Glow Plug Question...
 

1985 300SD AM I safe that if the Glow plug light is working proper on the dash, that the glow plug system is operating properly?

glow plug
 

you need to post the vehicle thats in question

1985 300 SD Glow Plug Question
 

Am I safe in assuming that if the glow plug light on the dash is on, the glow plug system is working properly.

Not neccesarily, one or more of the glow plugs may have failed and you will still get the light to come on.

The test is to check the resistance at the 6 pin plug mounted in the engine compartment above the drivers side tire.

http://dieselgiant.com/glowplugrepair.htm

that is a quick test kent, but it's not a SURE test of the glow plugs.
the only sure test is to actually remove the GP's and connect them to 12v and visually verify they heat at the tip and along the length of the element to a bright orange hot, and the tip should get nearly white hot.
if the .8ohm test passes, the next step is to verify voltage is reaching the plugs and the cabin light dims when the relay is lit.

No. I'm pretty sure that the diagnostic system hooked to the light compares plug #1 with the other plugs and signals a problem to the light if a discrepancy occurs. If all the plugs are out, there is no discrepancy hence the light would show everything is fine.
Another quick test is to use a non-contact thermometer pointed at the glow plugs to see if they are heating up.

Yeah, but there's something really exciting about holding a tiny piece of metal that is a Bazillion Degrees. got to heat the shop somehow.

X2 vstech !
Unless you can see the glow pattern you cant be sure. In you cold conditions, everything has to be 100% not near enough!!
Pull the plugs & test them on your battery with your jumper leads!!

Has anyone ever used and ammeter to measure the amps that are going through the bundle of GP wires or each GP wire seperately? I've done it some VW's when I was at the diesel shop. It worked pretty well. I've never tried it on the MB. The old VW GPs each pull 20amps. So if you had a bad one you'd read 60ish amps instead of the 80 for a 4 banger. Might be a better way to measure the GP condition without pulling them.

If the glow plug light comes on then off AND the car starts OK then you have a good chance that the system is working. The glow plug light may come on with 1 or 2 plug not working, so it is not a good indication that ALL plugs are working.

This is what I normally do, remove the cover to the GP relay box and unplug the big connector. Use your DVM to measure resistance from each connector pin in the plug to chassis. They should read less than 2 ohm. This check for continuity between the relay to the GP then to chassis. This is an indication that the GP is not open circuit but it may or may not glow.

The only way to check is to remove the GP and connect it to 12V. I would NOT go this route unless you want to do a visual check. It is a PITA to remove GP.

You can also put +12V thru the big connector one pin at a time and measure current drawn. I have never done it myself and I do not think you gain anything by doing it. The resistance test is good enough for me.

I have a weird issue with a 83 300SD. I believe others have had this happen but I have not seen a solution.

Replaced all glow plugs (new Bosch) as well as the new upgraded glow plug relay.

When you put the ignition to the glow position the light does NOT light, count to 5 or so and it starts no issue, in recent cold weather (-10C) I have has no issue starting the car by waiting about 25 seconds - and no need to floor the gas etc as the manual indicates may be necessary - so the glow plugs are functioning well. In addition I can notice the lights get brighter when driving after the extended glow cycle completes.

The real weirdness is that if you turn the ignition on wait 5 or 10 sec and then cycle the ignition again now you do get the glow light to light- full bright and it goes out after a relatively normal light cycle?

So the bulb obviously works and the plugs work, why would the dash light light come on only after a second switch cycle?

I'm just living with it for now as everything seems to be functioning - cold start etc.

Paul

Wow- that is weird indeed. The only thing I can think of is the ignition switch. Have you tried wiggling the key around while waiting for GP light on first try to see if it lights? I am reaching here but was about to tear out the electrical system on the heater blower that would work and then not work. Found the ignition tumbler was worn to the point that it would rest on the dead spot between run and start. If I jiggle the key, the heater blower would work....

Other than being annoying (no Light) sounds like your system is working.

Reviving a thread to add my own experiences. Car (87 300D) getting hard to start when cold. Pulled the relay cover. Measured 12.5VDC at each pin. Measured resistance:

G1- 140K ohms
G2- 51.1 ohms
G3- 1.0 ohms
G4- 4.86K ohms
G5- 1.0 ohms
G6- 18.5 ohms

It would appear that only two GPs MIGHT be good. :rolleyes:
Glow plug light on the dash lights up...sometimes...often every OTHER turn of the key. When it does light, is stays on approximately 5 seconds. I'm starting to believe the light means nothing other than the key is in the correct position and the bulb is good. :D

-Louis

With my 300D, I got normal glow plug light operation with one bad plug (#3 I think) and only noticed something was up when my car was hard to start one cold morning in Grand Junction. When I bought the car, I had two bad plugs (don't remember which ones), and the light would come on briefly and then go out. The car started with difficulty when cold. Since I got the car out of storage last month, the glow plug light doesn't come on at all. I vaguely remember a thread somewhere that said if the #1 plug, and only the #1 plug is out, that the light won't come on. The car starts normally in warmer weather, and with just enough hesitation in cooler weather to make me believe I may have one glow plug out. However, the car has other issues (vacuum, no dash lights, blower works intermittently) and I am lazy, so I haven't checked it out.