My 1980 300D won't start all of a sudden.
Been running/starting great all day.
Car starts really easy and fast even in cold weather.

After excluding vacuum issues (to make sure fuel wasn't being cut off)
I went to take all the glow plugs out and test
them, just to be sure they were okay.

This is what seems wierd:

I get 12 volts with key on, at the relay, on the pins for connector.

With all 5 wires off the glow plugs I get 12 volts at each end that
connects to glow plug.

When I connect the 5 ends to the glow plugs, and then connect my volt
meter and turn on the key I get nothing - no voltage with key on at the
glow plug. But I do get 12 volts with the 5 ends disconnected.

I am befuddled. I seem to remember that I used to get voltage at the
glow plugs.

All glow plugs passed the heat up test with 12 volts applied directly.
I do not have an ohm meter that goes below 10 for proper resistance
testing, so I test with full 12 volts off my battery charger.

I know that the glow plug creates heat by resistance, but I still think there
should be 12 volts at the glow plug.

Can anyone please explain ?

Thank you.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

There was a thread about 3 weeks ago with this exact problem. Look it up and see if the problem was ever diagnosed.

Glow plug testing question (w123)

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Just had the same thing happen to me with my wagon. All but one plug was shorting out from the carbon packed around it in the prechamber. Ream out the holes and you should be good.

__________________
1982 240D auto
1978 300CD auto
1985 300D auto
1983 300TD auto
1984 Porsche 944 5 speed
1973 Opel GT 1900 4 speed
1967 Chevy C30 350 Sm420
1973 Mustang Grande Convertible 302 C4
1981 VW Pickup 1.6D Turbo 5 speed
1983 Rabbit 1.6D Factory Turbo 5 speed
Plus parts vehicles...

Thank you!. What was happenning in that thread seems
to be exactly what I am experiencing.

Unfortunately the thread has no conclusion.
Many solutions presented, no resolution posted. Dang!.

The suggestion is that there is resistance somewhere sufficient
enough to cause the voltage to drop to zero when connected to glow plugs.

I will check the strip fuse and ground strap for decent clean connection.

Any other specific places to look for resistence ?

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

Hey DieselVolvo, Question about this.

I just had all the glow plugs out.

The center electrode (pencil tip) seems to be connected to ground anyway, so
how does the carbon cause this to short circuit ?

I did feel like the tips were hitting carbon, but did think too much of it.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

So I cleaned the connector at the body for the negative (ground)
cable. Did not look bad when I took it apart.
Still, took a wire brush to the bolt and cleaned the cable and body.

Then went to the strip fuse in relay. Fuse looked great.
Noticed as I was unscrewing that the screws were not real tight.

After careful inspection reinstered fuse and snugged up screws tighter
than they were.

Cannot say which condition caused the resistance.

But... I started connecting one glow plug at a time to the harness
and turned the key to on after each connection to check voltage
at the glow plug harness. After each connection I continued to get 12 volts
at the glow plug. I was waiting for it all to drop off again, thinking that
maybe one of the glow plugs was grounding/shorting.

Got to the last one and still get 12 volts when key on.

Buttoned it all up and car started just like I am used to.

Thank you everyone.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

The bad connection of the scews not being tight was the resistance.

__________________
84 300D, 82 Volvo 244Gl Diesel