine fan engaging. I also thought it was the trans when I first heard it.
I know when it would happen to me stock, it was like hitting a brick wall in the power dept.
Also with my truck there was no relation between coolant temp and fan engagement. Correct me if I'm wrong guy's, but I believe there is a sensor for air temp as it passes thru the intercooler that effects the fan.

Regards,

David

[More Power]

A bi-metallic coil on the face of the fan-clutch controls engagement. As the air temperature between the radiator and the fan-clutch increases, the coil expands, which in-turn engages the clutch.

You can read more about fan-clutch engagement here:
http://www.thedieselpage.com/reviews/fanclutch.htm

I've towed more than 5,000 miles with our stock 2001 GMC Duramax/Allison, in all types of weather - from 20 degrees to 105 degrees. I've never noticed any impact on power when the fan-clutch engages.

MP

[Kent Tuttle]

Only have towed a couple of times, once it was pushing 100 degrees and like Jim said I noticed no impact on the power when the fan clutch kicked in. I did notice it was quieter than the one on my 99 6.5 powered sub.

[Desert Diesel]

Hitting a brick wall was probably an overstatement. The times it happened that stick in my mind are when I was nearing the crest on a grade. When the fan kicked in my power stopped, no more - no less. Throttle was at 100% in 4th gear.

[konacat]

Thanks,

I feel a little better about the situation now. Did not realize that the fan would activate while the engine temp was still fine. The fan made a lot of unexpected noise, which shocked my wife and I.
I did not know it would be so loud.

[More Power]

I'm sure it does take power to turn the fan, especially at higher rpms. I've heard unofficial numbers like 20 horsepower at 3,000 rpm, but it's too hard for me to tell much difference.

I do know that on a long steep climb in hot weather and with a heavy load, not having the fan-clutch engage would mean losing 300 horsepower.

MP

[HD Hauling]

Are you sure that you might have started out in first and the tranny shifted to 2nd unlocked T/C and then it shifted to locked T/C?
It does feel different in 2nd because this is the only gear that the tranny is unlocked and when it does it a person could think that it was slipping.
Just remember fellows what the GM Engineers want us to do, don't lug the engine to much.
The HD must mean these trucks are only good for pulling trailers in muddy conditions just like in the Commercial's on TV.
That's all you see is the Chevy trucks playing in the mud on the TV!!
I could take my 87 with the 350 in it and do the same thing!!
Have a Good Day