Hello. I have a 2000 Mustang GT Conv. 5 speed in my shop right now with all 4 wheels locked up. The customer brought me the car after putting new front and rear pads and rotors on it. He said then the pedal dropped to the floor so he put on new calipers and a new master cylinder. He brought it to me with all 4 wheels smoking after a 6 mile drive to my shop. I put on new front pads, rotors, calipers, and hoses. Still did it. So I checked the system and found that when I released the front master cylinder bleed screw, the front wheels freed up. So I put on a new master cylinder and hydro-boost unit (per customer request on the hydro-boost unit???) It is still doing it. But now all 4 wheels are free until I start it up and then they lock up, even if I start it without touching the brake pedal. I removed the belt to see if the power steering pump was over pressuring the hydro-boost and they still locked up. The only things left are the Abs Block or the prop valve and I don't want to just throw parts at it. PLEASE PLEASE PLEASE SOMEONE HELP ME. If I don't get this car out of my shop by tomorrow afternoon, I have to pay for a rental car until I fix it. The customer is super pissed that we can't figure it out. Thanks for any help in advance.
Thank You for the reply. I have probably flushed 2 gallons of brake fluid through the system and I thought he might have contaminated it (haven't seen any evidence of it but the thought crossed my mind). I will try the master cylinder unbolt thing tomorrow morning. Also, I am not extremely familiar with mustangs, but, I have read about adjustable master cylinder pushrods but the one on this mustang isn't adjustable. It would partially explain the only sticking when the engine is on because of the pressure from the booster, but I'm not sure. My head hurts from thinking about this
The problem you're looking at points squarely at an ABS problem. The brake system pressures should NOT be increased simply by starting the engine. The hydro-boost system does not increase brake system pressure, it simply replaces the vacuum type brake booster. Do the brakes lock up by simply turning on the key, not starting?
It honestly sounds like something is wrong with the ABS system causing the different ABS pumps to engage all at once. This is the only thing that could cause all 4 wheels to be locked up.
Exactly what that problem is, I couldn't tell you. Could be a faulty ABS module, could be a bad PCM, could be 4 bad wheel speed sensors (unlikely, but possible), maybe a simple wiring issue.
In all honesty, and don't take this as me saying you don't know what you are doing cause that's not the case, but it may be in your best interest to sublet this repair to a Ford shop. It could potentially save your shop a LOT of money.
I have been thinking that too. The lead tech used to be a Ford Dealership Technician and he had been on vacation the past couple days (hunting season) so he should be in tomorrow and I'm hoping he can shed some light on the predicament. I will try unplugging the Block tomorrow morning and see if it has any effect on the brakes. And to answer your question, I haven't tried it with just the key on and engine off...
If you wanted to spend the time bypassing the ABS unit and pluging the holes in it to prevent contamination, you could do that. Using brass fitings and spair hard line to link the lines and remove the ABS unit from the circuit. I'd definately unplug the electrical connector on it if you do that.....