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

The function of the diode you marked is not just simple "one way valve" but it is acting as a "flyback" diode blocking the high reverse voltage spikes (~300 V) coming from the pump motor windings every time it is disconnected by its relay. No fault in the ABS system can blow this diode.

CTEK MX5.0, in addition to being a charger, also has a battery desulfation function - it sends very high voltage (~30,000 V) pulses to the battery. Although unlikely, but these pulses can blow the diode if you switch the ignition key to ACC (first position) while the charger is connected to the battery and the battery isolation switch is on. Are you absolutely sure that your battery was always isolated while the CTEK was connected to it?

 

Just in case that diagram is too small to read...

Latest F355 (2.7) Fig4_Teves_ABS_System

Looks like Miro has beaten me to it. Those diodes are normally reverse biased, so current shouldn't flow at all, but as Miro says, the diode is for protecting the circuits from reverse voltage spikes as the pump cycles on and off. Looking at the datasheet for that BY252 diode, it's capable of withstanding 400 volts reverse bias.

 

When installing a diode for "flyback" protection (relays, motors), I prefer to use diodes with 1000 volts maximum reverse voltage like 1N4007 (for relays) or 1N5408 (for motors).

 

The diode is permanently connected between the pump motor positive terminal and earth. The orientation of the diode is such that, when +12V is brought to the positive terminal of the motor, no current will flow through the diode. At the moment of disconnection of the power from the motor positive terminal (the relay switches it off), there will be a brief pulse of around 300V coming from the motor windings (induction) in reverse polarity to the polarity of the earlier applied +12V. The said reverse current will be shorted by the diode to prevent it from damaging any electronics connected to the motor's positive terminal (Pin 32 of the ABS ECU in this case). The reverse flyback spike, although it can be ~300V, is of rather low current so the diode can easily handle it.

 

Looking at the diode only, it does not seem possible for the ABS ECU to detect earth through the diode as it is in reverse to the path ECU Pin 32 to earth. But then, the diode has a minute reverse leakage of 20 micro amp so this could be sufficient. However, Pin 32 is also connected to the motor's positive terminal and, when the motor is off, it provides almost no-resistance path for the Pin 32 to earth so the presence of the diode is irrelevant. When the pump motor is switched on, there is +12V on the motor's positive terminal and on Pin 32 of the ECU. Again, the presence of the diode is irrelevant as far as the ECU is concerned. The only time when the diode has a function is at the moment of the motor switch-off.

The diode (if you have BY252), can handle up to 400V when blocking the motor's flyback spike and, even though it is a 3A diode, it can handle up to 100A of very brief forward surge (spike) current. So, it is quite unlikely that the diode is damaged by the motor's flyback spike which I don't think is more than 300V, probably around 200V.

The approx. 30,000 volts from the CTEK, if it reaches the diode, will be acting on the diode in its reverse polarity so no current should flow. However, as the maximum reverse voltage that the diode can handle is 400 volts. If higher voltage than this is applied to the diode in reverse, even a very small current will damage the diode, i.e. in this case it is the voltage that breaks down the diode's P-N element, not really the current. When this happens, it will probably first short the diode so it will become a conductor in reverse as well. When then the pump motor is switched on, the high current available in this case will just instantly blow the (no longer) diode open.

 

Have you always had this problem, or did something happen to the car before the problem showed up? Like someone was in that area of the car to install after market electronics, or some other unrelated repairs?

 

This is what I think is going on.

Sometime in the past, someone has done something to the wirings of the ABS relay and fuses block at the right hand side of the car. This is because it is often in the way of getting to something else like a shock absorber or other electronic modules in that busy area. They have moved the module around, disconnected it, and then reconnected to the wrong relay. Don't go down the CTEK path yet until you rule out everything else.

The diodes are safety devices and you should be able to remove those diodes without affecting the functioning of the ABS pump for a while (as a troubleshooting thing). That fact that it is blowing and the pump no longer working means they are in the wrong place of the circuit.

The symptom you are describing is that of an old ABS pump that is slowly going bad and now has gone totally bad by drawing way too much current.

The suggested solution is for you to use the color wiring diagram to make sure that all wires are going to correct relays. Thank you Ian.
Then confirm that the ABS pump is indeed bad and replace it. There is a thread on here about how to buy and mount a Jaguar pump in the 355. Get on that bandwagon before the Jaguar pump people jack up their prices too compensate for the Ferrari use.

On a side note, the same pump on my 328 has continued to function just fine, while many people with 355 have reported their pumps going bad. I don't have an explanation.

 

But that is just it, the diodes are not supposed to be in line with the power supplying the pump. Something is messed up with the wiring. I have seen a lot of crap with the old Ferrari wiring. Here is an example:

1. Someone on a F355 messed up the ABS wirings in previous work, either by installing big amp or something else in that vicinity
2. They disconnected the relays from their holders, and plugged them back in the wrong places relative to the wiring, thereby getting two relay wiring mixed up
3. Doubled down on the problem by replacing the two diode box with a regular relay because the diode box looks like relay
4. Causing the ABS pump to not work properly, so they wired the pump on permanently, triple down on the error
5. That pump died a year later because it is now on 100% as opposed to being on 5% of the time.

It took a while to undo all that mess. And it is the reason I said that when the circuit is properly wired, removing those diodes does not stop it from working for some time.

 

Good points. It should be noted that the Black and Yellow relays are special and must be inserted in the right sockets. This is how they are arranged on a 348 (the one with a white dot is the diode box).

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Interesting. The datasheet for that diode says the ability of the diode to "rectify" depends on the frequency of the voltage change. Whether the diode breaks down faster because of this, I don't know.

 

I checked and found out that desulfators inject pulses of some 50-60 volts which have a frequency of about 30,000 Hz so I was wrong when I earlier said "30,000 volts". Therefore, we can rule out the CTEK as the 50-60 volt pulses, whatever frequancy, cannot damage the diodes.

The diode functions are:

Left Diode - the Brake Warning light and Pin 27 of the ECU are connected to the anode (triangle) of the diode; the cathode of the diode is connected to the main (green) relay contact to which Pins 3 & 20 of the ECU are also connected. When the main relay is "off", the Brake Warning light and Pin 27 are grounded via the diode (the warning light comes on), and Pins 3 & 20 are grounded directly, through the relay contacts. When the main relay is "on", Pins 3 & 20 get +12V but Pin 27 and the warning light do not get +12V as they are isolated by the diode. The unaffected by the +12V link between Pin 27 and the warning light remains so that Pin 27 can light-up the warning light if there is a fault somewhere detected by the ECU while the engine is running. This diode can only be damaged by excessive current flow through it (when the main relay is off) from Pin 27 of the ECU to the ground due to some damage of the ECU circuitry.

Right Diode - connected to the pump motor; this diode can only be damaged by excessive flyback voltage from the motor but this is unlikely as the diode is designed to easily handle the flyback; if there is a short (or partial short) in the motor windings, the flyback voltage will only be smaller so no adverse effect on the diode. A serious motor overload, due to higher current flow through it, may result in higher flyback voltage on switch-off but this should not be such to cause diode damage, unless the flyback exceeds 400 volts.

"galactica", are you sure one of the diodes was blown? They are not easy to test with a digital multimeter. I always use an analogue multimeter when testing diodes as it clearly shows whether the diode conducts in one direction and not in the other or whether it is open or shorted circuit. If one of the diodes does blow, determine which one exactly by looking at the colours of the wires to which it connects and checking on the diagram. If you can confirm that the pump motor flyback diode definitely blows, then I would check whether the motor draws excessive current due to partial seizing of the pump.

While looking at the diagrams, I noticed that there is a unit called "Electronics for ABS system cut-off" (No. 10009). If faulty, this unit could disable the ABS pump motor as the pump relay operation depends on it (perhaps Ian can provide more info on this unit).