Glow plug normalness?
 

I was just troubleshooting the glow plugs in the '79 240D I bought a few days ago. The seller said the "glow plug light never goes off" (before starting) and I had given it around 30 seconds, and indeed it never went off in that time before starting the car. It does go off when I hit the starter.

Anyway, in the course of testing the voltage, the light finally did go off after about 60 seconds. This seems like a long time -- I am in Southern California and the temp is usually anywhere from 55-105. Or is 60 seconds (maybe 45 seconds) normal?

The voltage I measured at the glow plug fuse and at both #1 and #4 plugs was 10.7 volts. Is this low? I'm wondering if I need a new battery. The battery looks pretty old (and it is a correct white Mercedes-Benz brand battery!). Voltage at battery with car off is 12.75. Once the light went off I could also touch the end of the plugs with my fingers. They were warm to the touch, but not hot. Not sure what's normal... they certainly never "glowed".

Oh and there's been a veg oil conversion on this so there is an electric fuel pump energized along with the glow plugs.

Thanks!

*I don't have any first-hand familiarity with this particular car.*

That said, it definitely sounds like low current capacity in your battery could be part of the problem. Take a look at the battery voltage while you're glowing and see how far the plugs pull it down.

**EDIT: Just re-read and you already posted 10.7 volts with the plugs running, my mistake. Not sure if that's low or not, it may be alright.**

Glowplugs that are cool enough to touch with the fingers immediately after glowing them are probably not functioning up to par at all. My guess is that the glow plug light goes out when a sensor detects that the area heated by the plugs has reached a certain temperature... IF this is the case, then the finger-touchable glow plugs definitely aren't getting as hot as they should be. That said, it would forseeably take sixty seconds to get the sensor happy with the temperature if the glowplugs aren't making enough heat to start with. This could either be caused by dead glowplugs or a low battery issue.

That's my guesses, but I've never messed with any of the diesels except our newer one, so don't take any of that too seriously.

I don't know how much current an electric fuel pump draws, but the glowplugs themselves need quite a hefty amount. With that much engaging all at once, a weak battery could easily be failing to supply enough current to keep all those components happy at once.

ignore the light on the dash, it's useless.
45 seconds is correct time.
are you touching the tip of the glow plug out of the car, or the screw terminal on the glow plug that's in the head of the car? glow plug out in your hand, and warm to the touch, bad plug. wire terminal warm to touch after warmup, normal.

I was just touching the terminals (while glow plug installed in engine).

I'm surprised these glow plugs take so long. Growing up we had a 1984 Chevy Suburban diesel and the glow plugs never took more than 10-20 seconds, and that was in the middle of northern Illinois winter.

they do not require that much time, it's just the time available to you.
normally unless it's really cold out, 5 to 10 seconds is plenty to start the car, and the plugs shut off when you start.
low compression will require longer glow time.

Be careful with the glowplug light never going out though. I've had this problem and decided to ignore it. After a day I couldn't start the car - battery drained. Brought it over to a mechanic (before I started working on car myself) - he claimed that the alternator was burned out and I didn't need to worry about the relay.

He replaced the alternator and battery and off I go - guess what - next day I can't start the car again.

So he had to replace the glow plug relay, alternator and battery again.

I guess, the glow plug relay will pump insane amount of electricity to glowplugs to get the hot and is supposed to shut off after a certain period of time. And if the relay is bad and constantly pumping amps to glow plugs the current drain will be too much for the alt. and battery.

My $.02

Thanks for the tip -- next time I have the engine running I will test for voltage at the glow plug fuse to make sure they are turning off.

My Glow Plug Relay operates normally and with the key off I get voltages as stated in the pic.

http://i242.photobucket.com/albums/f...el911/ZZZ7.jpg

If you want to see if your Glow Plugs are on constantly pull the big connector on the left of the pic and check the voltage at the pins (+ probe on the pins and - probe to a good ground) that are sticking up. After your Glow Plug Regulator has gone through its cycle there should be no voltage at the connector pins.

I don't have that relay. My '79 has the earlier "silver box" relay under the driver's dash.

Hi, that was a good picture above. I am having issues starting my '83 300d also. I checked the voltage at the locations above, and I got 0 volts with key on and off, it didn't change. What does this mean? Do I have a bad relay, or fuse, or what? I also pulled the cap off the glow plug pins and didn't have any voltage from any of them to ground,(chasis).

Am I checking voltage in correct place?

Help and Thanks,

Bob

The terminals on the GP relay (where you measured zero volts) get their power directly from the battery via a heavy (4mm) wire – no switches or relays or anything to get in the way. If you are sure that you were measuring from the relay to a good ground, like the battery negative terminal, and that your voltmeter is good and all that, then that heavy wire may have a break in it somewhere.

That is an unusual failure so I would next check the heavy flexible ground wire from the engine block to the chassis of the car. It should be on the right (passenger) side, under the car at the rear of the engine.

If the ground is good, you could next try a temporary direct feed from the battery to the glow plug terminal – you could even use a jumper cable – and with key ON, connect battery positive to the big terminal in the GP relay. You should get a spark due to the large amount of current drawn by the glow plugs and, after 10 seconds or so, the engine should start easily.

Jeremy