1995 456 GT ABS prob: pump won't turn off

Any ideas from experienced Fchat members (regarding below described ABS problem) is greatly appreciated. I've had this 456 less than a month and it's a steep learning curve mechanically compared with my '74 911S.

Driving on an open highway Saturday, the Brake warning light came on. As soon as I could pull off safely, I applied the brakes. There was definitely something wrong...a buzzing vibration in the pedal. I turned around and drove home carefully after first checking the manual (which indicated the problem was ABS-related) and second checking under hood that fluids were good and no other noticeable problem. Brakes worked, but the brake pedal would exhibit vibration intermittently causing delayed braking power. Today on way to mechanic, I pressed pedal very slightly at higher speed I seem to get normal braking without the vibration. My first thought was that a speed sensor had failed and the ABS is engaging incorrectly under hard braking, or braking to a stop at low speed.

Today, though, my mechanic noted that the ABS pump does not turn off as if pressure is not being maintained, We pulled two fuses that shut off the pump, the vibration is gone from the pedal but now I have no brake boost.

Plan A, put in 1-3 new gray 30A relays (I've read elsewhere that these wear out and can cause the pump to run continuously, but doesn't seem to explain the ABS engaging incorrectly)
Plan B, pressure switch gone bad?
Plan C, ...

I know there are a some VERY expensive components in the ABS system, but so far the worst I've read is a pump rebuild. I haven't heard anyone having to replace the ECU.

John

PS. The 30A relays p/n 155437 are same as used for 360 F1 pump. I read that this relay would be better off rated at 50A to avoid known problem of overheating. If anyone knows of such an option, please post.

 

Taking it in for diagnosis tomorrow, but I like to think through these things to be prepared for discussion with the mechanic.

I have some questions

1. Are you saying that if my problem were "just" a bad ABS accumulator, pump or faulty relay, I would not get the vibration I'm feeling in the brake pedal? I would just loose boost and have a "heavy pedal"?

2. If the ECU is sending a signal to dump pressure, where is the pressure dumped to? Would I have any external visual indication of this happening?

3. Would the erroneous pressure modulation signal from the ABS control unit be interrupted by pulling the two middle fuses, which shut off the pump? The vibration went away as soon as I pulled those two fuses (out of the total of four).

I saw your other post that had a happy ending by taking the car out an "activating the ABS". That doesn't apply in my case, but I fear my problem is going to end far more poorly!

John

 

Thank you for the all of the information, I was driving on open road in dry conditions when light came on. Brakes were working normally up until the warning light popped on as my first indication. Battery/Alt seems to be working fine. I would say the vibration did NOT feel like ABS, and more like what you are talking about--related to the pump or accumulator... but I had read the ABS pulses are very fast, so not sure what it should feel like in a Ferrari. Never any trouble cranking the engine over, but will keep battery/alt in mind. I'm off to the mechanic now: luckily not far away. I'm thankful for your time in helping me get a better understanding of how the system works.

John

 

Mechanic has determined that ABS brake booster pump is not producing enough pressure (150 PSI vs. 1800 PSI), so recommending replacement (rebuilt $1,600 vs. new $3,600). I haven't talked to him yet to see how pressure measurement is taken and what components could contribute to the low pressure measurement. Any comments from more knowledgeable Fchat members are welcome. But so far, it seems that the ECU is not the culprit. Pump is running continuously and pedal has vibration and spongy low assist feel because pressure never builds to the specified value.

 

Thanks for the info. Yes, I asked the mechanic how he tested pressure and based on his answer I asked him to remove the pump (as recommended here on Fchat) and to check for a valve problem. All he tested was brake line pressure, I think, which was not zero, unlike I read on Fchat where the valve was stuck and there was ZERO brake assist pressure. The $1600 rebuild price was for a complete pump with accumulator, etc. If you have a name of any company let me know. The two I checked so far don't work on the Ferrari ABS system. I will check on who rebuilds Range Rover ABS and make some calls. I'm not sure who would make the search/determination of similar less expensive parts: my mechanic would charge me shop rate for his time and I'm self employed with a backlog of client projects. Time is money! Thanks again posting, it really helps to get Fchat perspective!

 

Thanks, excellent advice. The pump did run continuously for some time as this occurred when I was about 40 miles from home. Breaks worked with a heavy foot, and the owner manual indicated it was ABS problem but could be driven, so I turned around and drove back. Once in the shop it was evident the pump was running continuously. First check was going to be relays, so I will ask what was found regarding relays. I've read there is a better after-market relay, but will review Fchat for those references. Yes, I'm inclined to replace all of the key components to make sure I don't get stuck with this problem (I'm sure it will be another on tho!)

 

Mechanic pulled the pump. He found tool marks on valves, etc. So the pump had been opened before. No issues found with valve-sticking. I'm going for the Ferrari parts pump rebuild with new accumulator & pressure switch because there is one in stock right now to swap. It seems to me if the accumulator were the source of the problem it would still be possible to get full pump pressure at 1800 PSI during a test (vs the anemic 150 PSIon test). Pump gears, etc are inaccessible it seems. Now I need to look at my options on replacing relays.

 

Mechanic reports that brakes are working now, but ABS light comes on for a time after multiple pumps on brakes and then goes out. System was bled twice with a one-night interval in between (with iginition on/engine off). He's going to get a better instrument that can measure exact pressure drop between brake applications and compare to Ferrari max allowed pressure drop. Hopefully not a bad pump rebuild. Anyone with other ideas?

John

 

Mechanic let car sit overnight and re-bled system. Brakes were fixed. I'd hoped for an easier solution than $2,000 in parts/labor to replace ABS pump. Next, replace the brake pads to eliminate the brake squeal that can be heard for 2 blocks in every direction.