QI own a rear-wheel-drive 1999 Jeep Grand Cherokee with the 4-liter engine. I recently had both rear axle seals and bearings replaced. At the same time I had the mechanic check U-joints, shocks, etc. The noise from the failing bearings is gone, but now I have a loud "howl" that gets louder the more I accelerate. If I take my foot off the accelerator, the sound is gone. As soon as I accelerate, the sound returns. It is barely noticeable at speeds less than 35 mph, but the howl gets increasingly louder as the speed increases.

I returned to the mechanic the same day I picked up the Jeep. We went on a road test and I was given a story about gears in the differential and that despite the noise everything was mechanically sound. Even though the sound was not present before the seals and bearings were replaced, the only option given was a very expensive replacement. Can you suggest a fix? This howling is giving me a headache!

AYour mechanic is probably correct. You're hearing a howl generated by worn parts in the differential, likely the ring and pinion gears. The only "quick fix" might be to check the level of gear oil in the differential -- maybe enough leaked out from the worn seals that it needs topping up. Better yet, drain and refill the differential with synthetic GL-5 gear oil. While 80W-90 gear oil is standard, for heavy-duty use like towing you can use 75w-140, which may help quite the noise significantly.

If this doesn't quiet things down enough for you, you might be able find a used differential carrier assembly in a salvage yard.

QI'm 74 and have been working on cars all my life, and I can't figure this one out. I have a '99 Jeep Cherokee with the 4-liter engine and 150,000 miles. The other day a ripple in the concrete highway caused a major vibration in the front end. I pulled over to check, then drove home. Later, when I tried to start the car, nothing happened. The battery was destroyed by the shaking. I replaced the horizontal steering damper, and everything is fixed -- except the oil pressure is now pegged at 80 pounds per square inch instead of 40. Is this a plugged relief valve?

AI doubt that the high oil-pressure reading is mechanical; it's more likely a problem with the oil pressure sender/switch or gauge. On this engine the sender/switch, just above the oil filter, is integrated into the powertrain control module (PCM). Unplug, check, clean and reconnect the connector on the oil pressure sender/switch. If this doesn't help, try a new sender/switch.

QI have a 2002 F-150 with the 4.6-liter engine, and it has 194,000 miles. My air conditioning works great when the car is moving, but when you let it idle for a few minutes, the compressor kicks off and it blows warm air. As it continues to idle, the compressor kicks on and off and you get cool air for a few minutes then warm again. As soon as you start moving again, it blows cool air constantly. Any ideas?

AHave the refrigerant level and pressure checked. The system may just be low on R-134a. You can buy a do-it-yourself can of refrigerant and oil with a dispensing hose and small pressure gauge. Hook this up to the low-pressure side of the system and see if the pressure is above about 25 psi when the compressor is engaged. If not, add refrigerant until the pressure is in the normal zone -- generally 25 to 35 psi with the compressor engaged.

After that many years and miles, a slight loss of refrigerant is normal. Topping up the system may keep you cool for at least the rest of this summer. And don't forget to clean bugs, leaves and debris from the front of the A/C condenser and radiator.