Updated April 2, 2008
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The combo relay is the main power and control distribution point for the LJetronic system that is used in E12's. It's also the interface point between the fuel injection harness and the engine harness. In fact the combo relay is one of two junction points of these two systems (the other being the ECU to tachometer connection in the glove box).
The combo relay is mounted near the coolant tank and has two connectors on it. One is part of the fuel injection harness (only white and brown wires) and the other (various colored wires) is part of the engine harness.
(Older style metal cased relay- Photo by TR Hanson)
Below is information for the combo relay. The 530i relay is similar to the 528i relay but has an additional diode and one less terminal.
Combo Relay pinouts
*I=input O=output P=Power G=ground
Here's an internal diagram showing the operation of the 528i relay.
|528i Fuel pump operation:
Power is supplied to relay pin 88y through fuse 1 and pin 2 of engine connector (green w/violet wire).
Power from starter solenoid signal is applied to 86a which energizes relay coil A (fuel pump relay) through the diode which is a one-way device. This prevents the current from flowing from 86b to 86a.
When the AFM door is opened from air flow, voltage is applied to pin 86b (should have power) and through the coil 'A' and out 85 (should have ground) to the ecu which is actually grounded inside. If you hear the relay click when the AFM door is propped open with key on, then all that part is working. The diode blocks current from AFM switch on 86b from flowing to 86a and engaging the starter while the car is running
Starting or running:
When the coil A is energized by the AFM switch or starter solenoid signal, the switch between 88y and 88d (and 88c) is closed and power should appear on 88d and 88c. The wire on 88d runs through engine connector pin 8, through the fuse box and to a connector under the rear seat. From there it runs to fuel pump and in-tank fuel pump.
Also see the complete fuel injection diagram here in Adobe Acrobat format (PDF) here.
Here's an internal diagram showing the operation of the 530i relay.
If you suspect a bad combo relay (intermittent no start, bucking or loss of power to injectors or fuel pump, you can remove and check the relay. Most type have a black plastic cover that can be pried off.
Below are pictures showing the internal PC board (which holds the 2 relays and other components). Repeated heating and cooling and vibration cause the soldered connections on the PC board to crack or break (note the circular crack on the broken joint below) and fail. They can be re-soldered and the relay is good as new. A little liquid rosin flux (not acid) will make re-soldering the joints easier for the novice. Just paint a little on the joint before soldering.
|back of combo relay PC board showing broken solder joint|
Photo captions courtesy of Tom Hanson
Since this FAQ was written, I suffered a partial
failure of my combo relay not related to broken solder joints. I
started getting intermittent low fuel pump voltage, causing cold
running off-idle stumble. Swapped out the combo relay and cured
the problem. I'm guessing the half round contacts have worn flat
to the point where they no longer have the contact pressure per
area of two round contact meetings (more area, same force)
Now my car has been a daily driver since 1984. It's really the number of starts not mileage that causes the wear. Each time the pump cuts on, the locked rotor (motor initially not turning) current of the pump is much higher than running current and wears off a little metal. Each time the fuel pump shuts off, the inductive kickback of the motor causes the relay to arc a little. Being a daily driver I've had a lot of starts. Also the cold fuel demands of my 3.5L are probably a little higher than the stock 2.8 which reveals the delivery shortfall sooner.
Consider a new combo relay if you're not sure and it's original (what I should have done 3 months ago).
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