On a W123 how much voltage should be getting to the glow plug ? At the relay I'm seeing 12.5 v from the battery and 10.5 v at the glow plug itself. Seems like too much drop or is it ? Also, Is there a section on the GP relay on the Mercedes CDs ?

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David McDonald
1981 240D
1985 300D (wrecked)
1985 300TD 365,000 miles
1969 Mustang Fastback

DD,

10.5 volts sounds too low. You can put the voltmeter leads across check points to try to fine the ~2 volt drop.

Check all the points possible with your volt meter thru the GP system to see if you can find where the voltage drop is occuring.

Are all the GP the same voltage? If they are, its probably not the GP wires but some place in the relay. Have you checked both sides of the fuse in the GP relay?

The spot that is dropping the voltage will be generating about 100 watts of heat so if you leave the GP on, you might be able to find a hot spot.

P E H

If I measure the voltage across mine, I see 11V, which is the rating of the GPs. I assume the relay controls the voltage at the GPs, but I'm not sure.

Are you talking about your 240D ?

I don't ahve a 240 but I have read that there is a voltage drop before the first GP supposedly so they can use the same GPs as the 300D. You should have 12/5= 2.4 volt drop across each plug in series. But if you are talking about the '85 300D then you should have full voltage of the battery.

__________________
1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

if your checking the voltage at the glowplug with the glowplug attached you will see a drop of approx 1.5vdc.detach the glow plug and you will see 12vdc at the lead

I concurr with Michael...

The Vdrop is there for a reason...if you find any loose connections (battery/relay/plugs) fix them up and call it good.

I get about 11.5-11.0Vdc at my glow plugs.

Craig,

The GP relay does not control the voltage. Its either open or closed. What drops the voltage is resistance.

Since the GP pull about 70 amps, you need only ~14 milliohms in the entire GP circuit from the battery to the GP to drop one volt.

P E H

Thanks for all the replies. I checked my 300TD which starts quickly and it showed 10.5 v also with my meter and cables. Since I still hadn't found a circuit problem I went ahead and adjusted the valves ! They were all pretty tight I think. By the time I had finshed it had warmed up and the car started fine but I don't know how much that will help it when cold. I'll try in the morning and see how it does when cold outside.

__________________
David McDonald
1981 240D
1985 300D (wrecked)
1985 300TD 365,000 miles
1969 Mustang Fastback

Thanks,
That sounds right. I just measured the voltage from the GPs to ground while they were glowing and read 11V on my 300D. The GPs are also marked 11V, so I assume the system is designed such that you only get 11V at the GPs. 14 mohms sounds reasonable for the total system resistance.

I was talking about my 82 300D, which has 11V at the GPs (see PEH's post). BTW, the later 240D, such as my 83 have the GPs in parallel, just like the 300D.

Did you adjust your valves warm?

If so...I hope you compensated accordingly.

EDIT: Noticed you meant it warmed up outside...sorry!

Is the 12.5V at no load, or during pre-glow? Battery voltage will decrease from internal 'resistance' when under any significant load, and thus it will be normal to read less than no-load voltage. That's why the dome light dims when you turn the key, the battery voltage has dropped a bit.

Some voltage loss, therefore, has nothing to do with resistance in the wires, but resistance within the battery.

I think Jim H has it pretty well right. One check you can make is to read the battery voltage at the battery terminal with glow circuit on. From experience on one of my diesels it sags about two volts on the built in voltmeter with the heavy load.