What would a diesel be without glow plugs? (beside harder than the
dickens to start!)
Glow plugs are used to pre-heat the indirect injection chamber on some
diesel engines for ease of starting. VW has two different glow plug
systems, called slow and fast. Each system consists of a high current
relay (with integrated starting logic) and glow plugs. It is important
not to mix the fast and slow components.
The glow plug relay is the heart of the glow plug system. It gets input
from a temperature sensor on the cylinder head which it uses to adjust
the duration of the glow plug pre-heat cycle depending on the engine's
temperature. Longer when cold, shorter (or not at all) when its warm
(or hot). The sensor it located on the heater hose fitting on the end
of the cylinder head. It should be a brass sensor with a single
connector stud on the end, screwed into the body of the fitting. In
fact, you should actually have two such sensors, the other one is used
for either the coolant temperature gauge (or idiot light) on the dash.
An obvious question arises, which is which and what happens if they are
Well, it sort of depends. On some early VW diesels (maybe through
1984), the senders are different. If you have only an idiot light, you
can tell its sensor easily by use of an ohm meter. The sensor will
probably read open when cold and then close when hot (to turn on the
idiot light). The glow plug sensor will register some resistance
(between 100-200 ohms when cold). If you have a temperature gauge, its
sensor will register a higher resistance at any given temperature than
the glow plug sensor, probably 500-1000 ohms when cold. If you get the
two mixed up, you'll find the glow plug relay may cycle on and off
after starting before the coolant heats up. Since the other sensor
registers higher resistance, the relay is thinking it is *really* cold.
You may also get the little yellow glow plug light blinking on and off.
(Both of these symptoms happened to me with switched sensors).
On later VW diesels (around 1985) the two sensors (assuming you have
the temperature gauge) are identical and both measure about 1000 ohms.
So if you have this type of vehicle, you can't mix up the sensors.
Finally, on some of the latest VW diesels, the glow plug temperature
sender is now a simple on-off device. According to the Bentley '85-'92 Jetta Diesel manual, the glow
plug sender should be closed below 50°C coolant temperature and
open above that temperature.
So, what does this all mean? It means there are lots of combinations of
temperature senders and probably glow plug relays to go with them. I've
found some glow plug relays are very sensitive to the temperature
sender input resistance. Others tolerate a wide range of resistances
(they are probably a later design). Best bet is to find a relay design
that works and stick with it. If you experience "weird" glow
plug problems, this is just one more variable you'll need to address.
Troubleshooting the glow plug system consits of testing the various
system components in a logical order. The order to test the system in
needs to take into account the ease of accessing parts. You need to
make sure power can get to the glow plugs, then that the relay is
capable of supplying that power when needed and finally that the plugs
themselves are functional.
The above schematic diagram shows the electircal components and
connections in the VW diesel glow plug system.
Here are some handy troubleshooting tips for testing the various
aspects of the glow plug system:
The first (and easiest) component to check if you suspect glow plug
problems is the fusible link (see photo above), located above the
heater control valve on the firewall on the A1 platforms. It may be
under a black plastic cover (mine is missing:). On the A2 and later
vehicles, the glow plug fusible link is typically under the dash, near
the glow plug relay, under a clear plastic cover.
The link is a thin piece of metal that acts as a fuse for the system.
It should have two heavy gauge wires running to it. In some vehicles,
the fusible link is inside, under the dash. This would be the case if
you follow the heavy gauge wire from the glow plug bus to the firewall
and find that it goes through without terminating. In this case, find
where the wire comes through under the dash and follow it towards the
glow plug relay. There *should* be a fusible link somewhere between the
relay and glow plug bus. This is a safety item, this circuit is easily
capable of sourcing 50-100 AMPS, which could make bad things happen if
it were to short out to ground.
Make sure the metal link is intact and that the nuts are tight. It
can't hurt to remove the link and clean the contacts to ensure good
current flow. If blown, a replacement is available from the dealer.
Hint, if you have to buy a replacement, get a couple so you'll have a
spare next time.
The glow plug relay uses a coolant temperature sensor located on the
heater outlet on the cylinder head. This is located just below the oil
pressure sensor also attached to the head. Usually there are two
sensors screwed into the heater outlet. One is for the coolant
temperature gauge (or idiot light) on the dash (with a BL/Y wire), the
other is for the glow plug relay (with a BL/W wire). The two sensors
Pictured above left, you can see how to test the temperature gauge
sender, the ohm meter is reading 131 ohms with slightly warm coolant.
On the right is the glow plug sender test showing a nominal 645 ohm
reading. Both readings vary with temperature (that's how temperature
sensors work), I did another test at an outside air temperature of
60°F (15°C) and found the temperature sender read 1600 ohms,
the glow plug sender rear 1200 ohms. On some vehicles, the two senders
are the same VW p/n (068-121-145D) others are different and some later
models have a simpler open/closed sender for the glow plug relay, with
continuity below 120°F (50°C) and open above that.
If the sensor reads much outside this range, it is probably bad and
should be replaced. If the senders are different and the two wires are
swapped, you'll have abnormally low temperature gauge readings and
abnormally long glow plug cycles as best or the glow plug relay will
just stay on continuously. If the resistance reading is nominal at the
sender, check it under the dash at the glow plug relay socket. This
signal is present at the connector labeled "T", read
resistance from there to ground. If you get a different value than you
read at the sensor, there is a problem in the wiring or the connector
Another way to check that the temperature sender signal is reaching the
relay is to disconnect the wire from the sender and cycle the glow
plugs. If the relay stays on for a much longer period of time (than
with the wire connected) its probably working fine, if not, you
probably have an open circuit between the sender and the relay. The
other test is to ground the wire from the temperature sender. The glow
plugs should not come on with the pre-glow cycle. If either the LED or
glow plugs come on with this test suspect either a faulty temperature
sender wire connection or a bad relay.
So you have the little yellow glow plug LED on the dash, why do you
need another glow plug light? Actually, the LED on the dash has nothing
to do with the glow plugs. Its just an output from one part of the glow
plug relay, which takes the temperature sender input and produces a
delay to let the glow plugs reach the proper operating temperature. The
glow plugs themselves are powered by a relay which is operated by a
different part of the relay circuit and they generally stay on for a
time after the LED goes out. If the coolant temperature is warm enough,
the glow plugs and LED won't be turned on at all in the
"glow" position of the ignition switch. However, the glow
plugs are always turned on whenever the starter is operating,
regardless of temperature.
By running a wire off the glow plug bus bar to a light on the dash, it
will be possible to observe when power is actually reaching the glow
plugs directly. This indicator would be useful to diagnose a number of
glow plug problems.
Well, when it finally came time to install this secondary indicator
light, I had a second problem to solve. I found one of my windshield
washer nozzles was hopelessly clogged and would no longer spray. I
tried a fine wire to clean it out, but no luck. Then I found out that
the one nozzle I needed was NLA (no longer available). So looking
around, I found some of those lighted (LED) washer nozzles for a
reasonable price, around $10-15/ea. While those are popular in some
circles to have lit up at night, I thought this would make the perfect
glow plug indicator and would make installation/wiring a piece of cake.
I simply crimped on a large ring teminal to each of the power wires for
the nozzles and put one on each side of the glow plug fusible link and
then tied the two ground wires together and screwed those to some bare
sheet metal. Viola, two LEDs light up with the glow plugs. If only one
lights up, this means the fusible link has blown and if none light up,
the relay is bad.
During the day, can't really see the LEDs light up from the driver's
seat, but you can hop out and see then for troubleshooting purposes. At
night you can see the red glow easily. And to top it off, the washer
nozzles work very well, they have two separate jets that you can aim
separately for better spray coverage.
I've found that when I have one bad plug, I may not notice any starting
problem in warm weather and maybe just a little cold smoke. With 2 bad
plugs its harder to start, especially when cold or at high elevation
and you'll get rough idling and cold smoke until the engine warms up. 3
bad plugs and you'll be lucky to get going, and if you do you'll make a
most impressive smoke screen!
In the vehicle, you have a few options.
[Last updated: 21.April.2020]