Are W123 and W115 glow relays interchangeable? I feel as though I've asked this question before, and was told they were not. Thanks.

It depends on of the glow plugs in the cylinder head are the filament/loop type glow plugs are not.

The filament/loop type glow plugs if there is a really it is in an aluminum box behind the instrument cluster and works its own way.

The way the filament/loop type glow plugs are wired reduces the voltage to them and in fact trying to test a single glow plug at batter voltage burns the filament/look like immediately.
IF you W115 chassis diesel engine has been upgraded to the adapter pencil type glow plugs the stock aluminum box relay may or may not work.

You can use a later say an 84 300D fender mounted glow plug relay with the pencil adapter glow plugs.

With the pencil glow plugs there is 2 relays for on the fender well. One is I believe a Schuler or some company that starts with an S that has 3 mounting legs on it and the typical Bosch one as is on my 84 300D

Not sure on the wiring. The fender mounted relays (pencil glow plugs only) have a wire that feeds it from the Ignition Switch. When you turn to crank the Sarter there is also another wire that comes up from the Starter Solenoid and or a junction block that keeps the glow plug relay on during cranking.

The Schuler fender relay and the aluminum box relays if available cost over $250 and that was like 5 years ago. I don't know if any cheap aftermarket companies ever make them.

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84 300D, 82 Volvo 244Gl Diesel

Attached pic of an adapter pencil glow plug. And they are wired differently compared to the filament/loop type glow plugs.

The hex head is 13/16 (21.5mm)

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84 300D, 82 Volvo 244Gl Diesel

Some may think me a butcher, but I bought Kent’s simplified basic relay kit to replace the stock, elaborate timer relay. Works great.

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1986 300SDL, 362K
1984 300D, 138K

I think a lot of the Mercedes purist will be modifying their thoughts as real and even aftermarket parts become too expensive and in fact unavailable.

Did yours have the aluminum box relay behind the instrument cluster?

Below is a pic of the kit. The solenoid in the kit will likely outlast the car.

Someone using the kit needs to understand that there is no timer. They simply need to turn the key and keep it in the pre-glow position as long as the feel is needed before turning the key to crank. (Or is there some why to use the timer feature if the old relay is still working?)

My volve diesel had an aluminum box glow plug regulator under the hood and when it went bad, I went to the junkyard and got a used ford type starter solenoid for $5 and replaced the relay with that and a spring-loaded toggle switch.
The ford type starter solenoid is designed to handle the amperage of a starter motor, so glow plugs are not going to stress it at all.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the replies. Looks like I have some more research to do, but the input is very helpful.

People like me have no idea what type of glow plugs are in your engine as I have no idea when they started putting in the penciled type of glow plugs.

This is what is on my Volvo diesel see attached pics. Note my Volvo is a 6-cylinder diesel.

The fist pic is a ford type starter solenoid like in the 1970-1980s.

The drawing is how it is wired. I put a switch inside, and you simply hold the switch down as long as you want the glow plugs to heat.

It does not have a timer nor the feature that allows the glow plugs to be on during cranking. However, the glow plugs being on during cranking could easily be added by running a wire from the starter solenoid (the terminal that the ignition switch wire connects to on the starter solenoid).

In the drawing that would be a wire from that terminal on the starter solenoid to the S terminal on the solenoid I am using as a glow plug relay.

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84 300D, 82 Volvo 244Gl Diesel

The attached picture is from a 617 engine. Note "White Wire to starter solenoid." I'm thinking that this is what you are referring to...yes or...?

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Where did that come from? Mine is much more basic, no glow plugs included. I replaced those a couple of years ago, also reamed out the cavities, started so much better.

The only weak part is the little push button that came with the kit. I put in the bottom edge of the instrument panel trim. It’s not a high-quality item, it doesn’t have to be. I need to buy a spare and keep it safely wrapped up somewhere. I could always get by in a pinch by just holding the wires together, but that might be a drag.

I don’t recall any aluminum box relay. Do you mean standard equipment?

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1986 300SDL, 362K
1984 300D, 138K

If your car came with that element/indicator that is viewed the circular grill with round holes that is nick named a saltshaker there is no glow plug relay.

Other Mercedes with a diesel engine would have come equipped with a glow plug relay.

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84 300D, 82 Volvo 244Gl Diesel