I've done the reading on Glow Plugs...but my problem is slightly different.. - PeachParts Mercedes-Benz Forum

nathwalter · #1 12-02-2008, 10:30 AM

Hi All,

I have been reading all of the glow plug scenarios the past couple of days but my situation is a bit different. I have a 1991 300D 2.5 Turbo and for probably the past 2 weeks I have the intermittant problem of my glow plug light flickering sometimes and cycling sometimes. It is always a light glow (one that can't be seen in direct sunlight) but never as bright as it gets when I first start up the car.

Here are the facts:

New glow plug relay over the summer and recently replaced it again last week (it wasn't the problem)
New battery within the last week (not a result of the glow plugs draining it, just an old battery), battery tests good and alternator charges like it should
New alternator within the last two months
The glow plug light comes on normally and goes out according to the timer when starting the car from a cold start and also from a warm start
The glow plug light comes on only when the car has come up to normal operating temperatures and usually after about 25 minutes of driving
I am certain that the glow plugs are actually "glowing" because the headlights/dash dim
I have tested according to diesel giant and had continuity the same that he did for each pin in the plug connector so I think all of my glow plugs are good
The glow plugs were replaced over the summer with an extended glow plug (don't know the brand)
The coolant temperature sensor was also changed at this time

Thanks for all your help in advance. I have owned my old Benz for about a year now and with 279,000 on the ticker I smile every time I drive it.

Biodiesel300TD · #2 12-02-2008, 11:17 AM

Might check the grounds behind the instrument cluster. Bad grounds are always a suspect in intermittent electrical problems.

SD Blue · #3 12-02-2008, 11:43 AM

Hmmm.........take some of those keys off your janitor keyring set.

Seriously, I'm thinking ignition switch when you mentioned the dimming of lights when the glow light comes on.

(And yes, excess keys/weight on the switch will create problems.)

Bajaman · #4 12-02-2008, 11:47 AM

This is also what first came to my mind. For some reason the glow timer is getting the signal to cycle. Could be a problem with the ignition switch.

Jeremy5848 · #5 12-02-2008, 12:18 PM

I agree with the theory that the ignition switch is the culprit but, since this is a W124, there might be a problem in the wiring. Once you have eliminated everything else, such as the ignition switch, consider running a temporary separate wire to the glow plug relay. Get an in-line fuse holder from an auto parts store (just in case) and connect it to the battery positive terminal. Run the wire into the passenger compartment (you don't have to be fancy, just route it across the firewall, out of the hood and into the driver's door. Now attach a switch, any kind, just something that you can turn on and off.

Run another wire from the switch back into the engine compartment and attach it to pin #1 of the smaller plug at the glow plug relay. Pin #1 has a pink wire with a red stripe (PK/RD). If you have a spare pin like the plug uses, you can open up the plug and put in the temporary pin. If not, you'll have to temporarily cut the PK/RD wire. Attach your new wire from the switch at the driver's seat to pin #1 of the plug and put the plug back on the relay. Now you can turn the glow plug relay on and off with the switch.

A second thing to do while you're working with the glow plug relay is to temporarily remove pin #2 from the small plug. This pin has a violet wire with a white stripe (VI/WT). Tape it to the side of the cable so it can't touch anything. This is the "afterglow" mod and can be done separately or combined with the separate power to pin #1.

Now you have bypassed the control wires and the ignition switch. Try driving the car like this and see if the problem goes away.

Caveat: my instructions are based on the diagrams that I have for my '87 300D. Yours should be the same, I don't think Mercedes made any changes during the W124 model run except possibly introducing a separate engine temp sensor and built-in afterglow.

You're welcome.
Jeremy