This is for my father's W123. He has an '85 300D. His glowplug light does not come and and when he goes to start the engine, it takes forever for it to crank over. He lives in Virgina and has to actually plug the block heater in to get her started with ease. When my folks visited last, I ohmned out his plugs. #1 plug had an infinite reading, #'s 2,3, and 4 had about .5 and #5 had about 3. This was on a 1K scale. SoO if I read this correctly, #1 glowplug is dead and #5 is on it's way out. My father want s to just replace all the plugs as a preventitive measure. Should he just replace them all, or just the bad ones?

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

Sounds like the small flat fuse in glow plug relay is cracked. Common problem, cheep fix.

Rick, diesel engine 617.95 job no. 15-710 checking preglow system.

You say that the light is not lighting up and the car is not in the ready to start condition? Here is the test:

Pull 6-point coupler from preglow time relay, turn key into position "2"
Test voltage at jack 2(terminal 15) against ground by means of a voltmeter.

If there is no voltage(approx. 12 volts) on red/black line from fuse box(terminal 15, non-fused side) to coupler jack 2 of preglow time relay, check for interruption and remove interruption, if required.

If voltage is indicated, connect voltmeter to jack 2(terminal 15) and jack 4)(terminal 31) and test voltage.

If no voltage(approx. 12 volts) is indicated, test brown line from jack 4 to ground for interruption and remove interruption, if applicable.

If voltage is indicated, preglow time relay is defective. Replace preglow time relay.


Rick, I believe interrupted glow plugs would cause a flashing glow plug light after the car is started or attempted to be started. It would flash for 30 seconds.

Well, his glowplug light does not flash and the test I did was from a Haynes manual. They say to pull the wiring connector that goes to the plugs and check for resistance. An infinite reading meant a bad plug or plug wire. I will let him know about the relay fuse. Where about is this flat relay fuse? I am looking at a picture of the relay box, in the Haynes manual. At the top of the box there is a flat piece of metal held down by 2 screws, is this the fuse?

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

I think you found it. It should be an 80 amp fuse.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

Well since I am in NJ and my father is in VA, I'll have to ask him what condition this flat piece of metal, is in. I was just messing around with my glowplug system I have a few more questions. Do glowplugs wear out slowly or are they like light bulbs and suddenly go out? The reason I am asking is when I was checking my glowplugs I was getting readings that were higher than what I got on my father's good glowplugs. Also something doesn't make sense. The Haynes manual says to disconnect the glowplug connector at the timer relay, turn the ignition to on, then check for continunity at each terminal on the connector. Once the connector is removed, what difference does it make if the ignition is on or not. Does anyone know what ohm reading I should get.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

I guess what I meant was according to the Mercedes manual, testing the glow plugs for resistance would be what you would do if the light was flashing. The fact that the light does not flash, and the car is not glowing would mean you would do the test I described. You should of course always check the fuses first, the 80 amp fuse and the fuse in the fuse box which I believe is no. 4.

The glow plug light tells you what to look for by it's action.

It will light normally(normal), or not light but still be activating the glow plugs(possible bad bulb or temperature sensor), or not light and not be activating the glow plugs(your condition, fuse blown/broken wire/defective relay), or flash for 30 seconds after the car is started(the condition you are testing for, bad glow plug(s)), or flash for 30 seconds in key position "2" before a start attempt(bad 80 amps fuse/power interruption/bad relay).

All of these indicators point to different tests, according to the manual.

As for your testing question, here is what the 617.95 manual says for engines through 1980.

Measure current input of pencil element glow plugs with a DC clip-on probe. For this purpose, place probe over the individual lines on pencil element glow plugs.

Move key to position "2", the power input of each glow plug should then be 8-15 amps. after 10-20 seconds.

At a value above 15 amps, renew glow plug.

At a value below 8 amps, test electric lines or glow plug with ohmmeter for interruption.

I don't believe you remove any of the couplers on the relay, and you only have a short window to get a correct reading. You will have to turn the key off and on a couple of times perhaps.

Hey Patsy, thanks for the info. The reason I keep mentioning the Haynes manual and what it says to do, is because this problem is on my father's '85 300D. The Haynes manual I have covers the W123 series cars. The glowplug relay on my car, and probably yours is different than what is used on the W123 diesels. Like I mentioned earlier, when you pull the connector off there are 5 terminals, one for each glowplug. On my car the connector has 8 holes with a terminal in 7 of them. There is one in #'s 1,2,3,5,6,7, and 8. 2 of these go to 2 temp. sensors in the head. Well, I believe they're temp. sensors. Eitherway, I guess I'll have to do a continunity check to figure out which terminal goes to which plug. On my father's car the connector only has 5 terminals and they are numbered.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member

I believe your dad would still have the 617.95 engine, right?

So here are the directions for the 1981 and later(quick start system).

Testing the glow plugs is the same as I detailed in the last post, and if you find a value higher than 8 amps replace the glow plugs, if lower than 8 amps I will tell you how to test further below.

If lower than 8 amps pull 6 point coupler from preglow relay. Measure resistance against ground with ohmmeter one after the other at:
Jack 1 of coupler=pencil element glow plug cylinder 1
2=2
3=3
4=4
5=5

If you measure resistance, an interruption of the respective glow plug or supply wire is indicated.

If a smaller resistance is measured(e.g. at 68 degrees F< 1 ohm) the supply line and glow plug are fine.

Note: It is possible that an indicator lamp will indicate fault only after a failure of 2 glow plugs in cylinders 2 to 5.

To make sure that the fault indication in relay is not defective, disconnect 2 glow plugs of cylinders 2 to 5 in such a case and repeat preglow operation.

If the indicator lamp is now indicating a fault(not lighting up) the preglow relay is fine.

Does the car run poorly after it is started?

I'll have to print all these instructions, so that when I visit my folks again I can take another look at it. Do glowplugs act like light bulbs, in that they are either good or burnt out. Or do they slowly fail over time? I ask this only because when we checked for continunity, #1 plug indicated infinity (an open) and #5 indicated a fair amount of resistance, where as the others indicated very little resistance. This I would think shows that #5 is on it's way out. If glowplugs do that.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member