Questions and Answers
Dodge Dakota Vibes
Q. 1991 Dodge Dakota LE, 318 V-8, automatic, 86,000 miles, fuel injected,with P/S P/B C/C , antilock brakes on rear only, rack and pinion steering. My truck developed a terrible vibration and roaring sound at 75 mph that smoothed out completely at 80 mph.
I checked the driveshaft last summer, had it straightened and balanced and installed new U-joints and the worst of the noise went away.
Recently the problem reoccurred just as before and the driveshaft was found to be .300" untrue. The machinist said it was flattening from driving torque and built me a new shaft. I installed it and the noise was gone, but there was still some vibration coming in at a little over 55 MPH and fading away near 75 MPH.
I then checked the drive line angles and found the pinion centerline to be 3 degrees less than that of the transmission shaft. I shimmed the pinion upward 3 degrees and re-checked the angles which are now both 5 degrees upward slant toward the front of the truck. That's with the truck sitting on a dead level surface and the frame having a 1 degree upward angle toward the front. The drive shaft also has a 1 degree upward angle toward the front.
A test drive after the shimming showed all the same symptoms as before shimming.
The engine mounts and transmission mount all are free from any oil contamination and appear to be in good shape.
A fellow told me that he had fixed three Dodge Dakotas with this same problem and the fault was found to be the cooling fan clutch. When I told him the vibration is related to ground speed, not engine speed, and happens at the same speed with the transmission in overdrive or not he said, "I know, those three were the same way as yours." He went on to say something about the air coming through the radiator to the fan and the fan speed catching up to it or something like that. I do know, however, that sometimes when driving down the highway and I decelerate I can hear the fan kick in which doesn't seem normal to me.
What do you think about this problem?
JohnA. I think with the work you had on the driveshaft, we can just about eliminate it as a possibility.
It is possible the fan clutch is the problem. The fan drive clutch is a fluid coupling containing silicone oil. Fan speed is regulated by the torque-carrying capacity of the silicone oil. The more silicone oil in the coupling, the greater the fan speed, and the less silicone oil, the slower the fan speed.
The fan drive clutch uses a heat-sensitive, coiled bimetallic spring connected to an opening plate. This unit causes the fan speed to increase with a rise in temperature and to decrease as temperature decreases. They can engage and dis-engage at any time and do make a roaring sound when engaged. Since it is pretty easy to replace, I would go ahead and put a new one and eliminate it as a possibility.
It is also possible there is a problem in the differential, a bad bearing or chipped tooth for example.
I would also look at the tires. They could very well be in need of balancing. I would put them on a high speed wheel balancer and make sure they are in balance. Tires do go out of balance as they wear. Check them for unusual wear as well. Cupped or scuffed tires will make a lot of noise as you drive.
Additional Information provided courtesy of ALLDATA
