Bleeding the ABS unit on a 1996 Ford Thunderbird

Vehicle

1996 Ford Thunderbird LX 3.8L

It has the Teves MK IV ABS system with traction control.

 

Symptoms

I've been having a problem with my brakes for a few years now. The rear brakes don't have enough braking power. When I took my car in for safety inspection, the brake bias was way too far forward, meaning the fronts did most of the braking, and the rears barely did anything.

The other problem was that if I pressed hard on the brake pedal, I could get it to go all the way to the floor. If I did a panic stop, I could almost get the wheels to lock up on dry pavement, but this would require lots of pressure on the brake pedal. I was worried that I would have to press too hard in an emergency situation.

 

Attempt 1

So I replaced the master cylinder and bled the brake system. That didn't help at all. I was considering replacing the proportioning valve, but didn't because I suspected air in the ABS unit because of the excessive pedal travel.

I activated the traction control while bleeding the brakes. I activated the ABS in the snow, then bled the brakes. I bled the brakes again. Nothing helped.

 

Research

So I turned to the internet and the service manual. Both sources said that I needed to get my hands on a T90P-50-ALA tool. I didn't have the $300 that they wanted for one on ebay. So I found some wiring diagrams and a pin-out diagram. These were really helpful, but I still made a couple mistakes along the way. Hopefully you don't repeat my mistakes.

 

Attempt 2

This was a near disaster. I blew a fuse trying to hook up power to the ABS valves. I was connecting to the wrong end of connector C-104. Basically, I connected power to pins 3 and 33 of the ABS module without the ABS relay being activated. When the main ABS relay is not activated, it connects pins 3 and 33 to ground. Connecting power to ground is not a good thing. Something had to melt. It's a good thing there was a fuse between the battery and my connection to ground. 

After recovering from that mistake, I thought you run the pump with all the valves activated. Well, that made the ABS motor make an awful sound. I hope I didn't damage it. After putting it all back together, it still wasn't fixed. I was not having a successful day.

The ABS module is located in front of the front wheel on the driver's side. Here's a picture with the inner fender removed and the wiring harness detached.

 

More Research

Then I took a look at the hydraulic diagram. I realized my mistake. By closing the inlet valves, I blocked the path of the brake fluid from the pump, preventing it from circulating. (See dead head definition.) If I activated only the outlet valve, then the motor could circulate brake fluid, leaving the air bubbles in the reservoir. The inlet valves are normally open, while the outlet valves are normally closed. When you activate either valve, you change the state (open to closed, or vice-versa). Activating the valve is done by grounding the appropriate pin on the connector to the ABS module. 

This is an ABS diagram similar to my Thunderbird, but it doesn't have the traction control valves in the diagram.

 

Attempt 3

Here's the approximate procedure I used to bleed the ABS unit on my Thunderbird without the help of the T90P-50-ALA tool.

1. Remove the front driver's side wheel, remembering to implement all safety precautions.
2. Remove the inner fender.
3. Disconnect the wiring harness from the ABS control module. You pull on the metal clip and rotate it up to the top. Then lift the connector to remove it from the module.
4. Ground pin 34 to activate the main relay. This makes power available to the valves.
5. Ground pin 15 for twenty seconds to run the ABS motor.
6. Ground pins 2, 21, 36, and 18 to activate each wheel's outlet valves, one at a time while the ABS motor is running. Don't run the motor for more than 60 seconds. Maybe run each valve for 5 seconds each.
7. I'm not sure if you're done at this point if you have traction control. But here's what I did next.
8. I hadn't found a hydraulic diagram for the traction control part yet, so I opened both rear dump valves (pins 18 and 36) with the ABS motor running while I activated pins 37 and 40, one at a time to run fluid through the traction control part.

I was really hoping this worked. Unfortunately, it is mostly the same. But it was enough to get it to barely pass the brake skid-pad test at inspection.

 

More Research

I found this diagram on the internet. It looks like this is the traction control version for FWD (still not my RWD version, but maybe close enough to figure it out).

 

 

Only the outlet valves are normally closed. The inlet valves and the two special valves for the traction control are normally open.

When the traction control is activated, the special valves close to prevent the ABS unit from applying pressure to the  front brakes.

I'll probably have to work on this again to pass the next annual inspection. So here are the next two possible solutions I'm going to try.

Bleed one-way valves

Activate both traction control valves and dump valves for the back wheels while pressing on the brake pedal to bleed through the one-way valves.

Close the inlet valve and compress the caliper piston to bleed each one-way valve on the inlet valves. Do this for each wheel.

Bleed pressure relief valves

Activate both traction control valves and the ABS pump.

Hopefully this fixes it. I don't see a way for the $300 bleeder tool to bleed the one-way valves if there's any air stuck in them. So I don't see a point in spending the extra money if there's a chance it won't solve my problem. Inspection is coming up, so if the brakes don't pass this month, I'll be updating this post soon.