95 E300D glow plug light problems - PeachParts Mercedes-Benz Forum

Paul64 · #1 11-16-2011, 11:45 AM

I have a problem with the glowplug system on my '95 300D too in as much as when starting, the glowplug light stays on for an age though if I start the engine after say 5 secs from cold, it fires perfectly. The light then goes out and doesn't return. Same when the engine is hot - lamp stays on but engine starts perfectly after a few seconds pre-heat.
I checked the glowplugs to earth and all have 1 ohm reading which I think is correct so, I checked the resistance on the water temperature sensor at the relay end of the cable.
At 68C, it was 325 ohms and at 20C, it was about 200.

I guess this indicates that the sensor is dodgy and needs replacement however, as this is reading lower than usual when the engine is cold, wouldn't that fool the relay into turning the glowplugs off more quickly and thus the light would go out too soon? How would this affect the afterglow??

It's my first diesel and loving the 'relaxed' and 'soothing' performance but this has me a little baffled and I don't want to take the inlet manifold to bits, replace the sensor only to find the same problem....!!

Any ideas guys???

Thanks

Paul

Jim H · #2 11-16-2011, 08:36 PM

Quote:

Originally Posted by Paul64

I checked the glowplugs to earth and all have 1 ohm reading which I think is correct...

Yes, 1 ohm or so indicates that the plug may be OK. Only removing them, applying 12V and observing a strong white-yellow glow will prove them OK.

Quote:

Originally Posted by Paul64

...so, I checked the resistance on the water temperature sensor at the relay end of the cable.
At 68C, it was 325 ohms and at 20C, it was about 200.

I guess this indicates that the sensor is dodgy and needs replacement however, as this is reading lower than usual when the engine is cold, wouldn't that fool the relay into turning the glowplugs off more quickly and thus the light would go out too soon? How would this affect the afterglow??

This thread agrees with your assessment, where MBDoc states that most Bosch temperature sensors should read around 250-350ohms HOT and 2500 ohms COLD. So, if the GP relay thinks the engine is HOT it could be fooled into adjusting the light and afterglow functions accordingly. The next time your engine is cold you could place a 2000 ohm resistor inline using alligator clips or some such arrangement, and see if the glow cycle then behaves as you expect for a cold start.

That all said, the lamp does not necessarily stay on for the full duration of the GP cycle, it seems to be more of an indicator that the system is functioning normally, or that there is some problem. These are just my guesses, so I hope others will add their knowledge to this soon.

Paul64 · #3 11-17-2011, 03:14 AM

Hi Jim,

Thanks for the pointer - I'll give the resistor a go and see what happens.
Do you have a part number for the sensor? I couldn't find it on the internet as a listed spare.....

Paul

connerm · #4 11-17-2011, 08:10 AM

Classic sign of a deteriorated wiring harness. Search this forum.

Paul64 · #5 11-17-2011, 10:28 AM

I think I'm going to take the inlet manifold off this weekend and check it all out - I had to replace the harness top and bottom on the 500 so am used to it now....!

Thanks for that hint - more time bent over the engine bay coming up.

Paul

stcbenz83 · #6 11-17-2011, 11:00 PM

couldn't this be a relay issue? I'm sorta having the same issue as you are with my e300d. I'm confused to what sensor your talking about?

Paul64 · #7 11-18-2011, 08:44 AM

Hi,

This is the sensor - a temperature monitor - that measures the temperature of the engine prior to each start. It sends info to the relay which decides, depending on how warm or cold the engine is, how long the pre-heating runs for prior to your turning the key to start from the pre-heat position.
It's located under the intake manifold at the rear of the cylinder head on the left hand side from the driver's seat.

There is a link on Mercedes Benz Model 124 - OM606 Maintenance Manuals which gives you the readings you should have when testing the sensor with a multi-meter.
Not sure what the part number is for this part - does anyone have a reference?

Regards

Paul

stcbenz83 · #8 11-18-2011, 06:04 PM

From what that online manual says (which is great BTW) I think this is the part your thinking of

FAE Water Temp. Sensor,W0133-1628922 - PeachParts

kinda Ironic since i thought I originally needed one but later found out that the sensor I was referring to was for the climate control... turns out I probably could use one of those anyhow. from what the manual says anyways