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How to fit a manual Preheater circuit


On a diesel engine, the electrics are not very complicated. There's a preheater circuit (to warm up the preheater elements) and there's a starter motor circuit. Unfortunately, as with the loss of starting handle on modern cars, there's been a loss in the hands-on control of engines, and in the diesel that has meant that instead of the driver being able to make a conscious choice of how much preheat to give the engine, a key switch and circuit tries to ape the ignition system of petrol/gasoline powered cars. Well, we don't need to have that!

Instead, how about fitting a real manual preheat control, so you can decide by your own judicious feel how much preheat to put into the engine, and how much of your battery you're going to gamble? Then, instead of turning a key, it's much more stylish to have a Start pushbutton to press, don't you reckon?

The downside of car ignition key style preheat systems, typically found in diesel vans, is that the automated system rarely has a clue how much preheat to apply, and it makes a poor guess, which in cold weather doesn't start the engine, and in hot weather is a waste of preheat.

Instead, with a manual preheat, you decide. You choose, and you soon get the feeling of exactly how much preheat to apply. Your own experience tells you, and you know what you're doing.

(In case you're wondering what those things are that look like spark plugs on a diesel engine, they are the preheaters. They are only used for starting from cold, as diesel fuel won't burn at room temperature if you set light to it. Preheaters are electric elements that get hot enough to boil diesel fuel when the engine itself is cold)

How to fit a Manual Preheat Circuit:

First, find where the cable goes to the actual preheaters. It's usually quite a fat cable as the preheaters are at least forty watts each, which means there's double-figure amps going through the cable.

Next, get a nice chunky relay. It should be a 12 volt coil (if it's a 12 volt battery system, that is), with a set of switch contacts that's easily able to cope with the amount of current that goes through the preheaters.

The relay contacts should be normally-open, and when the relay is energised, the circuit is made, so as to allow the preheaters to be powered. So in effect, the relay contacts take the place of whatever was the switch before.

Next, the coil can be connected up so it can be powered through a pushbutton switch on the dashboard. The advantage of using a relay is that the switch can be any neat switch you'd care to fit, as it only needs to be able to carry enough current to power the relay coil.

In the Leyland Redline I had a GAMBLE pushbutton from a slot machine as the preheater switch. This had symbolic advantages, one of which was that it was always a matter of feeling how much to gamble on the preheat. The other thing was that the pushbutton switch had a 12 volt lamp inside, which could be powered from the preheat circuit, so it was possible to verify that the preheaters were being powered.

The START pushbutton, also good philosophically, has an easy implementation too. Fortunately, the current required to run the starter motor does not need to go through the dashboard. This is one of the reasons why control consoles generally do not catch fire and have to be extinguished in an old sci-fi style. Instead, the starter motor has its own relay inbuilt, and the current required for that is more reasonable. Even so, you may decide to put a further relay in, depending on your choice of starter pushbutton.

It's important in such circuits to keep the current through the switch down to a sensible level so as not to overload the switch. Also, make sure that whatever control switch you use, it's got to be one that turns off if you let go. This avoids various types of fire-related embarrassment. Spring-loaded pushbuttons are good, and they're available from Maplins as well as from various places that have slot-machines being decommissioned.

To sum it up: Get a nice chunky relay and use the relay contacts as the switch through which your preheaters are powered. Then use a neat switch on the dash to power the relay coil.

Other note: If a diesel engine has been running a while and it stalls, it's possible to restart it with no preheat. With a manual preheat system, you decide, and experience tells you how much preheat to give it, and when to avoid wasting any energy on unnecessary preheat. In contrast the ridiculous "let's pretend to be a petrol engine" ignition key systems force a preheat regardless of such things.

It has been suggested that a manual/automatic switch be added so that experienced drivers can use the manual preheat but novices can still use a keyswitch until they can learn the better ways of manual preheat.


Oops! Almost forgot: The advantage of having a key is the idea that it gives some security. If you want to have security even after you've replaced the key with a pushbutton, here's a sensible alternative: Connect up a hidden toggle switch in series with your start pushbutton, and fit it somewhere out of sight. The result is less easy for a thief to hotwire. It's safer than a key which is often too easy.


Fitting a preheater CIRCUIT does not involve fitting the preheaters themselves. They are screwed into the engine. However, if you would like to know how to fit preheaters, here's how:

How to fit Preheaters:

Carefully undo the electrical connections. Then, using the right-size spanner, carefully turn the preheater anticlockwise. Be careful not to break the ceramic part. When you've got the old preheater out, you can then insert a new preheater into the hole (with a bit of grease on the thread), and screw it in all the way. Then give it an extra tightening to a reasonable torque. It needs to be tight, because the engine involves high pressure, but it should not be so tight that you can't undo it again. Then, reconnect the electrical connections, making sure the contacts are good. There! Done! You've fitted your preheaters.