Help! F355 ABS Gremlins (one for the electrical gurus)

Milo has always been the man on this forum.
I have seen bad relays. I would just replace them.

 

It's not that strange when you look at Jaguar, Ford, and other ABS systems using the ATE ABS set up. It seems they could not find a readymade relay that can be used to drive a big pump so they just add more flyback protection diodes to the outside. Perfectly ok.

 

The motor is a big inductive load (a coil). When you turn it on and off by cutting its current, the inductive motor fights back by jacking its voltage the other way. To prevent the large negative voltage spike to ruin the rest of the electronic, the reverse bias diode shorts out the voltage spike and sends that energy to ground, thereby protecting the rest of the circuit. The corrosion on the contact is just the secondary effect of having high current switching on and off through the contacts.

The action above is technically called a Fly-Back phenomenon.

Fly back is useful in many other applications, just not in this one.

 

When television had cathode ray tubes (CRT), flyback was used to generate the large voltage to display pictures on the tube.
When cars ran on points and condensers, flyback is what gives you spark.

 

Glad you got this sorted. I'm guessing you didn't take your car to the dealer on Friday (and saved a fortune)

Yep, he's a bit of a legend on this forum... taught me everything I know

 

I read this whole thread and all along I was praying for an easy fix. You guys had me worried when you got deep into the electronics. Thanks for posting a happy ending. I just wonder how people that dont work on their own stuff survive. They probably spend their free time making money, so they don't have to worry about stuff breaking. What a terrible way to go through life. Good work Glactica!

 

The diodes in relays connected to the relay solenoid (pins 85 & 86) have nothing to do with arcing on the relay load contacts.

On the black relay, the diode across pins 85 & 86 suppresses the relay's flyback voltage (approx. 250-300 volts) at the relay switch-off and prevents it from reaching the ECU (pin 14). In general, this diode protects whatever circuit is switching the relay on & off, not the relay itself. You can experience the relay flyback voltage if you keep your fingers of the spades 85 & 86 while connecting and disconnecting 12v supply to the spades to switch the relay on & off. Be ready for a nasty electric shock when you remove a wire from one of the spades.

The other in-line diode at pin 85 is there to isolate / prevent backflow of current, again something to do with the ECU.

A diode across the inductive load switched by the relay (like the pump motor, one of the diodes in the relay-like housing), "kills" the motor's flyback voltage and protects the electronic circuitry and also contributes to reduced wear of the relay contacts. Without the flyback diode on the load, there would be two sparks between the relay contacts: one at switch "on" and another, considerably stronger, on switch "off" when the flyback voltage (~200V) would arc across the relay contacts as they start moving away from each other.

 

Get back to us on the spade connectors at 85 and 86 where you grab them.

 

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I am amazed by the level of expertise present in this forum and really appreciate the generosity of all of who who so freely and wisely contribute. Although I am no neophyte in the automobile world, alot of this is well above my punching weight and I feel like I am back in school.

Really great comments and astute observations. Salute !!