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I've decided to do a Cummin's conversion

FellowTraveler said:
Hello everyone, I've been playing catch up with this project and finalizing the cooling system heat circuit and the torque converter heat exchanger circuit and have a question.

All my previous chevy/gmc diesels had some sort of hot water shut off valves incorporated into the heater hoses but this 99 gmc just has the heater hoses plumbed to both heater cores w/o any shut off valves and truthfully I never had issues with the hot water flowing through the heater cores during hot weather and AC was so cold in the past I actually had to blend heat in to be comfortable and since I'll remove the ECM and wire the AC like FERM suggested do I need hot water shut of valves or?
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Are hot water shut off valves a factory option? I've never heard of that?
 
Nosferatu49534 said:
Are hot water shut off valves a factory option? I've never heard of that?
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The last GMT400 w/hot water shut water shut off valve was a 95 K2500 suburban BBC it had vacuum activated valve. I'm seeing some chevy cummins conversions that have the valve (s) too so they have to use a different exhaust manifold to clear the valve assembly.
 
GM installed a heater bypass valve in suv's that had rear heat but only front a/c. This way you didn't get the hot air coming out of the rear vents heating the back up. All of the 95+ control heads have a trigger wire to trigger a solenoid for a heater control valve. I've considered installing one in mine to make the a/c colder when I'm towing. When the engine temps break 195(the duramax likes to run 205-210 when pulling), the a/c performance begins to suffer. The evap is still ice cold(I get some ice on the evap box), but the little bit of air that makes it through the heater core really heats things up when she warms up. I need to pull my evap and replace the doors to get new seals. So you can install it or not, it's your call.
 
THEFERMANATOR said:
GM installed a heater bypass valve in suv's that had rear heat but only front a/c. This way you didn't get the hot air coming out of the rear vents heating the back up. All of the 95+ control heads have a trigger wire to trigger a solenoid for a heater control valve. I've considered installing one in mine to make the a/c colder when I'm towing. When the engine temps break 195(the duramax likes to run 205-210 when pulling), the a/c performance begins to suffer. The evap is still ice cold(I get some ice on the evap box), but the little bit of air that makes it through the heater core really heats things up when she warms up. I need to pull my evap and replace the doors to get new seals. So you can install it or not, it's your call.
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I have looked and can not find a hot water shut off valve anywhere on the Burb I know were one would normally be and it has me baffled that's why my question.......
 
FellowTraveler said:
I have looked and can not find a hot water shut off valve anywhere on the Burb I know were one would normally be and it has me baffled that's why my question.......
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If you have dual a/c, you won't have one. Single a/c but dual heat trucks installed a heater control valve over beside the turbo where the 2 heater lines criss crossed. I know years back somebody posted pictures of his 96 with one on it. The solenoid was mounted on the firewall, and it tied into the vacuum line for the boost control. I've seen them on 4 door tahoes more so.
 
OK, I've run into a glitch w/2 piece driveshaft in that I measured for the first shaft being level/inline w/transmission to center/carrier bearing then from center/carrier bearing angled down to rear differential however; GM upfitter best practices manual states that any two (2) or more piece driveshaft should be installed in a straight line from transmission tail-shaft to differential.

The illustration in the manual shows a straight configuration that must be confirmed w/string down from transmission tail-shaft to differential.
 
Well, looks like you'll need to fabricate a bracket that lowers and angles the carrier bearing to the proper alignment. Then re-measure and adjust both shafts' lengths accordingly.
 
I'm baffled since I built the 2 piece exactly like it came out of the Ram Cummins but for my GMC conversion like the image below however; I see many debates on different forums and trying to find out if what GM upfitter manuals state is indeed accurate, I seem to remember someone on this site commenting that some of the info in them is inaccurate.....
Ram_2piece-driveshaft.gif
GM's version has the transmission and differential the same as image but the driveshafts are inline like a single driveshaft would be.
(Image copyright unknown)
 
I think it may be to minimize the deflection angles of the u-joints that GM has the two shafts in a line vs. a nearly horizontal front shaft and a sharply angled back shaft. You can only wedge in so much rear diff pinion angle before causing other problems.
 
Yes to minimize angle is the reason, but always 2 sides...

The more inline the shaft is, the better for the ujoint in strength, drag efficiency, wear, etc.

Having a multi piece driveshaft also increases harmonic vibrational issues. Everytime there is a directional change it adds more harmonics.

Conversly
There is also the too perfect theory. Having a driveshaft perfectly true never forces the cup bearings to rotate and now instead of distributing the load and greae across more of the bearings, it is focused on a few and can wear then quicker.

Also the longer the shaft the longer the wavelength of all frequencies will be- like long piano string compared to short one. While instinct is so what if it is bass vs treble, the issue here is it is a stronger wave form that can carry further into other vehicle components and generate more issues.

So the problem of long and straight becomes don't make a shaft assembly longer than it needs to be, and have a minimum offset to it. Unfortunately it has been way too many years since I could remember the optimum desired angle.

When I owned the truck equipment shop, we used to stay on top of what each mfr says is desired for testing and certification purposes. Now, common sense says 1 would be best, but they all had their opinions. Funny thing is not one mfr ever met what they said was best. So in real life install we used the “bout” tape measure. “Oh, build that bracket bout there”.
 
Normal suspension movement negates your "non rotating of needle bearings and lubricant spreading in the bearing cups" theory. The long wave amplitude magnification of long drive shafts (and their inherent weakness/weight issues) and thus their excessive deflection, imbalance and ultimate failure, is eliminated by the use of two (or more) piece driveshafts in long wheel base/heavy duty applications. Driveshaft geometries are a happy medium between empty and fully loaded suspension parameters and one of the places things can go horribly wrong when majorly lifting vehicle suspensions, "slamming" vehicles or grossly overloading them.
 
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I called my driveshaft guy after reading the above and my understanding for a 2 piece drive shaft is that all 3 angles should add up to zero, the same as with a one piece shaft. He said that a lot of times the first shaft will be set on the same line as the transmission and the second shaft angle treated the same as if they were using a single shaft to make it easier to calculate.
He explained it as "if your trans if set 3 degrees down with your first shaft in that line and the pinion set at 3 degrees up, the angles of the two joints of the second shaft will be equal through the suspension travel".
Of course there will be some slight change in pinion angle through the loading and unloading of the axle, but that is normal.
 
Thanks everyone for the feedback I now have a better understanding two shafts inline equals less issues loaded/unloaded so the GM upfitter manual is accurate. Hopefully I will not need to get these shafts shortened then again I had no idea that inline is a better configuration than what I had in the Ram so my bad.
 
One or the other or both will need shortening for the simple reason that they are the two short legs of the triangle that the new configuration is the long leg of. Pythagoras.
 
@Husker6.5 quoted from your response:
“Normal suspension movement negates your "non rotating of needle bearings and lubricant spreading in the bearing cups" theory.

Not my theory- from classes taken. And you are correct for his pickup in this thread. But some rigs use fixed differentials so the driveshafts never move other than rotataion, which is why I bothered mentioning.
 
Of course with an independent suspension like in a Jag or Vette (or Hummer/Humvee) the differential would be fixed to the frame and the driveline remain static.
 
I'll know today if I have to modify the fixed cross member so I can go with the GM inline configuration. I'd like to know if any of you have a GMT400 that is using a two or more driveshafts and how they are setup as in "is the first shaft from the transmission inline with the transmission or is it inline (straight) with following shafts from transmission yoke to differential yoke?"
 
My question would be, why are you using a 2 piece driveshaft in a 134" wheel base vehicle? Even if you have a SYE, I can't see your driveshaft needing to be long enough to require more than a 3.5" .086 wall shaft. I know my rear shaft is not that long, and I would think a 2 piece rear shaft would be introducing problems, sort of like GM did with all the ECLB and CCSB trucks where they tried a 2 piece shaft, and ended up having to replace them with 1 piece aluminum to finally get the drivetrain to run smoothly.
 
THEFERMANATOR said:
My question would be, why are you using a 2 piece driveshaft in a 134" wheel base vehicle? Even if you have a SYE, I can't see your driveshaft needing to be long enough to require more than a 3.5" .086 wall shaft. I know my rear shaft is not that long, and I would think a 2 piece rear shaft would be introducing problems, sort of like GM did with all the ECLB and CCSB trucks where they tried a 2 piece shaft, and ended up having to replace them with 1 piece aluminum to finally get the drivetrain to run smoothly.
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Thanks for your input Ferm. I am running 2wd until I find a good DANA 60 kingpin and need a 60.5" center to center on the universal joint caps of driveshaft however; when the builder in Ocala (mostly big rigs) found out I was running a Cummins he suggested a 2 piece as a cost savings over a much larger diameter over 60" single drive shaft I agreed knowing I'll have to do another shaft anyway w/SAS install. When I find a good DANA 60 I can remove the 2wd O/D and replace it with the 4x4 O/D case, t-case and one piece driveshaft.

The first shaft I had built from scratch was a single unit but measured for the shorter 47RE (big screw up) when I went to install the 47RH & find it's 4x4 OD unit is few inches longer than what I had measured for that screwup placing the t-case in the smack in the middle of the torson bar cross member. Considered a modified torson cross member but in the end that would not work as the t-case occupied the d/s key & adjustment assembly area on the D/S drop case.
 
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