Hi, I am a new member here, this is my first car, I bought it used a few months ago for a great price, but it seems to have some problems.


When it drives at around 30-50 mph, there is a heavy vibration in the front. today, I put it up on jackstands and took all four wheels off. Then I drove it on the jackstands at the same speeds, and it still vibrated. When it is in two wheel drive, should the front drive train be turning at all? The four wheel drive indicator light comes on, but when I try four low, nothing seems to be different from four high.


My dad had only manuals, and these automatics are put together different.

 

If I'm not mistaken, you don't have a 2WD mode. You should have an AUTO - 4 HIGH - 4 LOW mode switch. Your drive system is ACTIVE at all times, thus everything in the front and rear is turning with varying amounts of torque to the front and reat axles via a center multi-disc clutch in your transfer case. Your vibration may be caused by a bad U-Joint in the front drive shaft or front axle. I would check them for excess play. You may as well check the rear drive shaft U-Joints while you're doing the front ones.

 

Thanks for your response.
What makes AUTO different from 4 HIGH if all four wheels are driven during AUTO operation?


The U-Joints are tight, I guess that only leaves the half-shafts. I didn't see any balance weights on the drive shaft.

 

In AUTO mode, the multi-disc clutch is used to progressively transfer torque back-to-front and front-to-back between the front and rear drive shafts as needed depending on what the sensors feed the computer. 4-HIGH mode tells the multi-disc clutch to lock, providing a permanently locked 50:50 torque distribution to the front and rear. 4-LOW does the same as 4-HIGH except that it tells the transfer case to select its reduction gearing.

So, in AUTO mode, you have controlled clutch slippage to avoid what is known as "driveline binding" and "torque windup".

In 4-HIGH & 4-LOW, the clutch is locked and that is where things break if driven on pavement or other hard surfaces.

 

That brake pedal feels funny as well. When rolling to a stop, the pedal pushes back, in a way that leads me to believe that the anti lock brake is activating, which could be caused by a bad wheel sensor.


What effect would a bad wheel sensor have when operating in AUTO mode? Is there a way to check the wheel sensors, with a multimeter?


Is the front axle just a normal limited-slip differential? (no computor controlled lockers, right?)

 

The front axle is an open differential (1989 was the last year that Ford put a L/S axle in the front of their 4x4's), the rear axle can be either an open or limited slip (mechanical clutch packs) depending on your rear axle code. The computer controlled locking is done in the transfer case.

The rolling to a stop issue is commom. You most likely have a breached hub seal on one of your hubs. If you remove the ABS sensors, turn the hub and look for grease on the toner ring teeth. If any is present, this is a sign that the seal is breached and the hub will need replaced. You will then need to either replace the other sesnor or hub as the new hub will come with a new sensor and the new sensor will send a stronger signal then the oler one, thus causing the same problem. A new sensor will cost almost as much as a new hub with the sensor already installed. Due to this, I normally replace both. Just double check the prices.