Is the glow plug relay necessary for 200tdi?


Well-known member
Can the existing set up in a 2.5 NA ignition switch be used to time heating up the glow plugs manually in a 200tdi conversion? If so, what is the advantage of having the relay?


Well-known member
Relay has a built in timer that glows the plugs for 7-9 sec every start then shuts them off .

You don't need the relay and can use the intermediate position between on and start with the ignition switch and wire them direct and choosing how long you glow them for yourself.

Im unsure when the intermediate glow position was available

If you want to build the harness this will help


Staff member
I wouldn’t add a timer. The simplicity of holding it at the intermediate position only when needed adds to the 200/300Tdi appeal.


Well-known member
I have read & reread about wiring the relay in & it sounds so simple. It's connecting to existing wiring, when I have never had my dash apart or looked at the ignition switch wiring yet, that worries me. I repair HVAC equipment for a living & rewire line & control wiring all the time, so this hesitation doesn't make sense. I think comparing the wiring diagrams between my 1986 NA & the 200tdi with the lack of plugs is making me overthink this. Wish I had one local to look at. I am so relieved to hear that my many hours of anxiety were unnecessary!


NAS-ROW Addict
Callsign: KF7KFZ
I used a Carling momentary switch. Only change I would make is I might want to get one with a cover.

Easy. Cheap. Reliable. And the only factory wiring I used was the single wire that goes from one glow plug to the other along the side of the engine.


Well-known member
No, not really needed.

The relay has a temperature sensitive timer that changes the glow time automatically based on the temperature. It is just a little idiot proof.


Well-known member
The voltage rating is a little higher on the TDI GPs, but not enough to cause any issues.
The advantage of a relay is that the manual GP position has a set of contacts that will eventually burn out requiring you to change the ignition switch.
We have a few heavy duty relays left if you decide you need one.