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Electrical guides and mods

Glow Plug Controller Fault

Modification Details


I have had the same email once or twice a year from 450 and 451 CDI owners.

"My glow plug light doesn't go out"

Normally I recommend replacing the glow plugs and checking the engine earth lead. Normally this fixes it.

However, sometimes I get another statement.

"I changed the glow plug controller and now it's working properly"

So, the immediate reaction is that the glow plug controller had failed.

However, it probably hasn't failed because it's not unusual to get the following from the same person a year later.

"My glow plug controller has failed again. This car eats them"

So the next assumption is that either the glow plug controllers have a weakness or the car
is damaging it with excess voltage, a voltage spike, a back-feed or a short circuit to it.

Although that's possible, there does seem to be a weakness that can be fixed.
This fix will save you buying replacement controllers as yours is probably OK.


Thanks to Tone for sending me his "failed" unit for inspection. I had a good glow plug
controller but there's little point looking at a good unit if you are looking for a failure.

Thanks to Tolsen who had already discovered one of the main issues.
I didn't totally understand the post he wrote until I looked at a failed controller myself.

Where Is The Glow Plug Controller?

Open the boot, remove the engine cover. Look to the right of the oil filler cap.
The black box with 2 connectors is the controller...

...and it's bolted to the engine cradle. Remove the bolt, disconnect the electrical connections and take it from the car.

The connectors can be tricky to disconnect. You have to press the tabs in on both sides of the smaller connector.
Don't pull by the wires.

Things To Check

This page will cover the following:

Power fault.
Transistor fault.
Solder fault.

Power Fault

The 1st thing to do is obvious. Check all of your fuses.
Pay close attention to fuse 9 in the 450 fortwo and fuse 10 in the 451 fortwo.

These fuses power the logic in the controller. They do not supply the amperage to the glow plugs.

The main power supply comes directly from the wire that runs from the battery to the alternator.
From the alternator, the permanent live connection goes to pin 11. The largest spade connection on the controller.

Let's open it up to check a few things. Here's the glow plug controller removed from the car.

Dig out the resin covering the Torx10 screws. The resin dries up and cracks anyway so it usually flakes off anyway.


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