Series 3 Glow Plugs / Pre Heater

Image SpacerLand Rover Series 3 Glow Plugs and Pre Heater circuits.

This page gives a detailed and practical work through for the land Rover Series 3 Glow Plugs and Pre Heater circuits changing over from the original series wired to the newer parallel 12 volt type.

This document is still in DRAFT format, or under construction, awaiting further pictures

Original circuit description

The Land Rover Series 3 was originally supplied with a series circuit, from an electrical point of view, meaning like christmas tree lights, one thing after another. If any one bulb fails, the entire circuit fails to operate.

Power from the battery was supplied to the dashboard mounted panel lamp via the ignition switch. From here it ran back through the bulkhead to the engine bay connecting to the Ballast Resistor. From here it transferred to the Glow Plugs, or Pre-heaters mounted on the cylinder head. There are two connections per heater plug, the body of the plug is insulated from the engine, the second wire carry’s the power across to the next plug etc and on the fourth plug is a thick black wire terminating on the engine block that provided the earth return back to the battery, via the engine block, completing the circuit. If any one component fails in this circuit the entire circuit fails to operate and there is no Pre-heat available for starting a cold engine.


Fault finding an Original Land Rover Series 3 Pre Heater Circuit

Where to start

Jump straight in at the middle. The easiest place to begin is on the Ballast Resistor. Its located here, under the metal strip on the bulkhead. Use the sticky up leg that has the thick wire that leads to the Pre Heaters:

Image of location of the Ballast Resistor

Ballast Resistor Location

Place your trusty Volt Meter positive lead on the terminal with the thick wire going to the Glow Plugs on the engine, and ground the other meter wire to the negative  battery terminal. Or better still, use a test lamp and place it where you can see it from the cab. Now switch the ignition on to position One.

There is volts

If you see the battery voltage or the lamp lights, then voltage is getting to this point. That means your ignition switch is working, the dashboard lamp is working and the Ballast Resistor is working. From this position work forwards along each Glow Plug checking each terminal, input/ output for voltage. As the circuit doesn’t work, you will see the full battery voltage at each point, unless something is seriously amiss. At the point where the voltage fails to appear to the next step, that is your faulty Glow Plug. If you get all the way through all the plugs to the last one, and there is still the full battery voltage the problem is likely to be the thick wire that connects from the last Glow Plug output to the engine block. That is the end that is connecting the last Glow Plug output to the engine block that looks like it is, electrically, isn’t. Take it off and replace or clean up the terminal and refit, then recheck.

There is no volts

With no volts appearing to the Ballast Resistor, its going to be something ‘up stream’. The item before the Ballast Resistor is the panel lamp, so regretfully the dash has to come out to get to this. Its only a little bulb and the glass will probably be blackened making it hard to check the filament visually, so use your multi meter resistance range to check this continuity.

If there is still doubt then be very careful to insert your positive probe of your multi meter, other black wire connected to battery negative into the bulb holder. You’ll probably need both hands for this so get someone else to turn the ignition. There is Risk here because there is no fuse’s in the the Glow Plug circuit and its going to be very easy to short the center connector of the bulb holder to the outer metal ring which will cause the Glow Plug circuit to come on if there is no other faults with it. It will spark and get too hot to hold the lamp holder. Obviously if the lamp has gone replace it with another of correct type.

If you see no volts there to the lamp holder, then the next thing before the warning lamp is the ignition switch itself.  The ignition switch provides the current to the Glow Plug dash lamp. As the Glow Plugs demand a large amount of current, the terminals we’re interested in are the larger 3/8 inch ones. Firstly use your Positive lead on your Multimeter, black negative wire grounded, to look for full battery volts on the …..[awaiting picture] terminal. This proves the battery volts are getting to the switch. You’ll need to ‘probe’ the ….[awaiting picture]terminal while turning the ignition switch to position One to confirm the voltage is appearing on this switched terminal.

If there’s still not volts appearing across the ignition switch and there is definitely input voltage AND you’ve confirmed your on the right output terminal, then its likely the ignition switch is burned out with age and isn’t working any more. Purists may wish to fit a new ignition switch  but other people may wish to wire an alternative switch and save all that trouble. For details on how I wired an alternative switch see ‘Steam Punk your ignition circuit’.

Foot note

The only time you’ll see the true voltages on a circuit is when its working. I think somewhere I read the Glow Plugs as originally fitted are rated at 1.2 volts each, so for four of them connected in series, to make them work, we need 4.8 volts. Clearly we have a 12.7 volt battery which means we’re over by 12.7 – 4.8 = 7.9 volts. As the circuit is not working, the resistance of each bit, the dashboard bulb, the Ballast Resistor and the Glow Plugs are not doing their job and so the full circuit voltage appears all the way along the circuit. Remember – there is no current drain in a circuit that is ‘open circuit’.

The dashboard lamp drops some of this voltage, the extra 7.9 volts, that is then fed to the Ballast Resistor. The Ballast Resistor then further drops the voltage and current to something the Pre Heaters can work with. The reason why your dashboard warning lamp is dim is because its designed to work on 12 volts, and really its being asked to work on something around the 7.9 volt mark and that’s why its dim. Its not really working correctly, its just existing on a menial amount of voltage. But it is having to pass all the current the Pre Heaters are demanding, and it really wasn’t designed to do this either.

What is this Ballast Resistor

Here is a picture of the guts of the Ballast Resistor, or whats behind the metal strip.

Image of the Land Rover Ballast Resistor

Land Rover Series 3 Ballast Resistor

That’s exciting isn’t it?! What is it? Its a length of coiled wire, that’s all. But, its a piece of wire that has a resistance, small, tiny even, but its there when there is a large current flowing in the circuit. The wire has to be quite thick to withstand the large current it is going to pass, and when everything is working, if you place your hand above the Ballast Resistor when the circuit is on, you will feel some warmth.

But wait – There’s more!

The wire is mounted by two bolts at each end, and the metal plate is bolted to the vehicle body, and that body is connected to the negative terminal on the battery. Therefore from an electrical perspective, for the current to flow from one terminal to the other and NOT back through the  vehicle body, BOTH TERMINALS MUST BE INSULATED FROM THE METAL BRACKET. On my Land Rover Series 3, both bolts were centered in their holes and clamped there using a hodge podge array of Silicone washers, spacers and clamping nuts to ensure the bolts stayed in the middle of their large holes ensuring they didn’t touch the sides.

Testing the Ballast Resistor

The only test you need to worry about is there is zero ohms between the two terminal legs sticking up from the Ballast Resistor, a shirt circuit, and that there is also no connection between either terminal and the bodywork. If your Pre Heaters are not working but your dashboard lamp has suddenly become much brighter than it used to be, a short to earth here is likely to be the problem, one of the bolts has moved and is touching the sides. If your Pre Heaters are not working, there won’t be a proper circuit and therefore the dash lamp won’t come on at all.

To Ballast Resistor or not

The verbal advice I got from friends was to continue to use the Ballast Resistor but the instructions made no mention of this requirement on the instructions with the new set  so I went ahead without it. The solution I came up with is a little more complicated than it would need to be because I chose to ‘Steam Punk’ my ignition circuit rather than going for a standard Land Rover system. Don’t be side tracked by this as all the information you need to understand for the standard system is presented together with some extra’s that if you follow it you might just like.

Removing the Glow Plugs from a Series 3 Land Rover

The photo below shows the only one of the four Glow Plugs that came out in one piece. The other three broke into pieces.

Image of the Original Land Rover Glow Plug

Original Series Type Land Rover Glow Plug

First things first. Disconnect the battery wiring. Do both sides to be safest. Negative first then positive.

Disconnect the old wiring from each of the Glow Plugs, that’s both leads. If your planning to keep or to continue to use your Ballast Resistor then you’ll use this wire to bring the 12 volt supply to the first Glow Plug in the chain of four.

From the front most cylinder, that’s the one nearest the radiator, remove the heavy Black wire that connects to the engine block. With the new parallel Glow Plugs each plug earths through its body to the engine block so this earth wire is redundant and is no longer needed.

Take your trusty wrench that’s a good fit and unscrew each of the dead Glow Plugs. Keep the unscrewing force at right angles to the plug body. The plug body is hollow so these plugs are prone to fall apart and are certainly far weaker than a similar sized bolt.

Needless to say, only one of my four plugs came out in one piece, and worse than this on the rear most cylinder, the end piece, that’s the little stub on the end with the coil of wire on the end, broke off and stayed in the hole!

Dealing with a snapped off Glow Plug

Firstly, get perspective. The reason it snapped off is because its ‘trapped’ in the hole. Its probably carbon build up that has trapped it and, as you try and pull it out with a long nose plier into a hole that you can’t see on to a stub that you can’t see, and the further you push the long nose pliers into the hole the thicker they get and the less you can get hold of the stub, you’ll seek advice from friends. Perhaps that’s why you’ve arrived here. They will tell you it doesn’t take much time to strip the cylinder head off a Series 3, but now your heading for a top down strip simply because you’re trying to change the Glow Plugs and what started out as an hours work is rapidly becoming a long Saturday under the hood.


Image of complete and broken original Land Rover Series 3 Glow Plug

A complete and broken original Land Rover Series 3 Glow Plug. The nearer item is stub that broke off in the cylinder head

A more than knowledgeable friend said he’d had exactly the same thing happen to him on a van. It took 9 months just driving around as normal before it finally came out with a bang. His comments were that the exploded exhaust stroke is far more powerful than the induction stroke on a Diesel engine and therefore there was every chance it would be blown out before it got sucked in. As long as your willing to understand this risk, the carbon that’s holding it in is unlikely, but its perhaps not impossible for it be an unlucky starting up on the induction stroke of that cylinder, to let the broken bit be sucked in, rather it will hold it there waiting for the expanding gasses to blow it out. I fitted the other three Glow Plugs, and with reason in my head and my heart in my mouth we started her up.


We ran it for 15 minutes and guess what happened?

Absolutely nothing.

Cock sure the next morning she was started up and there was an almighty bang after just a few revolutions of the engine, just like a gun shot, leaving my ears ringing, and a new gaseous sound to the engine note. Yep, that was it, the end had come out. Engine off and in went the fourth Glow Plug. Job done.



3 comments to “Series 3 Glow Plugs / Pre Heater”
  1. Hi, I would just like to add a comment which may be of help. I have been having doubts about my old in series heater plugs for my series 3 diesel. I decided to install the parallel plug option and would mention that the dashboard lamp is in parallel to the ballast resistor so doesn’t form part of the heating circuit. In my case the bulb was not lighting up however I was still getting 9.9 volts at the ballast resistor feeding the plugs when the ignition switch was in the heater position. The conclusion is that there must be an open circuit to the bulb which has checked out to be in working order. I shall be rewiring the bulb circuit.

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