(Home) Glow plugs

Many owners have experienced problems with their glow plug systems, sometimes resulting from incorrect reconnections by garage mechanics following major remedial work to their engines. To be fair, the myriad of wires and pipes in this area can be a little daunting even for so called 'experts'. Here is a discussion of the glow plug circuit in a (right-hand drive) Townace circa 1990 with 2CT engine.

The system is easily accessible on the passenger side of the engine. After the starter motor, it is the system which draws the second-highest current from the battery so the some of its wiring is very thick, it is protected by larger-than-normal fuses and requires a little extra care.

In case you're new to Diesels, there is a glow plug for each cylinder and from the top they look a little bit like spark plugs. However, instead of individual wires going to each one their top terminals are joined together by a flat metal strip or "bridge" through which they are all energised at the same time. Don't confuse them with the fuel injectors though, also one per cylnder, joined together by a round metal pipe to collect surplus fuel plus an individual pipe to each injector from the injector pump.

The business end of glow plugs is a black (carbon?) cylinder about 15mm long and 5mm diameter which, as the name suggests, "glows" red hot when energised. Glow plug failures are rare. Getting them out is quite a job but if/when you have one out you can test it by connecting it briefly to a car battery. Use wire suitable for the 10 Amps or so which it should draw, and be careful not to short-circuit your battery. The tip should glow red within about 5 seconds but the body remains cool and there is no risk of shock so this test itself is not difficult. In use only about 10V reaches the glow plugs so do not leave one connected direct to your battery for more than about 10 seconds.

The glow plugs are not simply switched on or off. They are controlled by the Pre-heating Timer unit which is in the door pillar behind the front passenger's left shoulder. This gets an input when the driver turns the "ignition" key "on". Immediately the glow plugs are fully powered via No. 1 Glow Plug Relay (on the bulkhead above the fuel filter) and the glow plug symbol lights up on the dashboard. After a delay controlled by the Timer's input from its own Water (/coolant) Temperature Sensor, the glow plug symbol disappears from the dashboard and then later there is in some 'Aces a warning buzzer too, to get the driver to start the engine. Once started the Pre-heating Timer unit must get an input from the alternator or somewhere and it keeps the glow plugs at "half" power, via No. 2 Glow Plug Relay and the Glow Plug Resistor, for a further temperature-dependent period around a minute to reduce the risk of stalling the cold engine.

When tracing faults in the circuit, you should refer to the RM025E Engine Repair manual, available from Toyota dealerships for less than £10 and well worth it. But remember that manual is not specifically for Townaces.

Click here for Dave's wiring schematic showing the glow plug wiring. (Click on the image to enlarge it). This shows where the Townace appears to differ from RM025E.

There are 3 terminal posts on the inlet manifold ...
1. The first terminal post is the earth at the clamp that holds all four injector pipes.
The other two terminal posts have insulating washers and sleeves so that the terminations don't connect to earth/the surrounding metalwork ...
2. The second one is the vertical connection post which has a metal bar linking across to the top of the nearest glow plug.
3. The third one sticks out sideways away from the engine and is complicated by having a resistor sandwiched between the connections.

There is one completely removable wiring component - a Y-shaped cable link about 100mm long. The base of the Y has an eyelet which goes on the vertical connection post. The push-on connector at the end of one branch of the Y is for sensing the voltage on the glow plugs - its mate is on a branch of the loom that also goes to the injector pump and the EGR valve. At the end of the other branch (#) of the Y is a large eyelet which goes on the third terminal referred to above, at the end of the resistor nearest the engine.

The branch of the loom which supplies the glow plugs has two thick wires with eyelets which go either side of the resistor on the third terminal - the eyelets are different sizes and can only go on in one order. The larger one goes on first, nearer the engine. That branch of the loom also has two thin wires going into an eyelet that goes on to the earth terminal. Those wires are white and black.

Click here for Ian's close up photo showing the Glow Plug Resistor (right of centre), the (vertical) connection post (top left), the "glow plug resistor to connection post (#)" wire connection in the place of the Glow Plug Current Sensor referred to in RM025E, and three other connecting wires.

Be aware that except in really cold weather the engine will start farly well without any glow plugs working! This means that the system might be re-wired incorrectly in the spring and apparently unrelated bad starting symptoms appear 8 months later. To test all is well, connect a voltmeter between the top of one of the plugs and a good earth. The glow plug circuit is working correctly if, when you first switch on, you get a reading of about 10.5V which after a few seconds drops to about 7V, which coincides with the glow plug light on the dash going out.

The common problem areas appear to be:
1. A short causing a 80A fuse/fusible link to blow.
2. Incorrect fitting of wires to the glow plug resistor.
3. Incorrect fitting of wires or insulation to the vertical connection post and onward connection to the glow plug connector or 'bridge'.
4. Failure of glow plug(s) or No 1 glow plug relay.

The following descriptions may help when trying to overcome the above:

1. As you can imagine it takes a lot of current to blow one of these fuses so find the problem before simply replacing the fuse. They are not cheap, typically between £2 - 5 each! The fuse for the glow plug circuit is located in a rectangular plastic box, anchored on the underside of the bulkhead and accessed by the first of the two removable inspection covers. The box is secured to the bulkhead by 2 x 10mm bolts on slotted holes. After disconnecting the earth connection on the battery, loosen the bolts and pull the box away. Unclip the top and sides of the box to reveal a row of cross headed screws on both sides, holding down the appropriate wires. The centre black fuse is for the glow plug circuit. (Note the screws are of different size to prevent incorrect reassembly). Remove the two screws and wires, one from either side of the box and the fuse will then slide out. Use the old fuse as a pattern when obtaining a replacement as some 'universal' type fuses do not fit correctly. Refit in the reverse order of removal.

2. The glow plug resistor is located between the fuel pump and the EGR unit. Refer to the engine manual RM025E for details of how to test but note that it's value is only a fraction of an ohm so it looks almost like a short circuit. There are three wires connected to the resistor. Two wires with large connection rings fitted first at the back of the stud. One of these wires connects to the No 1 glow plug relay and the other to the vertical connector post on the manifold (this wire is referred to in the manual as the glow plug current sensor). The resistor is followed by the silver coloured ring, then the smaller connector (which wire goes to No 2 glow plug relay) and all is held in place by a 10mm nut.

3. The vertical connection post (sorry I have yet to see the correct terminology for this in any Toyota documentation) on top of the inlet manifold is there simply to connect the wires from the glow plug resistor and pre-heating timer to the glow plugs via the rigid bar which Toyota refer to as a connector but is sometimes known as a bridge. I suspect that the reason for the rigid nature of the fabrication is to ensure that the electrical connections and components are made solid so as not to come into contact with earth during the vigorous shaking that the engine undergoes. Essentially you have to ensure that all three connections that sit on the post make a good contact with each other but NOT earthed to the post itself. Preventing these connectors from earthing on the engine appears to be a common source of failure. With mine I fabricated an insulating sleeve from some neoprene tubing and made a thick fibre washer between the base of the post and the 3 connectors. It should be also realised that this area gets very hot so if you do have to fabricate new parts, ensure they can withstand temperatures of about 100oC. Ensure you refit the plastic cover that fits on top of the post and each of the four plugs so as to prevent any metal component touching the connectors and shorting them. Further problems can arise if the connection from the post to the bridge or the bridge itself has had its outer insulation damaged to reveal bare metal, which again can permit a short if it's distorted or badly assembled.

4. Finally the glow plugs themselves can fail and cause a short. If you suspect the plugs or the No1 relay to be at fault, again refer to the RM025E manual and refer to the procedure for testing them (I don't propose to repeat the procedures here). A new relay will cost about £100 and glow plugs vary but can be between £10-20. So a wrong diagnosis can be expensive!

- Ian Dunse and Dave Mason, December 2004.