'95 E300D Intermittent Glow Plug Dash Light
 

Recently I have started experiencing intermittent functioning of the dashboard glow plug light on my '95 E300D. It may work for several starts, then fail to light for the next couple, or alternate between lighting and not lighting. I am trying to determine if this could be caused by the glow plug relay, indicating it needs replacement, or if it may simply be a wonky bulb. I haven't yet pulled the instrument panel to check the bulb. I have not had any issues starting the car when the bulb does not light (tho I'll wait for a couple of seconds before starting if the light doesn't come on), but the weather hasn't been wicked cold yet. Has anyone experienced this issue?

Note that all the glow plugs were replaced with new earlier this year and the engine wiring harness was replaced last year.

Tom
Closter, NJ

I have had bad alternator brushes cause this , one brush badly worn and hardly making contact.
In my case I had sometimes only the main battery and abs bulb working.

It comes and goes depending on brush contact when engine stopped and a telltale is the interior lights can appear to flicker at tick over.

Hey Old Barn Guy my GP light was acting up similar to how you describe yours a few weeks ago. I did nothing and waited a few weeks. Then the lamp would not turn on till after the engine started so then I knew the problem.

If everything is operating corectly the glow light indicates pre glow time only. A flashing glow lamp indicates a short and if the glow light comes on after the engine has started then at least one plug is not working. I then used a ohm meter to find the faulty GP and managed to change it without removing the intake manifold.The bad GP had a high resistance maybe it was in transition for a while when the lamp was acting strangely?

Small possibility it could be the temp sensor. The temperature sensor for the 60x glow plug is midway down closer to the front of the engine on the passenger side. I used to have the temp vs time graph but can't locate it basically at -20 the plugs glow for max time 30 seconds (?) and after glow is I think 90 sec. As the engine water jacket temp rises the glow time is reduced as is the after glow time.

'95 E300D Intermittent Glow Plug Dash Light
 

Thanks for the responses to my dash light issue.

Murkybenz: No other electrical issues that I can see. All other dash lights come on as normal and there is no visible flickering of any lights.

Dieselbenz1: The glow plug light doesn't light up at all when it is not working. It doesn't flash and it doesn't come on after the car has started. It acted like that until my recent change-out of all the glow plugs--that was an adventure! Then the bulb worked as normal. Those times it does work now, it lights up when the key is turned on, and stays on until the engine starts, then goes out immediately. The temp sensor sounds intriguing: if the sensor is not working, would that cause the bulb to not light at all?

For couple of weeks i suppose when my GP started to fail my GP light sometimes would not illuminate pre and post startup. That is not normal and is not described anywhere in the service manuals.

I think a short circuit or very low resistance on the temperature sensor may cause the lamp not to come on. The colder it is the higher the resistance for that sensor.

A different year and model. I had my Glow Plug light stop going on at all. But, the Engine started normally and ran normally once it started. So I know that the Glow Plugs were working OK.

I checked the resistance of the Plugs at the Glow Plug Wire connectors and found no issues. So I just drove it that way. (A basic place to test when ever your Glow Plug Light is acting oddly.)

Not quite a year later the Glow Plug Light started working normally with out any intervention on my part and it has been working OK for 6 months. (No temp sensor on the Engine. Note that if you had a temp sensor sometimes the connections need to be checked.)

As long as it started normally I do not care about the Glow Plug Light. The hardest part of the issue was convincing my Wife not to worry about the Glow Plug Light not going on.

For 602's, the documented GP lamp codes are:

Fault indication by preglow indicator lamp

- Lamp fails to light up even during preglow,
lights up for approx.1 minute while dnving.
Fault: One or more glow plugs defective.

- Lamp fails to light up during preglowing and
also while driving.
Fault: Indicator lamp defective, line
interruption to the indicator lamp, preglow
time relay defective.

-Lamp lights up permanently.
Fault: Preglow time relay defective (relay
sticking).

- Lamp fails to light up, engine is hard to start
or fails to start altogether.
Fault: Short circuit on one or more glow
plugs, line interruption, preglow time relay
defective.

The way the system works is that the current draw of the plugs is checked by circuitry in the relay. The expected problem is that one or more plugs go open, and that will cause one of the standard signals documented in the FSM. I think it's possible for something odd, like a partially shorted plug, to cause an undocumented signal. So my suggestion is to begin by checking the resistance of each plug. Replace any plug that's far from the average reading. Recent new plugs mean nothing. These days, even new parts out of the box are suspect.

I'm sorry to add that it could be going the way my '95 E300 did with totally decomposed glow plug wiring harness.

That's how mine started. Now I just plug in the block heater everywhere I go (mostly to work with it) when it is cold out.

'95 E300D Intermittent Glow Plug Dash Light
 

Diesel911: Don't you hate it when the car mysteriously heals itself without any action on your part? I'll definitely check the connection on the temp sensor.

I understand the wife thing: After convincing mine to wait for the glow plug light to go out; now she has to count to ten before firing it up. Her response: "Is this car safe to drive? Am I going to get stuck somewhere?"

Mxfrank: Thanks for the error codes. I'll need to determine if the codes for the 606 engines are the same, but the second code you offered seems to fit. Is it possible to test the pre-glow time relay, short of swapping out with a known good one.?

dtf: Yes, the dreaded biodegradable wiring insulation. My engine harness had deteriorated to such an extent I had no choice but to replace it and eventually tracked down a new, upgraded harness about a year or so ago, resolving that issue.

Bad glow plug(s) or bad wiring harness (or an issue with your work replacing the glow plugs, like a poorly connected harness at one or more glow plugs). It's also possible that the pre-glow relay is going bad, but that's not a very common issue.

Resistance check of the plugs is a quick-n-dirty test that will usually find a bad plug, but not always.
A better test is current, which should jump to about 15 amps on a cold plug, and then drop off to around 8 amps as the plug warms up. Get an old school amp meter gauge (meant to be mounted in the dash) with a range of 30 amps or so, use some heavy copper wire like house wire, and put 12v to each plug, one at a time, at the connector to the pre-glow relay. Testing at this point (connector at the relay end of the harness) checks both the health of the plug AND the condition of the harness. Any position on the harness connector that is different from the rest is suspect. This test takes about 10-15 minutes from hood up to hood down, but you will need to spend $20-$30 on a amp meter / gauge.

'95 E300D Intermittent Glow Plug Dash Light
 

A brief update to follow-up: Since my 12/2 post, the glow plug dash light in my E300D has started working normally again without my doing anything, so I have not tested any of the suggestions provided. I will say that there seemed to be no apparent difference between startups when the glow plug light was working and when it was not. In each case, the car started easily, without vibration or roughness, and without the presence of any exhaust smoke. Should the dash bulb stop working again, I will run through some of the suggestions and report back to the forum.

Thanks to all who responded to my questions. Sorry I could not provide a more definitive response.