Lack of glow plug light & no start - PeachParts Mercedes-Benz Forum

johnathan1 · #1 09-30-2008, 11:12 PM

I posted about this problem in the "my latest 300D" thread, but I don't think that will get much traffic...so I'll post it again here.

I just bought a 77' 300D, and it won't start...The glow plug light doesn't illuminate at all when the key is in the on position, and the engine will not start, just cranks. The glow plug system on this car is the early version, and I'm having a hard time figuring it out. I know the glow plug relay is under the driver's kick panel, and I don't hear it click at all when the key is turned...

I suppose it's possible that all of the glow plugs are bad, but isn't that unlikely? And if they were bad, wouldn't the relay still make a clicking sound?

kerry · #2 09-30-2008, 11:44 PM

It's got a series glow system if it is the original system. One bad glow plug causes the whole system to fail. It also has a fuse on the firewall at the back of the engine on the firewall.
The best way to deal with this system is to replace the series glow plugs with the retrofit parallel plugs. You can remove the heavy squiggly wires, remove the ground wire from the first glow plug at the front of the engine, put in the retrofit plugs, hook glow plug 5 up to the hot wire from the firewall and put jumper wires between each plug.
Searching on here should give you the part number for the retrofit parallel pencil plugs.

johnathan1 · #3 10-01-2008, 02:20 AM

So I should start by replacing my glow plugs? If my #5 glow plug (or all of them for that matter) was bad, then wouldn't I still hear the glow plug relay click on when I turn the key? Or would the relay stop functioning entirely when a (the) glow plugs die?

The strip fuse on the firewall appears to be intact.

toomany MBZ · #4 10-01-2008, 05:16 AM

I would check the glow plugs anyway. Don't know if it's the same with the early version. Check for resistance, less than 1 is ideal.

kerry · #5 10-01-2008, 09:49 AM

In my opinion, the pencil plugs are so superior, that if I started working on a car with series plugs, I'd simply replace them.
I don't think the resistance test will work on series plugs in the same way it works on parallel. Checking the continuity between the first plug and the last should though.
It could be your relay. I don't know for sure if the relay will click if a plug is bad, but I doubt it, because a bad plug typically breaks the circuit so the relay would not be seeing a complete circuit. I don't recall hearing the relay when the strip fuse burned out on my 77 300d.

Abadjay · #6 10-01-2008, 10:27 AM

It could be just that relay fuse itself. I had that problem on my cars even though I knew all the glow plugs were working. Simple $1 replacement.

kerry · #7 10-01-2008, 10:31 AM

He checked the fuse.

Since one bad glow plug causes the whole system to fail to energize, the likely source of the problem is one bad glow plug. In a pinch, jumping over that glow plug will allow current to flow to the other 4.

johnathan1 · #8 10-01-2008, 02:12 PM

Can I get the updated pencil type retrofit plugs at Autozone?

kerry · #9 10-01-2008, 02:40 PM

I doubt it. Maybe if you search and find the Bosch part number on here, they could order them.

qwerty · #10 10-01-2008, 02:50 PM

Quote:

Originally Posted by johnathan1

Can I get the updated pencil type retrofit plugs at Autozone?

You can get the plugs there, but that's not all you need to make the transition.

kerry · #11 10-01-2008, 02:54 PM

Quote:

Originally Posted by qwerty

You can get the plugs there, but that's not all you need to make the transition.

Apart from the jumper wires from plug to plug, it is all you need. The jumper wires are pretty easy to make. The rest of the system can stay as it is.

johnathan1 · #12 10-02-2008, 08:42 PM

I tried testing the glow plugs resistance, but it just shows infinite/open between the plug itself, and battery ground on every single plug...I guess this means every single plug is bad? I had a thought...is it possible that my engine grounds are bad, and that's why it's showing open? Where are the block ground cables on this engine?

Thanks guys!

kerry · #13 10-02-2008, 10:51 PM

I don't think the resistance test used on parallel plugs will work with series plugs. I would check the continuity thru the plugs by putting an ohmeter on plug 1 and plug 5. If you get an infinite reading, one or more of the plugs is bad.
I think you could check each plug by checking the resistance between the input wire and output wire. An infinite reading on any plug would show it was bad. I don't know what a good resistance reading is for a series plug.

TheDon · #14 10-02-2008, 10:54 PM

Quote:

Originally Posted by johnathan1

Can I get the updated pencil type retrofit plugs at Autozone?

pfft.. Autolight plugs are total junk. I don't even keep a spare in my tool box.

Dieselgiant has a great DIY on converting to the updated system

http://dieselgiant.com/mercedesdieselloopglowplugrepair.htm

johnathan1 · #15 10-03-2008, 12:11 AM

Well, to eliminate the guesswork...I went ahead and removed all of the glow plugs, and they all read infinite individually, when testing them with an ohmmeter, with one probe touching the wire shaft, and the other touching the threads that contact the head...I know this is the proper way to test parallel plugs, is this the correct way to test series plugs? I guess this means they are in fact ALL bad?..wow.

Am I correct in thinking that the squiggly wires carry the positive current, and the glow plugs ground in the head? If so, then what is that ground strap for at the number 1 glow plug that attaches to the block??

A little help please...lol.

Oh, and about the Autolite plugs...I know their SPARK plugs work very well in Supra engines running at high boost, in fact it's what almost everyone in the community recommends...I suppose it's possible that they don't make they glow plugs as well, but it really doesn't matter, since they don't carry them anyway...