When I started my car this morning I noticed the glow plug light on the dash did not come on. Despite that it started immediately but ran rough so I turned up the idle then turned it back down to normal after about a minute. Went to a store and I noticed the glow plug worked when leaving the store but it has not come on since then. I've done a lot of driving around today so the engine has remained hot and had no starting problems for the rest of the day despite the glows not glowing.

Obviously I want to get this fixed but most of the threads on the forum seem to be about the older loop style plugs. Mine are the pencil style ones. Do the pencil style ones have the relay under the dash? Or is it the box in the engine bay just just behind the drivers headlight assembly?

I'm assuming from the fact that the glow didn't work, then did work the next time, then hasn't worked since that it's probably an electrical issue. What do you think?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

I don't speculate, I measure the resistance of each glow plug with a multimeter (free @ HF). Easiest at the output connector of the relay box, but if you can't find that just touch the end of each glow plug w/ the red probe. Hold the black probe on a good engine ground. Should be <1 ohm, but if you measure anything other than infinity it is probably fine.

There's a HF just down the road from me. Are you saying they give these things away?

When measuring these is that with the key turned on to the glow position?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Free with a coupon from their ad.

Might head down there tomorrow.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

I must have 30 of the things.....hey they give them away


Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

So I got the multimeter at HF and here's a pic of my relay.



I've measured 12.86 V across the battery terminals. 12.86 at the two screws that hold the fuse in. And 12.86 at the power terminal. Power connection. That's the large cable on the right. That cable does look a bit ratty though. Wondering if it may be the problem. But if so why would I be getting 12.86V when touching that silver metal goodie in the middle of it?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

If you go to the HF website, you should find a coupon you can print off that gets you one free or almost free.

Like this: http://www.dealsoff.com/images/multimeter-0301.jpg

More coupons in this list: http://www.dealsoff.com/harbor-freight-coupons/

No. You want to measure the resistance by putting the red lead into each female receptacle in the large plug at the end of the harness that runs from the relay to the glowplugs. The black lead goes to a good ground, preferably the negative battery terminal if the lead is long enough. The engine block is NOT a good ground.

Before anything else, check that you can hear the relay engage when you turn the key to the preglow position. If you don't hear it click, try tapping it on the side or on top. Also make sure that the strip fuse inside the glow relay is intact.

__________________


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

Is the glow relay under the dash on the 81 with the pencil style plugs or is it in the engine compartment?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

On a 1981, it should be on the driver side front inner fender.

__________________


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

Why is the engine block not a good ground? It is where the glow plugs are grounded so it better be a good ground. The engine block is grounded via the ground strap on the driver's side bell housing and if it is not making a good connection, it could affect glow plug operation.

BTW, measuring resistance is not polarity sensitive (unless you are measuring the resistance of a diode), where you put the red and black leads does not matter. You want to make sure power is off while measuring resistance otherwise the reading will be wrong.

__________________
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

The glow plug relay is highlighted.

Attached Thumbnails

 

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83 SD

84 CD

I think Zacharias meant that connecting the multimeter probe to the block can give a poor connection due to rust, dirt and oil. The engine ground does need to be good for the glow plugs to work properly. Since the starter draws about 250 amps I would expect to see problems cranking with a poor engine ground.

OP Because your glow light isn't coming on I suspect a strip fuse or glow control relay failure. If your strip fuse is blown your relay may be sticking. Even if every glow plug fails the relay will still operate and turn on the glow light for the appropriate amount of time.

If a single glow plug is bad the glow control relay will function normally and light the remaining plugs. However, after the car starts the glow light will flash. This is to warn you that a glow plug has failed. The control unit determines that a plug has failed by comparing the resistance of pairs of plugs. It can be fooled if both plugs in a pair are dead but that's uncommon.

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K

Good question. I tried using the block when I measured mine and got strange readings.

When I asked for advice here, I was told that only reliable ground was the negative cable. By several respondents.

When using a multimeter or test light on other cars I have always used the engine, or something attached to it, as a ground.

__________________


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

Here you go Peter

http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/615/15-510.pdf

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!