I have been searching all over and apologize if this has been covered. Below is the issue I am running into with my pre-glow system. It weird and I can't seem to locate anything related to this any where and this to comes and goes, sometimes the system works perfect other times it does not, about 5% of the time.

When I insert the key and turn into the glow position, the light comes on...95% of the time. Sometimes, the light flickers or does not come on. The weird part is when the light flickers, I hear a strange noise coming form the firewall area under the speaker that sound almost like a clicking noise but is more of a deep stepping buzz behind the seat belt relay box.

Initially, I thought to change the plugs, so I did all 5 to eliminate those. Checked the plug relay with a meter, all seemed fine, as well as replaced the strip fuse. Now I am thinking the glow plug relay could be bad or maybe the ignition switch might be failing. Wanted to check if anyone here had the same experience with their w123 and could maybe offer some advice about what I may be missing in this equation.

It sure sounds to me like the glow plug relay is going bad.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

That's what I'm thinking but the only thing is the the weird clicking noise by the firewall. The relay is by the front left headlight, I know the older style w123's had relays by the firewall, and this one doesn't.

On My Year and Model behind the Instrument Cluster is a Power Window Relay and the Rear Window Defroster Timer/Relay.

You could take the Glow Plug Relay apart and see if somethng inside is fried.

Mine is wired as below. If the Black Wire is the one comeing from the Ignition Switch then you should be able to cut that Wire and apply Batter + Voltage to the end going to the Glow Plug Relay and if it works every time then I think that is an indication that the Ignition Switch is working.

Glow Plug Electrical Connector Wire colors on the square Connector and where they go.
Here is a pin out description:
1)rd/bk (Black) ignition switch
2)vi (Violet/Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On.
3)blu/wt (Blue with white strip)glow indicator Light
4)br (Brown)ground

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84 300D, 82 Volvo 244Gl Diesel

Always best to rule of the simplest items first, dirty ground?

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Nosce Te Ipsum "Know thyself"

And make sure the nuts that hold the glow plug wires on the glow plugs are good and tight.

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Marshall Welch
Seattle, WA
1982 300D-T

I wonder if you have an issue with the ignition electrical switch at the back of the steering lock assembly (not the ignition tumbler, the actual electrical contacts).

When these wear they can cause odd behaviours. Before I replaced mine, I used to sometimes get a sound like a relay clicking inside the dash, when I turned the key to the preglow position. Also in the run position I sometimes had to nudge the key back just a hair, to get my accessories to work.

Just to clarify, the nuts on the glowplugs should be snug, but stop tightening before the wire connectors start to twist.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Thanks for the replies everyone. I will check the grounds and will take the relay apart to see if there is anything fried on the inside.

Was this coming from the left speaker side? If I take the key out or try the preglow 3-4 times, then it works. It sounds similar to what I am experiencing with the clicking. Will try to test moving the key back to see if this points the switch.

No in my case it was the center of the dash.

However, now that you mention it, when I turn my key to the preglow position, when I have the hood open I could swear I hear a distinct click at the firewall on the driver side. Almost like an echo of the relay clicking on.

Which makes no sense in terms of component locations, as far as I can tell.

But I don't hear it in the car.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

what year is the w123?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

It's a 1983 300d.

I think the problem is with the ignition key electrical switch in the dashboard.

when it flickers, can you turn off the key, and back on, and have it work right?

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Usually takes a couple tries. Sometimes it takes 3-4 tries to get the light to illuminate and stay on after it flickers or doesn't come on.

Have you checked and cleaned the glow relay connector contacts? Many ways to do it, one way is use a bamboo Shishkabob stick and WD-40 and burnish the corrosion off.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Same deal currently in my 1984 300D. I usually have to turn the key 2 or 3 times before the glow plug lamp comes on firmly. I suspect the key switch. I have a spare, but a pain to get at those 3 bolts. I also have a spare g.p. relay box. Not currently driving that car, so a back-burner fix.