W124/OM603 Afterglow DIY

[Jeremy5848]

Having read the many excellent afterglow threads posted here, I decided to try to modify the preglow system in my 1987 300D Turbo. The idea of using a temperature controlled relay to "cut" the violet wire was attractive: no need to burn up the glow plugs if the engine is already warm. I ended up with a circuit that uses one relay and four pieces of wire. No diodes, no switches on the dash, no serious modifications.

The '87 300D has a switchover valve (Y29) in the transmission control plumbing; its purpose is to cut off turbo boost pressure to the blue flying saucer when the engine is cold (below 50C). This switchover valve is controlled by S25/6, a 50C temperature switch in the water jacket. The same switch can be used to operate a relay and cut the violet wire when the engine is cold, thus leaving the preglow relay "on" for the entire 30 second cycle.


Here is Y29, the transmission vacuum amplifier switchover valve. It lives on the left side of the engine bay next to the blue flying saucer:



This is the 50C temperature switch, on the right side of the engine, down low:



To do the modification, you need a single relay with a 12 Volt coil and a normally closed set of contacts. That's important: when the engine is cold, the coil is energized, the (violet wire) circuit is interrupted, and you have afterglow. The relay can be mounted on the left wheel well next to the preglow relay. Two wires run to the preglow relay and two to the switchover valve. That's all it takes.

First, remove the electrical plug from the switchover valve. To get some slack in its cable, also disconnect the plug from the turbo boost safety valve Y30:



Now you can pull Y29's plug up to where you can get at it. Open up the plastic case and pull out the wires on their two pins. (One pin has two wires, one pin has one wire.) Now solder an extra wire on each pin and put the plug back together. Put Y29 and Y30 plugs back where they belong and dress the two new wires down near the preglow relay.



Next, disconnect the battery negative cable. Then remove the large red power wire from the preglow relay and unplug the small four-pin connector. Now you can pull the cable up to where you can work on it.



Open up the plastic connector and remove the pin with the violet wire. You may have to first remove another pin to get the maximum amount of slack in the violet wire.



Now unsolder the pin from the violet wire. Solder a new wire onto the violet wire and solder another new wire onto the pin you just removed. Then put the connector back together, making sure you get the wires in the right order so the cover will close. The two new wires are to be connected to the normally closed relay contacts; in DIN terminology these are pins 30 and 87a. It doesn't matter which wire goes to which pin. Here is the completed preglow relay plug with the new relay hanging off the leads.



Now bring the new relay down by the preglow relay where (in my car) there is an unused stud to which the relay can be mounted. If your car still has the original alarm horn (mine is long gone), you may have to move it out of the way. Attach the two new wires from the switchover valve's connector to the coil terminals of the new relay. In DIN notation, these are pins 85 and 86. It doesn't matter which wire goes to which pin. Insulate any leftover relay pins (I forgot to do that before taking the pictures).



You may need to rotate the new relay a bit on its mounting stud as it tends to interfere with putting the cover back on the preglow relay.



With that done, you can reconnect the battery negative cable and try it out. A voltmeter attached to any one of the glow plugs will show you that the circuit is working correctly. With the engine cold, the glow plugs will stay on for the full 30 seconds regardless of when you crank the engine; with a warm engine, the preglow relay will switch off the glow plugs as soon as you hit the starter.

Fail-safe: if the 50C temperature switch or the relay fails closed, the preglow system will operate in its original manner – no afterglow. If the switch or relay fails open, you will always have afterglow. Either way, the preglow system will still work so you will be able to start your car.

Edit: The 1985 (only!) 300DT (W123) has a similar setup – 50C temperature switch and all that – and can have afterglow installed the same way. As for other models and years, I don't know.

Jeremy