fuel pump relay resistor?

[tgrandahl]

I have a 95 525i that wont start, well it will if i jumper power to the fuel pump fuse and that was enough to get it into the garage and out of the cold. It looks like my fuel pump relay is dead. looking at the bently it shows something additional between 86 and 85 on the relay. If i open the relay this looks to be a diode, but while looking to buy a new relay i came to a few people selling "30 amp., 4-prong relay without a resistor".

So is the what looks like a diode in mine really a resistor that actually isnt nessecary?

Thanks,
Tyler

[SRR2]

The diode across the relay coil is used to suppress the high voltage spike produced when the coil drive is removed. A resistor will do something similar but its effectiveness is related to resistance value. Lower values will reduce the magnitude of the spike, but will waste proportionally more power when the coil is energized. The obvious advantage of the diode is that suppresses the spike and consumes no power.

There's a second order effect though. The diode causes the relay dropout time to increase significantly. A resistor will cause some increase, but due to its power-dissipating advantage (in the transient state after deenergizing) over the diode, the relay will drop the load quicker. The practical effect of this with DC loads is that generally speaking protecting the relay drive circuitry with a diode across the coil will cause the contacts of the relay to fail sooner due to the prolonged and slower dropout, which causes more arcing with inductive loads like motors.

Also, generally, you do not want to run relays on DC without some kind of coil transient suppression, particularly when they're driven from solid state sources. So in your case, I'd go for the relay with a diode first and a resistor otherwise.

If you can't find a relay with suppression, you can add it yourself by choosing an ordinary 1A (minimum) diode. Something like a 1N4001/2/3/4 would work just fine. The cathode of the diode should be connected to the terminal of the coil that gets the positive voltage.

[tgrandahl]

Thank You!