The glow plug harness can go bad, it is cheap to replace.

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i read thru most of those links yetserday and the restg just a minute ago. i'm realizing that i may have tested the wires wrong. i need to know which socket in that 8 socket head leads to which GP and what the extras are there for would be helpful too. any info?

also any idea on why i got readings the way i did?

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

what is the gp harness and can it affect only a few gp as my problem seems to indicate?

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

I will add a little ,, I just bought my first Mercedes to tinker with 1979 300SD. Just got thru with a valve adjust today.. I went thru the glow plugs last weekend. Here is what I found..

I pulled the plug off the relay and checked the resistance to each plug , .7 ohms was the average. Use the ohm meter to find which pin goes to which plug..
Then checked the voltage at each plug while the relay was heating.. 11.2 volts to each plug.
Next step was to use a regulated Hewlett Packard 50 amp power supply ,set at 11 volts ,and test each plug at the relay plug harness.
Each plug pulled about 24 amps to start with and then dropped to 14 amps after about 5 seconds.

Since I was doing a compression check thru the glowplug holes, I was going to change the plugs anyway.
With the old plugs out I hooked them to the HP supply to observe the effects compared to the new ones.
The new ones instantly glowed red on their tips. The old ones, pulling the same amperage as the new ones, only smoked the carbon on the tip but never glowed red like the new ones.. Both the old and new were Bosch,, the new ones had a different tip,, the last quarter inch was smaller.
With the new plugs installed the startup was much quicker and smoother,, I could tell a huge difference.

As to which socket in the harness plug goes to which glowplug,, If you look down into the holes of the plug,, starting at the bottom where the wires come in,, on the right side row going counter clockwise
1,2,3,4,5 go to the plugs,, 6 is blank and 7,8 go to two sensors on the engine.. This is the way I remember it from last weekend... Also check the tightness of the 8MM nuts on each plug before doing any tests.. I found two of mine a little loose before I started doing my testing.

well, im hoping that 7 and 8, since they drew 5.6 and 7.1 volts are to the sensors. my #4 is blank - just a difference in model years? so that means 1,2,3,5,6 are my GP connectors.

so each pin except the sensor pins are testing the same as battery voltage, and #1 and #6 holes still test open for resistance.
does that mean no problem with the relay and the problem is wiring between the large connector and the GPs since the GPs themselves are new?

by the way, thank you all so much for the incredible help! having little $ to my name and relatively little mechanical knowledge, fixing my car with your help is the only way i'll be able to visit family for the hoplidays...

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

You got me wondering Bill,, went out and looked at my plug.
It is numbered 1,2,3,4,5,6,7,8
1,2,3 5,7 go to the plugs.. 4 is blank 6,8 go to the sensors. Use a volt meter and check the voltage at each glowplug tip while the relay is applying power. this is where you will verify if each plug is getting voltage.. If you have the proper voltage , 11V, and the proper resistance, .7ohms, then you are going to have current flow.

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Second owner 1979 300SD 250K miles.
1995 Ford Lightning
66 Mustang
87 5.0 Mustang
08 Ford Edge
46 Willys Jeep
89 Transalp
97 Dodge Ram 2500 Cummins..

This is an answer in a thread about a 1980 vehicle....
but those are ways to see how long your plugs are drawing current...
The advice about leaving it on...as compared to ' glowing it twice' is still good since many people don't realize it is suggested in the FSM...

thanks! that checks out, i mis-typed earlier. 6 and 8 are getting less voltage

ok, to summarize, i know little about what any of these procedures mean but i am getting a better grasp:

Voltage Measurements:

all but 6 and 8 (sensors) are getting 11 to 11.2 volts. that is, measuring voltage with negative lead on battery and positive on each pin with the ignition turned to preglow.

all GPs except the 2nd from the front of the car get 11-11.2 volts, with the head re-connected to the relay and the ignition on preglow. that is, measuring voltage with the negative lead on battery and positive on the threaded part of the GPs.

Resistance Measurements:

measuring with the negative lead on the battery and the positive in each of the relay head sockets, each measures .5 or .6 except the #1 socket which indicates open. i don't understand measuring between these two things (battery and sockets) because it seems they are disconnected. (got the instructions from http://dieselgiant.com/glowplugrepair.htm) yet the numbers still check out...

it would be more helpful to test the resistance between each socket and its corresponging GP. but i can't sigure out which goes to which. measuring the #1 socket, all of the GPs read open. measuring the #2 socket all of the GPS read between .4 and 1.6 ohms. wtf mate?

i don't understand exactly what this all means...help!

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

If you are communicating this correctly?

The wire from relay to glow plug one is broken, and you may have a short inside the harness.

Call Phil at FastLane for price and availability of a new harness.

Questions about Parts?
Call Phil for help at
1-888-333-4642
Mon-Fri 9:00a-7:00p ET
(NOT for technical support)

thanks!
so the harness is the "cap" that goes on the relay with all the sockets in it?
and is it safe to say that the wire going from socket #1 leads to the GP that is 2nd from front of car (the one not getting any voltage)?

would the failure of just 1 GP to get power cause the preglow light to flash after cranking and prevent the car from starting at all? seems like such a minor problem but i know so very very little...haha. thanks again!

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

I believe you have a multiple failure in this harness.

I suspect there is a broken wire and SHORT CIRCUIT inside the harness = few or none of the glow plugs are getting correct voltage.

If your fuel filter is not plugged and valves are adjusted correctly; there is a 90% chance your starter is bad, my 1985 300SD will start at 40F (no glow plug) with ten seconds of cranking.

called fastlane, i guess they'll get back to me tomorrow on the harness. i'm not so sure about the starter. it's 60 degrees F here and i just tried to crank it again. starter turned over very quickly (it goes slower in freezing temps), and kept turning over for a full 20 seconds or so. a good amount of white smoke, engine still won't start.

this makes no sense, if all but one of my GPs are getting proper voltage, its warm outside, and the starter turns over quickly, shouldn't i be able to start it with friction from the starter?

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.

Bill,

Before U go spending money on a new GP cable, test the old one.

Here is the straight poop about the cable connector numbers and cylinder numbers, at least for a '79 300SD:

Cylinder Hole #
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1...........2

2...........1

3...........3

4...........5

5...........7

So do a continuity test from each GP terminal to the metal in the GP connector using the chart above. If U get a low resistance reading for each of 5 tests, the GP cable is OK.

P E H

that makes sense PEH, since the #2 cylinder wasn't getting voltage and the #1 socket in the harness showed resistance. problem fixed - the GP light no longer flashes on and off after i try to crank the car.

SO the car just started once i fiddled around with some wiring. i have had it plugged into a block heater in case i had to go somewhere today so it is possible that is what allowed it to start. i will let it cool down over the next couple hours and try again.

i think you were right whunter, the starter may be weak. however i have about $100 to my name and that will need to go toward fuel and expenses for the 9 hr drive to see family this week. i am hoping that with strong GPs the starter will be inconsequential.

if it does turn out to be a problem, does anyone know how easy it is to put in a new starter?

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1980 300CD 281,000 miles. 9,000 mi on custom WVO system.