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.

 

New ones en-route. I just had to be 100% sure. These relays aren’t cheap.

I’d be surprised if regular normal non Ferrari ones don’t work. From what I’ve read, the diodes in these relays are only there to prevent arcing on the load contacts when the relay turns off. Arcing can cause this problem over time as well. Strange it supposedly requires a specific Ferrari part?

 

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.

 

Fly back to what though? Just the contacts on the load circuit?

the only point of this is to prevent arcing/carbon build up over time, isn’t it??

 

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.

 

It’s so weird these aren’t just generic parts. Why not just manufacture these off the shelf, or even more logically why not just manufacture all relays this way.... these damn things cost a fortune and take weeks to come from Italy.

not sure I want to use my fixed one in the interim or not...

 

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

 

yep that’s right... at least a grand or two... I’ve got a relay on order so I’ll just swap it out and I’ll be good to go, even though this fixed one is probably fine.

Not only the money saved but also the satisfaction of getting to the bottom of it and getting a ‘free’ education on how your car works. I probably know more about the abs/brake system than our local mechanic now!

Plus a greater understanding of how relays work..

 

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!

 

All in a good place now. Still works fine this morning and the relay continues to be luke-warm after being used, unlike before where the pins scolded my hand when I removed it.

 

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.

 

??

 

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 !!