I always thought GP light might still come on with 1 down, but not 2. I was surprised to find the 1983 300D with TWO GP down and the light still comes on as normal. It is by chance that I measured the resistance and found 2 are down, #3 and #5 cyl.

What is your experience on the GP light scenario?

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

The Manual has One or More Glow Plugs will casue the Glow Plug Light not to come on. But, it also said along with rough start symptoms.

However, right now My Glow Plug Light never comes on due to I believe a burned Bulb but My Engine Starts and runs as it should with the exception of the Cyliner I have that is 50 psi lower then the others (runs OK cold but nails on that Cylinder till the Engine is warmed up).

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84 300D, 82 Volvo 244Gl Diesel

I'm pretty sure it's one plug but always remember to check the glow plug harness and relay to make sure it is the glow plugs or the light causing to issue. My car had been starting up fine and had new glow plugs but the light wasn't coming on. I figured it was the bulb, pulled the cluster and it wasn't. I then check the glow plugs again, and the harness, nothing wrong. I then checked for voltage going through each pin on the relay when the relay was on and sure enough one pin was not getting and power. Opened up the relay and a coil resistor was melted.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

When I had a 123 with the new style glow plug relay the light came on as usual with 2 failed GPs. Rough starting clued me in. With my 124 one GP failure had caused the light to not come on.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

I just went through a scenario with bad GPs (probably counterfeit Bosch) and can confirm my preglow light worked normally with two dead GPs and one half-alive.

Once the light came on and stayed on for about 5 seconds, after the glow cycle. I found that weird as I thought it was only the second-gen system with the "three-legged" underhood relay that would do that. But it only happened the one time.

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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

The 123/124 use a current balance relay that looks at the difference between (some multiple of plug #1's current) x (negated sum of the other 3, 4, or 5 currents). This is supposed to equal zero in normal operation. But you can lose a plug or 2 and still pass due to the slop in the sensing circuit especially if #1 is one of the bad plugs.

The 210 (and probably later models) use sensing for each pair of plugs. It knows if there is the right amount, half the right amount, or no current - or both working, one working, or none working in that pair. This relay will throw an OBDII code per pair of bad plugs. It knows if you have lost only 1 but can only tell you which pair has the problem.

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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