355 Bosch ABS Light On

Okay guys, I'm at the end of my rope. 1999 355 Spider with Bosch ABS. 16K miles. Millionth owner. So ever since I got the car at 11K miles, the ABS light would come on randomly, but seldom. Sometimes it seemed when I hit a bump. The LR sensor plug was dangling and it looked like the female socket ends were cracked and maybe not making contact. I went so far as to tin the male ends so make them slightly larger, yet since moving to Arkansas where there's a great deal more humidity than in Arizona, the light came on more and more until now it comes on after driving a short while and stays on even on start up. I removed the wheel sensors, which are clean, and tested the resistance and AC voltage at the ABS plug. The resistance of each is 1.68K ohms for the rears, and 1.71 K ohms for each front axle sensor. All produce AC current when the wheel is turned. The plugs all appear to be making good contact, and are securely in the clips attached to the chassis. The computer was shipped off to a guy who rebuilds them, and though he doesn't have the software to test a Ferrari part, says it passes the physical exam of broken connections, and is the same general part as used by Volvo. I've cleaned the plug carefully and verified voltage at the large red wires, and ground at the large black wire. The brake fluid level is correct, the brakes work fine, and again, sometimes the light goes off, but when the engine gets hot, the light comes on. I'm thinking the rear plugs are in a hot engine bay and near the manifolds? Sometimes the light comes on when you apply the brakes, sometimes while simply driving along. There is nobody in town who can read a Ferrari ABS code, at least who answers their phone, and it's hard to verify who sells a code reader that can get me a meaningful code for this year and model. Help!!!

 

I see you've been struggling with this problem for quite a while...

https://www.ferrarichat.com/forum/threads/intermittent-abs-light.650945/#post-148205291

Unfortunately, the system can only be interrogated with Ferrari SD tools. The Bosch system is a bit of a mystery and, apart from the sensor wiring and dirty sensors, the system seems quite reliable.

Did you check the sensor resistances from the ECU plug or at each wheel? Did you have someone to help you shake the sensor harnesses whilst measuring the resistances?

On the surface, electrically, the Bosch system seems quite simple, but the ECU is monitoring the operation of the valves, relays and pump, so you really need to find someone with the proper diagnostic tools. Maybe there is a clue in the fact that it sometimes happens when applying the brakes and when simply driving along? If it was an engine compartment heat problem, you wouldn't think it would specifically happen when applying the brakes.

Did you look at the fuseholders? I'm wondering if the 60 amp pump fuse suffers from the same fate as the "hidden" melting AC fuses?

I rechecked the wiring diagrams and see that the fluid level sensor is not even hooked up to the Bosch ABS system electrically, so unless the brakes are not working properly because of fluid loss, I don't think it should trigger the ABS light. Anyway, your BRAKES or (!) light should illuminate if the fluid level was low or your front brake pads were worn.

 

I thought you plug a diagnostic fuse in the holder and the system will flash its light at you and tell you what the error code is.

 

That's the 2.7 I believe, which also has a totally different ABS system.

 

+1

The Bosch system has no test fuse slot

 

I'm still thankful for your help fixing mine. Still no light

 

I would like to thank all who answered, especially Quavion, who led me to the solution, indirectly. You're right that the later Bosch 5.2 system is reliable and fairly straightforward. What I did to check the wiring was to read the resistance of each wheel sensor at the ABS plug with an analogue voltmeter, to show real time changes as I wiggled the wires to each wheel. Nothing emerged, but while I was in the area of the ABS pump and removing the brake fluid reservoir to get to the LF sensor wire plug, I saw a ground wire attached to the chassis with a 13mm bolt. I reached in on the off chance to see if it was tight. It moved! The car had had extensive front end repair work under a previous owner, and some guy forgot to tighten the ground lug. To see if was the ground for the ABS pump I undid it, and sure enough, it was what went right to the pump a foot away. When I checked ground at the plug it would read okay, but when the car was moving and current was needed, it would lose contact. My daughter has a Honda minivan, and the dash and the left front corner lights all went haywire. I traced it to a lose ground wire that was not tightened when the tech did some body work on that corner of the car. These guys do things finger tight, then forget to go back with a wrench. So I'm over the moon to have the only mechanical problem this car has ever had fixed. It's my daily driver, and I jump in whenever the wife wants milk or eggs. Moved to Arkansas from Colorado and Arizona to be at a lower altitude and near 93 octane gas. The first place was 7,300 feet, the second 4,000. This place is 600 feet, and the car flies. Thanks again.

 

Thanks for the update. We'll add this solution to our "database".

Glad you fixed it. Ground issues can be sometimes hard to find. The wiring diagram shows two major grounding points, but it's not clear where they are.

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Looking at photos of the wiring harness (P/N 172956 for the LHD car) I see one is about a foot away from the main connector and the other about a foot away from the fuses (#60 and #61).

https://exoticautorecycling.com/product/ferrari-f355-anti-skid-wiring-cable-wire-harness-used-p-n-172956/

 

Qavion strikes again.