ok folks here it goes 1980 w123 240D w/pencil type glow plugs 12mm no kit. installed from factory...

problem: when i put the key in the ignition the pre glow light blinks on and off, instead of the steady glow

I replaced all 4 plugs and the light is still blinking on and off.

When the vehicle turns on the light goes off.

Yes the vehicle has a hard time starting

Any help

thanks

Can you hear the relay click on and off?

yes i hear the clicking

How long between the "on" and "off click?"

Watch your interior light. It should dim a little for the duration of the glow cycle. Since it is hard to start did you check your fuse strip? Or I should say check it if the interior light does not dim as it should.

will resume tomorrow morning to dark and cold

thanks to all

could not help it went outside interiour light does not dim at all will check strip fuse tomorrow is it located inside by the firewall

Yes I believe the fuse on your year it is where you think it is. Inspect it very carefully because if not that then probably it is the relay.
The relay is located on the pedal support arm above the cover panel ahead of the drivers seat if you did not know. Is a silver or aluminium box held by one machine screw and nut. You might try for a junkyard unit if any older 123s are there. I cannot see one of these older mercedes landing up in a salvage yard because of a bad glow plug relay.

1980 was a "transion year" so your relay is most likely under the hood, black box, driver side on the fender. Pull the top cover off and you'll find the strip fuse in there.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

as of today i conducted the following tests

voltage reading on all 4 pins leading to glow plugs on the relay 12v constant

glow plug light stopped blinking no light at all

voltage check across strip fuse 12v checks ok

also conducted an ohm check on the wires leading to the plugs checks ok .05 ohm on each

unable to hear relay click and there is always a constant voltage reading on all four pins leading to glow plugs

Just to clarify. You should not read 12 volts across the fuse. I2v to ground from each side is ok. Actually across fuse should be next to nothing voltage wise perhaps a quarter of a volt.

Just to clarify. You should not read 12 volts across the fuse unless it is open. Yet I2v to ground from each side is ok. Actually across fuse should be next to nothing voltage wise perhaps a quarter of a volt. Also should not read a constant voltage on the plugs. I suspect you may not be using your meter properly. Tell us how you are readuing various voltages if you wish. We might be able to help somehow. Or you may just not be describing things properly.

im sorry i assumed yes the black to the battery negative and the possitive to the test areas (such as the pins and the fuse ends

when this is done i get 12 v on the fuse both sides fuse not compromised

pins from relay that powers the glow plugs pin 1 2 3 4 constant 12v dc

no clicking noise from relay

glow light not working (last night it was blinnking instead of a constant glow prior to turning on the car.

when vehicle was turned on yesturday the light would go off *hard starting)

Since you obviously know how to read and use a meter. Hook up the relay plug and see if still 12v on glow plugs. If close to 12v and the circuit is complete it should be dragging your battery down.
Since the starter will crank the ground should be intact .
Something is getting odd here. If under load(the glow plugs hooked up)you have very low voltage on the glow plugs there is a high resistance point somewhere. Could be the relay or something else. I mean only a few volts or less than say eight volts if the batery reads 12.5 under static load conditions. Keep feeding findings back as a lot of fellows have more experience than myself on site and some voltage reading will indicate something specific soon.