OK baffling Glow plug issue 1984 300SD - PeachParts Mercedes-Benz Forum

meltedpanda · #1 11-07-2009, 01:04 PM

So my glow plug light has been out a few days.
So I
tested idiot light - good

tested relay , volts to it - good, timer relay on 20 seconds, 12+ volts to each pin

tested from plug to each glow plug, all ohmed out below 1

when plug is diconnected from relay , light on dash glows

reconnect it , does not come on

what am I missing, too much time under the SL hood has me stupid

seems that if the plugs are good, relay working --what is left???

pelon · #2 11-07-2009, 02:24 PM

tested from plug to each glow plug, all ohmed out below 1

roberto

Brian Carlton · #3 11-07-2009, 02:30 PM

With the key in the #2 position, and during the 35 second glow period, please check each plug for 12V.........you'll need to be quick.

Additionally, is there any possibility that the meter (presumably digital) is not being operated correctly and you've actually got some open circuits for one or two plugs?

Brian Carlton · #4 11-07-2009, 02:31 PM

Quote:

Originally Posted by pelon

tested from plug to each glow plug, all ohmed out below 1

roberto

He checked from the plug sockets at the relay to ground.

kingdoc1 · #5 11-07-2009, 04:00 PM

The diagnosis you have performed would seem to indicate that one
of your glow plugs or glow plug wires is faulty. Remove the glow plugs from the engine and test them with jumper cables to make sure they are all actually working correctly. Check all your glow plug wires if one of the plugs does not turn out to be bad. If everything checks out, you may just have a faulty sense circuit in the relay, the glow plug sytem will function but the indicator light will not illuminate.

The glow plug relay has a reed relay circuit that senses the current draw of the #1 glow plug compared to the current draw of the #2-5 glow plugs. If all the plugs (and glow plug wires) are operating correctly the relay will allow the glow plug light to illuminate. If there is a fault, the unbalanced current flow will cause the relay to extinguish the glow plug light. A fault in the relay's sense circuit could also cause the light to not illuminate.

toomany MBZ · #6 11-07-2009, 04:05 PM

If one of the wires were faulty, he'd get an open, or infinity reading, not a "below one".

kingdoc1 · #7 11-07-2009, 04:18 PM

Quote:

Originally Posted by toomany MBZ

If one of the wires were faulty, he'd get an open, or infinity reading, not a "below one".

True, but if the wire were partially broken it would still read ok with a meter but be unable to carry sufficient current. This would trip the sense circuit in the relay.

meltedpanda · #8 11-07-2009, 04:28 PM

I pulled plug one on the hopes that it was at fault, and BTW easiest to get to
I got continuity between base and wire end. As I understand that ,that is not good. I put 12 volt to the plug itself, got warm , but did not glow.
Does the #1 usually give up first ?
I plan to replace that one if you all agree with diagnosis. How much "stuff" has to be removed to get to the others, especially the one behind the oil canister

Brian Carlton · #9 11-07-2009, 04:32 PM

You've discovered the prime example of how all the testing in the world cannot determine the cause of your problem...........only pulling the plug and watching the tip can do it.

Resistance is good.........12V are present.........relay is good..........and the engine won't start properly.

Change #1 and see your results. The difference may be so obvious that you don't need to do 2-5.

If you do attempt the others, the use of a gearwrench makes life much easier. Also, removal of the hard lines is mandatory, IMHO, although others will forcefully disagree. They either have much smaller hands or don't mind the bleeding.........

toomany MBZ · #10 11-07-2009, 04:32 PM

You want continuity between the relay plug end and the loop at the gp, if it was open, you'd have a broken wire. Try not to touch the leads when you measure.
I now remove the fuel rack, makes things much easier. Test all your plugs.

kingdoc1 · #11 11-07-2009, 04:34 PM

The glow plug tip should glow red/orange. If it doesn't it is bad. If you have any extra plugs, put a known good one in and see if your problem goes away.

As for the other plugs, I don't remember them being too hard to access, but my cars are 300d's, maybe the sd has more stuff in the way.

toomany MBZ · #12 11-07-2009, 04:42 PM

Quote:

Originally Posted by kingdoc1

As for the other plugs, I don't remember them being too hard to access, but my cars are 300d's, maybe the sd has more stuff in the way.

No, same engine and same set up right there.

meltedpanda · #13 11-07-2009, 04:51 PM

thanks gang, will go with the #1 for now see what happens, my guess it it gets the voltage first , dies first, seems like i read this somewhere.
It did not glow , just warm

funola · #14 11-07-2009, 06:12 PM

You can reliably test a glow plug without removing it. It is almost as bomb proof a test as powering it up and watching it glow. If you have a DVM with a 20 A scale, connect one lead to the glow plug and the other to battery positive. If the plug is good, it should draw around 18 A and as it heats up decrease to around 15 A. If it draws nothing it's open.

I bought a 96 VW TDI direct injection about 2 months ago . As the weather got colder, I noticed lots of white smoke at cold start even though it fires right up within the first rev of the starter. I just checked the glow plugs with the DVM and all 4 were open but it starts right up no problem at 40F. Pretty amazing compared to my 300D wouldn't have started if all the plugs were shot. The VW is only 17.5 to 1 compression and has a mechanical VE rotary injection pump. I guess the direct injection has something to do with it.

jbach36 · #15 11-07-2009, 09:55 PM

I had a similar problem, and when else but during cold days of a winter month last year.

We did everything right, but the car would still not start. Finally found out that one of my glow plug wires had been tighted to the point where the wiring harness for that one plug went around, and touched the engine, shorting it all out.

Check to see if that might be one of the problems.