Glow Plug Timer Repair

The glow plug circuit was the next thing on the list to get fixed so I had a look at the relays under the seat. There are 4 relays in a line and above the one furthest away from the door is a timer relay. After looking at the wiring, I worked out that the second one in from the door is the glow plug relay. (It’s got a thick yellow wire at the relay which is the same as the wire at the glow plugs and it has a black with red stripe wire which also connects to the timer as well).

I took out both the relay (MM431-2820, 59A, Jideco) and the timer (NGK, MM431762, S81NJ) and checked for damage at the plugs and local wiring but nothing seemed obvious. I managed to find an engine manual online for the S3L engine (Get in touch if you want a copy) and under the glow plug section it showed how to test the timer. The timer provides a connection to earth for the glow plug relay coil. So I rigged up the test circuit (see wiring diagram) and gave it a go.

On closing the switch, the lamp glowed for a micro second and then went out. It was supposed to give power to the lamp for approx. 6 seconds so obviously the timer was u/s. Back online I found Diamond Diesels in the UK (They seem to be able to get anything for Mitsubishi engines old or new) and they said they could get me a timer but it would be a special order from Japan and would cost about £140. Too much for me. So, I opened up the timer and had a look at the printed circuit board inside. One of the resistors looked a bit black so I thought it might be the cause of it not working. Online again I found circuit diagrams for 2 similar NGK timers and descriptions about how they work. So, I thought I’ll have ago at repair and ordered a new resistor for the one that looked blackened (I actually had to buy 10 for £1).

I de-soldered the old resistor and put in a new one, tested it and it worked fine giving about 7 seconds of power on. I put this lot back in the machine and at the same time wired in a manual button to activate the relay should the timer burn out again. So, all done and working great. Glow plug light on the monitor panel comes on for a few seconds, goes out and then I turn the key to start and she fires up. Great.

If anyone wants to know how the internals of the timer works then have a look at the circuit diagram below. I couldn’t find the diagram for the S81NJ timer but this is the nearest.

Firstly, transistor Q4 acts as a switch and if voltage is applied via pin C4, D1, R8 and R9 to the base pin of the transistor it will allow current to flow from the relay pin 6A to ground 3D. Thus, providing a ground connection for the glow plug relay, which then turns on the glow plugs.

After a set time Q3 is switched on via Q2 and this grounds the R8/R9 connection and so Q4 drops out and breaks the earth connection for the relay and removes power from the glow plugs. The timer is provided by R1 as it charges C2 which takes 6 seconds (in this case) and once charged Q1 switches off which then switches on Q2 and then Q3.

In my case R8 was replaced and this sorted out the problem. It seems that whenever the engine is running there is current going through this resistor and as it’s only 580 ohms it’s a relatively high current. I used a 1W replacement resistor.