Well my 240D glow plugs failed this morning - PeachParts Mercedes-Benz Forum

pj67coll · #1 02-21-2015, 08:51 PM

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.

BillGrissom · #2 02-21-2015, 09:01 PM

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.

pj67coll · #3 02-21-2015, 09:31 PM

Quote:

Originally Posted by BillGrissom

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.

BWhitmore · #4 02-21-2015, 09:35 PM

Free with a coupon from their ad.

pj67coll · #5 02-21-2015, 10:10 PM

Quote:

Originally Posted by BWhitmore

Free with a coupon from their ad.

Might head down there tomorrow.

- Peter.

Zacharias · #6 02-21-2015, 10:23 PM

Quote:

Originally Posted by pj67coll

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

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/

Quote:

Originally Posted by pj67coll

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

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.

pj67coll · #7 02-21-2015, 10:38 PM

Quote:

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.

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

- Peter.

Zacharias · #8 02-22-2015, 12:16 AM

Quote:

Originally Posted by pj67coll

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

- Peter.

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

Stretch · #9 02-22-2015, 02:11 AM

Here you go Peter

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

pj67coll · #10 02-22-2015, 02:26 AM

Thanks folks. Stretch especially. Hopefully I'll get it all sorted out tomorrow...

- Peter.

Skid Row Joe · #11 02-22-2015, 06:39 AM

...

gatorblue92 · #12 02-22-2015, 07:32 AM

I would check the strip fuse before I do anything else. I think it is right by the glow relay on the inner fender on the later cars.

funola · #13 02-22-2015, 09:43 AM

Quote:

Originally Posted by Zacharias

I.........
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.

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.

toomany MBZ · #14 02-22-2015, 10:49 AM

The glow plug relay is highlighted.

VT220D · #15 02-22-2015, 01:17 PM

Quote:

Originally Posted by funola

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.

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.