VW Diesel Glow Plug Control Circuit

General Description

The glow plug system is composed of a 50 A glow plug fuse; glow plug relay; head temperature sensor and 4 Bosch quick glow plugs. The function of the relay is to turn on electricity to the plugs until they (or the head) is warm enough to start the engine when cranking. Access to all these parts is easy except for to the glow plugs and bus bar connecting them. Two of the glow plugs are behind the injection pump, and the other two are on either side of the head coolant connection.

The relay turns on power when turning the ignition key to pre-glow, and turns off power when one of the following conditions are met:

  1. The temp sensor indicates that the head is warm.
  2. The engine is started.
  3. 60 seconds (approximately) has passed.
There is a glow plug light on the instrument cluster. Note that this light goes out about 7s before power is actually cut.

How to test if the glow plugs work

The mechanic's approach

With the fuel injectors pulled:
  1. Look down in each pre-chamber and check that the glow plug tip is undamaged. Tip should look smooth and round, not broken off.
  2. Turn on power (With ignition key). All four glow plugs should turn cherry red within a few seconds.
  3. Wait a while and check so they actually turn off.
Reinstall injectors with new heat shield washers (Important! Otherwise they will most likely leak!). This procedure can be done whenever the injectors are pulled for other reasons, e.g. compression test, checking injector spray pattern, dripping and opening pressure.

The electrical engineer's approach

  1. Turn ignition key.
  2. Measure voltage at glow plug 1 terminal. If 12 (or so) volts its ok.
  3. Measure current between the relay and plugs. Should be about 40-50 Amps.
  4. If not, measure current either:
    1. To each glow plug individually. Should be about 12A.
    2. or measure the current through the bus bar. Should drop about 12A for each plug. No drop means the plug between the measuring points is bad. This is most easily done with a "tangamperemeter" (sorry, don't know the English word for this devise. It's measures current by measuring the magnetic field around the conductor. Thus no need to break the circuit, just "clamp" this device over the conductor.)
An alternative is to take off the bus bar (tedious) and measure resistance to each glow plug. However this is not an as accurate test as the voltage and current test.

The home tinkerer's approach

Several not-so-precise methods:
  1. Turn on the interior courtesy light. When turning the ignition key, notice how the big current draw dims the light slightly. Make sure the light comes back to original brightness after a while. (about 7s after glow plug indicator light goes out).
  2. With the engine cold turn ignition key. Feel with the hand on the head around each glow plug. Should warm up.
  3. At night yank out the glow plug relay while pre glowing. No spark? Then probably no power to the glow plugs.

Things that go wrong and how to fix them

Individual glow plugs broken

This is often caused by a "dripping" injector (an injector which does not completely shut off below the threshold pressure), drastically wrong injector spray pattern (dirty/worn nozzles) or some other injection system malfunction. Not much to do other than replace the broken glow plugs (and fix the injectors).

If you need to remove the glow plugs behind the injection pump, or the electric bus bar, it helps to first remove the injector lines.

Glow plug electrical control system acting up

Common problems here are:
  1. Broken fuse (Look for hair line cracks). Replace the 50 A "metal strip" fuse.
  2. Corroded terminals. Typically on the fuse or relay. Take off and polish. Reassemble. Protect with grease.
  3. Malfunctioning relay: Replace (or if electronics wizard, take apart and fix. The relay is expensive!)
To start the car despite one of the above problems you can jump power directly from the battery to the (No 3 or 4) glow plug. Just attach a jumper cable from the battery, but be prepared for some sparks! And don't sue me if you short the cable against the block! (Tough luck with that anyway. I'm not American!)

If the problem is related to the temperature shutoff, you can just pull (and leave unconnected) the cable to the temperature sensor on the head. More precisely the temperature sensor is on the fitting which connects the radiator return and the thermostat bypass hoses. See the picture to the right for a detailed view. For a general view see the overview picture first in this document. This will inhibit the temperature shutoff. Instead of waiting for dash board pre glow light to go out just count 10 seconds (or 15-20 if it is cold), and start the engine.

Some more advice by Jake Russell

Also as Harvey Chao has found out, the Relay can malfunction in a way so the dash LED goes out but the power to the glow plugs remain on (during driving). Read about his extra glow plug light


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Martin Jägersand