FUEL INJECTION - BUS AND BEETLE - '75 thru '79

copyright  Jim Mais  2001

  Simplified schematic here.

The double relay provides power to the fuel pump and to the ECU (Electronic Control Unit). There are two magnet coils , one for each relay, labeled CR1 and CR2. Each relay has one normally open contact which is shown separate from the coil in the diagram. There is nothing magical about a  "double relay". VW just decided to stuff two ordinary relays into one box
along with a few other components. Here's how it works:

When the ignition switch is turned ON, relay CR2 energizes.
CR2 contact closes, applying power to the ECU thru terminal 88b.
Power is also applied to the ballast resistor bank thru 88b.
The ballast resistors speed up the response time of the injector solenoids.
The other side of each injector solenoid goes to a switching transistor in the ECU.

When the ignition switch is advanced to the start position, power is applied to the starter solenoid and also to CR1 relay thru terminal 86a, 86. A diode in series with CR1 coil prevents power from flowing back out. The cold- start valve also energizes as the starter cranks. Cold start is under the control of the thermo-time switch. The cold start valve will only inject fuel if the temperature is low enough to require it and the injection will cease after a preset time due to heating of the thermo-time switch during cranking. If you crank for a long time and the engine still doesn't start, cold start injection will be turned off to prevent flooding.

CR1 contact powers the electric fuel pump and the Aux Air Regulator.
When the engine begins to run on its own, the pump switch in the air flow sensor (air box) closes 39 to 36. This contact closure provides a "holding" circuit to relay CR1. Because of the holding circuit, power can now be removed from terminal 86 as the starter is de-energized. The holding circuit has a resistor built into the relay enclosure which provides sufficient current to keep CR1 energized.

This circuit provides a safety feature in that the fuel pump will automatically shut off if the engine stalls. In addition, turning ON the ignition does not start the pump. The starter must actually be cranking before the pump comes on.

The injectors are fired by the ECU in time with impulses sent from the ignition points.
Note: If the wire to terminal #1 on the ECU is not connected to the correct ignition coil terminal, the engine will not run.  Injectors fire on alternate openings of the points. All injectors fire at the same time. How long the injectors stay open is controlled by the ECU after it has calculated factors such as engine temperature, RPM, air flow, and throttle opening.

Differences between years:
Shown on the diagram are the controls for the EGR valve, and the full throttle switch. Only some years had these. Also, early years did not have the temperature sensor located in the air flow sensor.

A test light or voltmeter is handy for testing various parts of the circuit. If you use an ohmmeter to test the double relay out of the circuit, note that the diodes will affect reading of the relay coil resistance.   The actual resistance of the relay coils is from 50 to 80 ohms. However, measuring the resistance from the pins on the double relay, the resistance will read between 100 and 500 ohms depending on scale used and type of meter.

If you get an open circuit reading, reverse your meter leads and check again.
(The polarity of the battery inside your ohmeter will affect the reading.)
You can also check the holding circuit between pins 86b and 85. This should also measure somewhere between 100 and 500 ohms.

The air flow sensor potentiometer operation can not be accurately measured with an ohmmeter because of the way in which the resistance elements are constructed.   So if you try to measure it and get erratic readings while operating the air flow flap, there is probably nothing wrong with it after all.

The following resistance measurements can be made on the air flow sensor:
terminal 6 to 9: 200 to 400 ohms (1975) 100 to 300 ohms (1976 and later)
terminal 7 to 8: 120 to 200 ohms (1975) 80 to 200 ohms (1976 and later)
terminal 36 to 39: should go to zero ohms when the air flow flap is opened slightly.

If the air flow sensor has 7 terminals (late style) you can also measure the resistance of the air temperature sensor. At room temperature the reading should be about 2500 ohms between terminal 6 and 27.

Failure of the ECU is very rare. Look elsewhere first in case of trouble.

The double relay is a frequent cause of trouble but with a little patience and use of the test light you can isolate problems in its circuits.
A quick test is to have someone crank the engine while you feel the electric fuel pump. If running, you will be able to feel the pump motor spinning.

Another frequent cause of a dead engine or poor running is the temperature sensor in the left side (#3) cylinder head. When cold (room temp), the sensor should measure about 2500 ohms. At running temperature, the resistance will drop sharply to about 100 ohms. If either test is way off, replace the sensor. In an emergency you can substitute a variable resistor for the sensor (Radio Shack).

Don't overlook poor connections when hunting troubles. Any one of the many push-on connectors could be loose or making poor contact. (Don't forget the two push-on connectors buried on the fuel pump.) Note that there is no fuse in any part of the FI circuitry. It is worth while checking the ignition points.
Points that are badly pitted or with improper gap may produce a spark but still not trigger the ECU reliably to fire the injectors. Also visually inspect the rotor/cap for carbon tracking and replace if they look doubtful.

For more detailed description of other control devices on the FI system, see the Robert Bentley manual covering your car.