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

[sixto]

Jeremy,

The trans port on the vacuum amp has a line through a green cylinder continuing to the transmission. In your pictures, it looks like a line splits towards the firewall. Do you have a vacuum gauge measuring the vacuum signal to the transmission or do you have magical tranmission control setup you're not sharing?

Thanks,
Sixto
87 300D

[Jeremy5848]

Sixto, you're too good! Can't I get away with anything any more??? Yes, the "extra" vacuum line feeds my "economy" gauge. The 4-gauge panel was 'borrowed' from an '88 300E gasser I found at Pick and Pull; the other three gauges are identical to the diesel versions I removed. The vacuum gauge warns me if my right foot gets too heavy and, should I have shifting problems down the road will help with the diagnosis.

Jeremy

[sixto]

Did you black out premium unleaded fuel? It's a very clean job if you did.

Mine's hooked up to the ALDA.

Sixto
87 300D

[Jeremy5848]

No, I substituted the dial from an older gasser, a 126 I think, that didn't have the "premium unleaded" label.

[redassag00]

NOTE: If you do this vac gauge mod on a W126, the fuel gauge will read 2X high, i.e. 3/4 tank= 1/2 tank, 1/2 tank= EMPTY.

Good write up Jeremy, I wish you did this 2 years ago before I put the 91 SDL Afterglow relay on my car!

[sixto]

Originally Posted by redassag00

NOTE: If you do this vac gauge mod on a W126, the fuel gauge will read 2X high, i.e. 3/4 tank= 1/2 tank, 1/2 tank= EMPTY.

Why? Nothing electrical is modified by applying pressure rather than vacuum to the economy gauge.

Sixto
87 300D

[Jeremy5848]

Maybe he means that the different models (E-class vs S-class) have different electrical properties in the fuel–temp–pressure sections of that 4-gauge panel? My swap was 300E to 300D so there was no problem; the only S-class component in mine is the faceplate, to eliminate the unleaded premium labeling that came with the 300E.

[redassag00]

Originally Posted by sixto

Why? Nothing electrical is modified by applying pressure rather than vacuum to the economy gauge.

Sixto
87 300D

By that, I meant that I took a FUel/Oil/Temp/Vac gauge out of a 560SEL, put it in my SDL cluster, and reclocked the Economy Gauge to act like a boost, and it worked great, until I realized the fuel gauge on the gasser W126 is wired differently, i.e. always reads double the amount of fuel in the tank.

You know what would be kinda cool is if you added a light betweent he SRS and glow plug light to indicate that the afterglow was working and when it shut off.

[babymog]

Are you not happy with the afterglow relay mod?

Originally Posted by redassag00

NOTE: If you do this vac gauge mod on a W126, the fuel gauge will read 2X high, i.e. 3/4 tank= 1/2 tank, 1/2 tank= EMPTY.

Good write up Jeremy, I wish you did this 2 years ago before I put the 91 SDL Afterglow relay on my car!

[redassag00]

Originally Posted by babymog

Are you not happy with the afterglow relay mod?

No, not at all, I'm very happy with it, I just wish that our resourceful friend Jeremy had written this thread earlier, so I could have saved some coin on a 350SD/SDL relay, coolant sensor with double heads etc, over doing this much less expensive mod!

But I dont regret doing it, I basically have an OE setup from a 91 car in the 87. I still have short Glow plug light cycles though. Go figure.

[Jeremy5848]

Originally Posted by redassag00

. . . You know what would be kinda cool is if you added a light betweent he SRS and glow plug light to indicate that the afterglow was working and when it shut off.

I agree. I checked the engine hot and cold with a voltmeter to make sure the new installation was working but it would be nice to have an in-dash indication, like having the glow plug light stay on (without causing confusion, since it does that to indicate some failures).

Maybe a bi-colored LED to indicate "glow plugs still on after engine start" or something like that. I want a clean installation above all. If you have any ideas of what you would like to see, let me know . . . and BTW what is your cute little video all about, anyway?

Hey – look at the photo of the cluster with my post #3. In between the SRS light and the glow plug light is an empty spot. You could put a lamp in there and wire it to any one of the glow plug wires at the connector in the glow plug relay. It would light up in the dash any time the glow plugs are energized (would also notify you of a stuck-on glow plug relay). Let me look into that, would work for W123 cars too. Really neat if I could get a transparency that said

AFTER
GLOW


Jeremy

[Jeremy5848]

Hey, this is gonna be easier than I thought.

First, get an extra idiot light socket for your instrument cluster. You can use either the single wire kind with a metal shell that grounds through the cluster or the insulated kind that has a separate ground wire. The first picture is that second kind because it's all I had in my parts box. Run a wire from any one of the pins in the "big plug" of the glow plug relay. Make sure the wire is well insulated and route it alongside the existing wiring harnesses, then stuff it through a convenient hole in the firewall so that it ends up behind the instrument cluster. Connect the wire to the new light socket and ground the other wire, if you have a two-wire socket. Put in the same bulb as the other idiot lights use and test the light by turning on the ignition switch. The light should glow as long as the glow plugs have power. If you have afterglow, they should stay on for the full 30 seconds or however long it takes your relay to time out.

The light goes in hole #2 of the instrument cluster. In the 124 cars and, AFAIK, the 123 models, there is nothing in that hole from the factory.

Now remove the strip that has the labels for the idiot lights on the right side of the instrument cluster. The right-most one has a glow plug symbol – yes, the one in the picture says "Check Engine" because my parts bin cluster came out of a gasser. Either way, the place in the strip for the #2 bulb has nothing written on the strip. All you have to do is put a label on it so that it says "AFTER GLOW" in sillouette. Any color will do, it will look black anyway.

You can write on the strip with a felt-tip pen but if your writing is as sloppy as mine it will look really bad. If you're good with tiny stick-on letters, you can try that. I plan to experiment with a sign made on my computer and then photographed with a film camera (I still have one). White letters will turn out dark on the film and if I can get the size right, I can cut a piece out of the film and glue it onto the strip that goes into the cluster. If that works out, I'll offer them free to the forum since I have to burn a whole roll of film anyway.

I'll post results in a week or two after I figure out how to scale the sign to get the size right on the film.

Jeremy

[babymog]

That scale is a classic piece.

[redassag00]

zomg!!!!111oneoneone one!

You are sofa king resourceful and fast! I've been wanting to do this for a year!!!!!!

W126 is the same way, space between the SRS and Glow Plug.