The Glow Plug Guide to Troubleshooting

----------- Nov. 21, 1996
WallyG wrote in article
> I have a 85 Diesel Jetta which is very hard to start when cold. This is becoming
> increasingly worse as the weather gets colder overnight. The hard starting only
> occurs in the morning, the rest of the time the engine starts and runs fine.
> The engine was rebuilt a few months back, the rings and rod bearings were
> replaced but I believe the glow plugs are original. Is there anyway of
> testing them?
> regards,
> Wally

Hi Wally

Good decision to test your glow plugs. Many people jump to other
conclusions as to why their vw diesel's aren't working. A lot of the times
it's a problem with their glow plug system. When it gets cold, it is
important to have good compression but more importantly, your glow plugs
must be working to give off the heat needed for spontaneous combustion.
Even with lousy compression a good glow plug system (not to mention a good
battery) can start the car on multiple glow cycles.

Here are the components of your glow plug system:
* Glow plug Relay mounted in passenger-compartment fuse box
* Glow plug fuse (mounted on firewall on A-1's. Not sure about A-2's.)
* the four glow plugs themselves
* there is also a coolant/head temperature sensor, but if it's bad your
system should still work OK (It will just cycle your glow plugs longer than
necessary every time you do a glow cycle.)

Here are the steps for glow-plug troubleshooting:

First thing to do is to test for voltage at the glow plugs. When you turn
your ignition to the pre-glow setting you should get a positive voltage
reading for at least a few seconds. Ground to the engine block. If you
get a bad reading try it at the car body - it means your engine ground
strap is shot. The voltage reading times can range wildly, and even if
they're glowing for a really long time (your instrument cluster goes out)
it's OK - they are actually disconnected from the light and are probably
working by design. I should mention that VW also has a quick glow system
which takes less time. Also all components in each system are unique and
can't be mixed.

If you don't get voltage at the bus it's time to check the fuse. Check for
voltage on either side of the fuse. If there's none there it's time to
blame your relay.

If you find voltage on one of the fuse terminals, replace your fuse (don't
forget to use the two washers so your fuse doesn't twist and break when you
tighten it.)

If you replace your relay and still get no voltage, check for voltage at
the relay. It must be there, or else your battery has no voltage, or bad
wiring / connections.

If there is voltage at the bus, that means the relay and the fuse are OK.
The next step is your glow plugs. Most thorough way to test these is to
remove the bus strip (very tight quarters) and test each glow plug for
resistance. You should get a value which is less than 1 ohm if I remember
correctly. The proper reading is pretty small. Also all glow plugs should
have the same resistance reading. I think you'll find any bad glow plugs
as having a very large resistance reading. These are the bad ones. Time
to replace.

I've heard another method for testing glow plugs which I thought sounded
good but I haven't tried it myself yet. It was to go out to your car in
the morning and cycle your glow plugs about 3 times. Then you feel the
cylinder head for warmth. If the head is warm around some glow plugs but
not others, you've found your bad plugs (the ones which don't warm up the

If all your plugs are good and getting voltage (Thus getting hot) and
you're still having hard starting there is still another option for
glow-plug system malfunction: intermittent error. This is likely caused in
the relay, fuse connections, or bus connections. I'm sorry but it's really
hard to troubleshoot this. My only suggestion if you find yourself in this
situation is to make sure your glow plug bus connections are tight, your
fuse connections are tight, and as a last resort you could try replacing
the relay which appears to be working. (I've taken mine apart and found
the contacts to be highly worn - a distributor "points" file can help get a
good connection here. Plus it's a LOT cheaper than a new relay, which run
about $40.) Inspect the fuse for hairline fractures, especially if there
are no washers on the screws holding down the fuse. Once I had an
intermittent glow plug problem. It turned out to be a loose screw on the
glow plug fuse. Tightening the screw revealed a hairline fracture in the
fuse (there was no washer on the screw which worked itself loose.) Too bad
I didn't touch that screw before I changed my plugs ):-<