Confusing Problem

[Cdawg]

Hopefully someone will have something to offer ... I think I'm going crazy. I have a '91 k2500 with 24,000 original miles on it. The issue I'm having is there is a vibration and a thrumming noise between the speeds of 30 and 50 mph. Here's the catch: It only does it when the ambient temperature is around 80 degrees or higher (the warmer it is outside the worse it is), or the truck is warm and the vibration instantly goes away as soon as you pull the 4x4 shift lever into 4 high. As soon as it goes back into 2wd, the vibration and thrumming comes right back, usually worse.
Maybe someone can steer me the right way ... my best theory I have going, (although I'm not extreamly knowledgable on the subject) ... it is my understanding that the 4x4 system on this model of truck is vacuum controlled ... so I am guessing there is a vacuum leak somewhere that is made worse by warmer temperatures that is causing the 4x4 to partially engage ... is this a possible theory, or am I way off base? I had someone ask me if I had had the recall work done in which the vacuum system was converted to electronic ... I didn't pay much attention to the guy at the time, he had a '94.
Thanks to anyone with any input,
~Cdawg

 

[canuckz71]

What engine do you have? Found this on consumer guide.

Automatic transmission: Trucks with the 6.5L engine may have a transmission shudder when the torque-converter clutch applies and releases. (1991-94)

Automatic transmission: 700-R4 automatic transmissions may shift late or not upshift at all. The problem is a stuck throttle valve inside the transmission. (1990-92)

 

[Cdawg]

I have the 5.7 liter engine ... the transmission (which is the 700-r4) was just professionally rebuilt less than 200 miles ago due to a bad torque converter. This problem existed before the issues with the transmission, and is exactly the same after the transmission/torque converter rebuild.

 

[MIHELA]

The shaking almost sounds like a bad u-joint, but I can't explain why that would change with temperature.

 

[94 Silverado]

I know the switch on the front differential is thermal activated. It heats up when you shift into 4x4. I wonder if it's not retracting all the way in 2wd and causing some binding. I don't know if you can drive with that switch removed without gear oil flying out everywhere but if you can it would probably rule that out. Maybe there is an ohm reading it's suppose to have and you could test it. Or you could just replace it. It probably needs a new one anyway.

 

[silverado_06]

The shaking almost sounds like a bad u-joint, but I can't explain why that would change with temperature.

my first thought as well.

 

[noremac]

the u-joint would be my first guess... how the temperature comes into play... i dont know.


it also could be something in the transfer case. my reasoning behind this thought: as the oil gets thinner, the internals might have a little bit less resistance to moving, and when you activate 4wd, the chain meshes with the internal gears and stops the movement. (seems semi-logical to me)

 

[barebones 1500]

sounds like that to me to, but it was on a ford i had. i replaced the whole front axle and made it fresh and the noise was still there. i know a ford isn't a chevy but a u-joint is a u-joint. that switch sounds like it could be it too, or it could a quwirk that you truck has and it really is no problem. check it out and see what you find.