I have a 1994 XLT, automatic trans with overdrive and 302 engine. Mainly on the open road when going up even a minor incline I get a lull and a slight vibration in the higher gears. I can even hear a hmmmmmmmm sound. It's an annoying sound. There is also a power loss. I have to push down on the gas pedal to drop it into a lower gear. At first I thought it was the overdive as it happens most in overdrive gear on the interstate, but it also happens in the other gears but not as much. Transmission seems to change out good and I've just changed the fluid, put in Mercon V and cleaned the filter. Does anyone have any suggestions on what can be causing this lull in power????
thanks guys for your help!

 

Is your truck still completely stock? (i.e. any changes to tire size, lifted, gearing in the axles?) I only ask before making suggestions because these changes often make a big difference. One other question while we are on the subject. Do you know what engine RPM's you are turning at highway speeds in overdrive?

 

How many miles on the truck? The driveshafts in Broncos usually need rebuilding and balancing by 100K miles. They will definitely cause vibration when worn. The driveshaft won't usually cause a hum noise though, that sounds more like a worn pinion bearing in the rear end, or a wheel or axle bearing.

Those are the first things that I would think of when hearing your symptoms; but it could be many things, even a bad tire.

 

Thanks guys for replying. My Bronco is completely stock except for custom wheels and tires are just a little larger than stock and I've had them on it for a long time. There has been no alterations in gearing and no lift kit. It's all stock with 152,000 miles. I did have the drive shaft replaced at around the 100,000 mile mark. This problem just started a few weeks ago and I hadn't made any changes to the vehicle. I can best descibe it as a "lull" or "drag" and a loss of power. It mostly occurs in overdrive and I'm accelerating and going up an incline. I have to disengage the overdirve or drop it into a lower gear and it stops immediately. Not sure of the exact rpm but it is not too high. Speed is usually around 65 - 75 mph but sometimes occurs at lower speeds also. I don't believe it is a tire or wheel bearing.
Your suggestions are greatly appreciated.