Glow plug light
 

I'm sorry if this question has been previously answered, I looked through the archives (I admit not every single one) but I couldn't find this particular problem.
I bought a 1984 300SD last week and everything seems to be fine except for the glow plug light in the instrument cluster. It doesn't come on when I turn the key for preglow. If I leave the key on and wait for the glow plug relay to click off then turn the key off and back on again, the glow plug light works as normal. It stays on for some seconds then turns off. If I wait for a few minutes and try again the light does not come on. So far the car starts very well, but its the middle of summer in Los Angeles.

Thanks for the help. Also thank you to all the people who answer all these questions. Sometimes I feel guilty that I get all this help for free. Its as if I have a Factory Service Manual that can talk back when you ask it a question.

Gilbert

GP + Multimeter........
 

Looks like no one has an answer to your question Gilbert!

If I understand your question, your glow light doesn't come on at first key, but does on the second key?

You may confirm if it is, or isn't coming on with a multimeter. Connect the positive lead of the meter to any one of the glow plug wires, the other to ground of course, and see if you get voltage upon the first key!

That should tell you something!

Good luck with that, and welcome the forum..................Sooty

Thanks for replying to my question sooty. The glow plugs do get voltage on the first key. I checked the resistance of the plugs at the glow plug relay connector as recommended by Gilly and number 2 and 3 had no resistance. I then pulled plug 2 and tested it by connecting a positive wire to the lead and a negative wire to the body as recommended by LarryBible. The glow plug got red hot right away, so I don't understand the low resistance reading. Tonight I guess I'll try to get plug 3 out and see if it heats up also.

Again thanks for your help sooty,
Gilbert

Glow plugs should have low resistance, this allows a lot of current to flow, heating it up.
If there are some with a high resistance, replace them, as they will not get hot enough.

Similar Problem
 

I have a similar problem with my 81 300SD w126. Sometimes the light comes on, sometimes it doesn't. THe glowplugs get power every time, stays on every time for the long period and you can hear it click off after awhile.
Most of the time the light never comes on. Is it ambient temperature dependent?

Thanks!

In contrast my 81 SD glow light would come on sometimes. I didn't check voltages but when the light didn't come on, it took a lot more cranking to start and a lot longer for the idle to stabilize. I figured it was a bad relay and didn't do anything with it until I sold the car.

> I checked the resistance of the plugs at the glow plug relay
> connector as recommended by Gilly and number 2 and 3 had no
> resistance.

What does no resistance mean? A short across the glow plug? A plug won't glow if the + and - terminals are shorted.

Sixto
95 S420
87 300SDL

gbrebes,

What type of ohmeter are you using?

The GP that you say have no resistance may be the good ones. Good GP have about 0.7 ohms cold resistance so if you don't have a sensitive meter, you may think they have 0 ohms resistance which is a short circuit.. The usual GP failure mode is to have infinite resistance (open circuit like burned out light bulb). If a GP is shorted, the fuse in the GP relay would melt and open the circuit before the wires start burning.

If the other GP read higher than 1 ohm, test them by connecting wires to them as you did the first one.. If any don't glow orange hot in about the same time as the others , that GP is NG and needs to be replaced. You might get some new GP first so you don't have to put old ones in and out again. You should always carry a spare anyway.

Hint: Just loosen the terminal nut on the GP, don't remove it until the GP is out. That way you won't lose the nut and its easier to R&R when the GP is out of the head. I always wondered why new GP don't come with a new nut. I use a combination wrench and tie a cord to the box end so if I drop the wrench, I don't lose it. And sometimes pulling on the string is an easy way to turn the wrench in tight places.

Do a search on cabin light.

P E H

I have had glow plug light problems in the past. Sometimes you would get the light, sometimes you would not. Sometimes you would have to cycle the key several times before you would get the light.

It always turned out to be the same thing.

One, or more than one, glow plug was not working properly. Change to a brand new glow plug and the dash light functions as designed.

BTW, one bad glow plug and the 617 will give you fits trying to start it when the temps get down below 25 F. or so.

Oddly enough, I developed a GP problem this week. No light, relay fuse blown. I thought I'd seen the GP light not being on for very long lately, not sure yet if that's an actual issue.
Is the light on for three seconds or so on a warm day normal?
Replaced fuse and it's functioning again, GP resistance all around .8 ohms... Original GP's too..

Problem Found!
 

Pulled all my glow plugs today, since the dash light was funky and I was having severe hard start problems.
The brand new set of glow plugs that the P.O. had put in the car (less than 50 miles) were already bad. Autolite...crap brand it seems.
4 of the 5 were not working...going to head up to the inport store to get my some better ones. P.O. must have not wanted to spend a little money to prevent super hard starts :D

Thanks!

4 out of 5 not working!!!

Amazing it would start at all without ether!

Depends on the ambient temperature. My 81 SD would start without the benefit of glow during daylight hours in San Jose, CA from Feb to Oct. I didn't have to crank it for more than 10 seconds for the engine to start. The SDL isn't as forgiving without glow but the glow system has been reliable.

Sixto
95 S420
87 300SDL

Yes...amazing
 

Yeah.
To get it started you had to do a few cycles or 5 (lol)
Then it would just start to kick over..but you could tell it was running atleast one becuase there was diesel smoke eminating.
It'll be nice to not have to wait 10min for that one glow plug to heat up all cylinders
(was #5 too)

Reminds me of the time I was up in CT, at a hotel, trying to start the SD at 12 degrees. No start. Not enough crank time on old battery. Try and jump it. No start. Charge battery for 15 minutes with other vehicle. No start. Shot of ether. No start.

Leave it sit for the day until the temps climb a bit.

Try it again at 3:00, ambient at 28, with a 100 amp battery charger. Two glows and it kicks and bangs right over with clouds of smoke.

Turns out that one plug was out, one was weak and one battery was good for starting a go-kart. Live and learn. If you want to start a 617 at 12 degrees, everything has to be perfect.

Hi all,

Wow I'm glad I'll never have to deal with freezing temperatures unless I take the 300 SD snowboarding (I live in Los Angeles). On my car three of the five glowplugs were no good so all of them were replaced. I'm still not sure why the glowplug dash light did not flash indicating bad plugs. Maybe because more were not working than were working. Again thanks to everyone at this wonderful forum for helping with all the replies.

Gilbert