VW Diesel Glow Plug Control Circuit
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:
There is a glow plug light on the instrument cluster. Note that this
light goes out about 7s before power is actually cut.
- The temp sensor indicates that the head is warm.
- The engine is started.
- 60 seconds (approximately) has passed.
How to test if the glow plugs work
The mechanic's approach
With the fuel injectors pulled:
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.
- Look down in each pre-chamber and check that the glow plug
tip is undamaged. Tip should look smooth and round, not broken off.
- Turn on power (With ignition key). All four glow plugs
should turn cherry red within a few seconds.
- Wait a while and check so they actually turn off.
The electrical engineer's approach
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.
- Turn ignition key.
- Measure voltage at glow plug 1 terminal. If 12 (or so) volts
- Measure current between the relay and plugs. Should be about
- If not, measure current either:
- To each glow plug individually. Should be about 12A.
- 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.)
The home tinkerer's approach
Several not-so-precise methods:
- 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).
- With the engine cold turn ignition key. Feel with the hand on
the head around each glow plug. Should warm up.
- 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:
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!)
- Broken fuse (Look for hair line cracks). Replace the 50 A "metal strip" fuse.
- Corroded terminals. Typically on the fuse or relay. Take off
and polish. Reassemble. Protect with grease.
- Malfunctioning relay: Replace (or if electronics wizard, take
apart and fix. The relay is expensive!)
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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