Glow Plugs Have Power and Zero Resistance But Don't Heat Up...Help - PeachParts Mercedes-Benz Forum

350SL4spd · #1 05-10-2005, 05:18 PM

Hello Diesel Fans I've got a weird one today:
617 Diesel Coupe ('84 turbo) is being a stinker about starting. The owner got it running w/ starting fluid (

) to get it to my house (where she suggested to me that the car might need new spark plugs

).
The relay and glow plugs pass all tests: Relay clicks on, glow plug light in dash comes on, all fuses check out good, I have 12V+ at every single glow plug w/ ignition on and .01-.03 ohms at every glow plug (btw the relay harness and the block). But the darn things don't get hot! Far from smoking a little oil on them, you can hold onto them for the whole glow cycle! What am I missing?!?
I hope someone has some insight...12V and <1 ohm should get the plugs hot....right

Thanks!
-Matt-

trontek · #2 05-10-2005, 05:47 PM

glow plugs. You can still measure 12v. in this instance but not be able to source enough current to do the job required. For it to be common to all the plugs, the trouble would have to be between the battery and the relay contacts for those plugs....

mbenzgoodyear · #3 05-10-2005, 07:16 PM

Just replace the glow plugs, save A LOT of headache and confusion. I had an issue today, and I had a bad relay and plugs so replaced both, but start with the plugs. ( you can read the post Glow Plug Relay and see what I mean)

estod · #4 05-11-2005, 12:18 AM

I just did mine on my 240 a couple of months ago -- relay worked, had voltage to each plug, .8 ohm each, big relay fuse was good ---- had to put a worklight under the hood all night & crank forever to start it anytime it wasn't already hot.
I replaced the glow plugs & haven't had a moment's trouble since. I didn't test them out of the engine to see if any of them heat up, but I suspect that none of them do.

compress ignite · #5 05-11-2005, 01:16 AM

'could the ETHER (starting fluid...incredible flash heat in a G.P. diesel) have
burned out your glow plugs??

"Only use ether to start upon armeggadon occurence"

rg2098 · #6 05-11-2005, 06:40 AM

Pull them out and physically test them directly with the car battery. Jumper cables work well for this. If they still don't heat, replace them.

Glowplugs at Fastlane

TonyFromWestOz · #7 05-11-2005, 11:21 AM

GPs can develop a short circuit, where they have a low resistance and as the voltage from the battery is too low at the GP, although the current is high, they cannot heat up. Instead, the wiring does. That is why the GP relay had the heavy fuse. If a GP were short, you would have a blown fuse, or smoking wiring.

I agree, use jumper leads to apply 12V to the GP on the bench. If the GP glows red, it is OK, if it doesn't replace it.

Another thing to check is the engine main earth. This has been known to cause hard starting issues. If engine does not turn over FAST, it may not start.
BTW, What did you pay for the Coupe'?

Tony
The Wizard of Oz

350SL4spd · #8 05-11-2005, 11:42 AM

Thanks for the replys guys. I will bench test each plug to rule it out. The mystery to me is that enough of them could quit at once to keep the car from starting in 80deg weather. Of course, I guess it's possible that one or two were already bad and another just sealed the deal.
I was planning on cleaning up the ground as well. My next guess as to the no-start issue if the plugs are good is the starter is dragging and not spooling fast enough and I remember my "always clean the ground when testing the starter" mantra.

As far as how how much I paid, I didn't: The car belongs to a client and I'll take just about every opportunity to get diesel on my hands

. She does desperately want to me to buy it from her, so all interested parties contact me. I wasn't going to post it for sale here because she wants way too much for it. It is an odd beige/charcoal color (that is very handsome) over black w/ +/-175k mi, but it has been ignored and driven hard for the better part of its life w/o any love. The engine is covered in oil and dirt, fuse box has no cover and bad american fuses. All the wood trim inside needs to be replaced, radio doesn't work, two good sized dents in the body, and some rust along the rear @ the end of the trunk/under the taillights. That being said the rest of it not rusty and overall a solid, very nice looking car that needs some TLC. Problem is she wants $4500 for it. Ha! Good Luck!
I'll let you all know what I find w/ the GPs.
Thanks!
-Matt-

michael cole · #9 05-11-2005, 11:52 AM

check to see if your getting a voltage drop at the battery when the glowplugs are activated.voltage at the battery should drop to 10 or less vdc.if no voltage drop is present sufficient current is not flowing.could be a bad fuse or hot lead to glowplug relay or the relay contacts inside could be corroded

whunter · #10 05-11-2005, 12:01 PM

Here is a thread to read.

Glo Plug Testing?
Glow Plug Testing?

350SL4spd · #11 05-12-2005, 06:57 PM

Hi all,
Pulled all the GPs today and not one heated up. I checked an older spare that I had just to make sure that I wasn't an idiot and that one lit up like an Xmas tree. I'm a little shocked that all 5 were bad...what are the chances?
Anyway, new ones are on the way from Fastlane, along w/ all filters and front pads since they're almost into the rotors. No one has adjusted the valves on the poor thing for at least ten years, so that will be done too. Luckily the starter was new or it would have been killed by all her attempts at starting it. it's life is much shortened though, I'm sure...Who knows when the tranny fluid was changed...
I really hate people who neglect a good car...

Thanks!
-Matt-

Brian Carlton · #12 05-12-2005, 08:40 PM

Quote:

Originally Posted by wdba123190

Hi all,
Pulled all the GPs today and not one heated up. I checked an older spare that I had just to make sure that I wasn't an idiot and that one lit up like an Xmas tree.

Would you be kind enough to perform a little experiment:

Take one of the bad glow plugs and measure its resistance. Ground the threads and check the tip with an ohmmeter.

Then check the older spare the same exact way using the same ohmmeter.

Thanks in advance.

This will provide very useful data, no matter what you bring back.

Your initial measurements of .01-.03 ohms are not possible. A properly functioning circuit would read .8-1.2 ohms. Glow plugs that are completely burned out will read infinity.

83-240D · #13 05-12-2005, 08:48 PM

the plugs got pulled on my car yesterday and they are heavily carboned up(not only are the heat tips black but there are peaks of carbon) and they just went in last september and there is about 2000 miles on the car

it has sat since feb and was driven little inbetween since it would not startw/o ether so 1000 miles might be closer to the truth.

suspicion is that the sellor/po did not clean the prechamber before installing them. i have ordered a glow plug reamer from performance prods. and it will be here is a couple days. these bosch plugs tested well at the tip but were not lighting and the car fired on ether but not plugs. the carboning caused a grounding preventing heating up.

just an idea.

psfred · #14 05-12-2005, 10:40 PM

Pencil type plugs don't ground out like the loop style, the "pencil" is grounded via the screw fitting.

They are shorted internally, most likely -- the element will droop and weld to the side, so the point doesn't get hot.

The soot is from either low compression or bad injectors, most likely bad injectors.

A nasty spray pattern will prevent starting, so will very late injection timing. check the chain stretch while you are in there, a new chain may improve things sight out of mind!

Peter

Brian Carlton · #15 05-13-2005, 12:14 AM

Quote:

Originally Posted by psfred

They are shorted internally, most likely -- the element will droop and weld to the side, so the point doesn't get hot.

Peter, could this explain a very low resistance reading, such as .02 ohms?
Have you tested any at this level?

If they are this low, how does the 80 amp fuse survive? I don't believe that it can with resistance that low.