Glow Plug Relay



I am new here and have recently bought a 1988 Fourtrak F75. I have trouble cold starting when it needs lots of churning and starts with clouds of smoke (hot starting seems ok - the starter motor is sticking but I intend to replace that). I am getting a voltage at the glow plug, but I have noticed that the relay cuts out the voltage very quickly - after about 1-2 seconds of current, before it stops for a while and then clicks to allow a one second burst of current again.

Is this enough to heat the glow plug for a cold start? How long should the current pass before cutting out? Is there any way that I can test the relay other than substituting it? Are they expensive?

Sorry if I'm asking dumb questions, but I'm new to problem solving on diesels!

Many Thanks

Check the glow plugs resisitance.

Cold resistance of the glow plugs should be around 1 ohm. I suspect there will be one of the front 3 that's gone high resistance (more than 3 ohms). This messes up the glow plug timing circuit which relies on sensing the current via that chunky resistor between the back glow plug and its neighbour.

Use 'search' to find out more about glow plugs (sometimes I have called them heater plugs). This topic crops up quite regularly.

if u feel the relay is cutti

if u feel the relay is cutting out too quick, turn ignition off when the light goes out and then on again to reheat before trying to start. on a really cold morning 3 or 4 lots of heat may help.

Glow Plug Relay

Appreciate the responses. The resistance check on my multimeter is broken, so I'll pop into Maplins and get a new one tomorrow & and check the Glow Plugs as suggested over the weekend.
Many Thanks

Glow plug electrical test.

Being an Electrical Engineer, I thought I would try to come up with a quick and easy go/nogo test for glow plugs.

Disconnect all the wiring from the glow plugs (they don't need to be removed from the cylinder head).

Take a piece of insulated wire (between 1 sq mm and 2.5 sq mm area) about 2ft (600 mm ) long and connect it to the outer body of a 12V 21W indicator bulb.

Temporarily connect the other end of the wire to the battery positive while dabbing the centre solder pip of the bulb onto the connection on top of the glow plug.
Safety note If the wire is accidentally short circuited to earth it will quickly glow red hot and cause injury. This is a known problem with heavy-duty car batteries .... who hasn't dropped a spanner in the electrical works and caused unintentional meltdown before now ! Really, the wire should have a 10 Amp fuse fitted in series to limit the fault current, should there be an accidental short-circuit.

The indicator bulb should light with near enough full brightness. If the indicator bulb is connected directly to the cylinder head, its brightness should be only fractionally more. (When I tried this out on my 'trak, I found that the connection between the aluminium rocker box and the cast iron head was strangely high resistance which made the bulb glow LESS bright than when it was connected to the glow plug. Connect the bulb directly to the cast iron head to get a good circuit for the second half of the test above.)

If you have a voltmeter, clip the positive to the glow plug top and the negative to the cylinder head. When the bulb is feeding current to the glow plug, the voltage should read about 0.75V. (This corresponds to a glow plug resistance of 0.45 ohm).

Note on glow plug resistance versus temperature.
As most metals heat up, their resistance rises. For example, the resistance of an ordinary light bulb at normal brightness is about 10 times its cold resistance. To measure the cold resistance, a small current must be used which does not heat the wire too much. This can be done using a multimeter, but the specified cold value of cold resistance (0.1 ohm) is so low that the resistance of the meter leads of an average multimeter is quite a lot greater, so it is difficult to know the true value of the glow plug resistance.

At least the method above gives a representative value of resistance under something like the operating value of current.

Glow plug electrical test.

Thanks. I have got a new multimeter today, and the plugs read 0.4 ohm resistance which, judging by your comments, looks OK. I have the starter motor out at the moment, but once I get that sorted I'll go through your test and also the other tests of timer / relay etc. as suggested in the workshop manual.