I had thought my glow plugs were bad on my 85 300SD even though they were only a few months old. So I put new Bosch plugs in. Still no go. I used a non contact thermometer, and the plugs are as cold as the air. But the light comes on as normal and I hear the relay shut off a while after the light goes out. So can I assume it's the relay? Is there a way to make sure? All the fuses, including the relay fuse, are fine.

Quick, no-tools check: leave the driver door open and turn the key. Does the cabin overhead light dim? The glow plugs draw about 10 amps each, so if they are funtioning they should dim the lamp.

I have 12 volt injection heaters installed, so the lights will dim when I turn the key regardless. I am going to check the voltage coming out of the relay tomorrow. I would have thought that if the relay is shot, that the dash light wouldn't work properly, but perhaps I am wrong.

What are injection heaters?

The easy way to test the glowplug relay is this:
Pull one of the glow plugs and hold it on a ground (with the + wire attached) while a helper turns on the key.

Or, if you have a volt-meter, just check the voltage across a glowplug wire to the block.

Injection heaters????? I gota keep track of this thread

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

The injection heaters are from fattywagons at www.fattywagon.com. Ok, I finally found my test light, and there is voltage to both the relay and the glow plugs. I am assuming the glow plugs are good since I just bought them last week, and there was no difference in starting from after putting them in than before. So, is it possible that the relay isn't putting out the proper amount of juice? I have a multimeter somewhere (sorry, I'm not very organized!) and I could check it, but what value should there be?

If there is voltage at the glow plugs, the relay is applying voltage to the plugs. The relay applies 12 volts, or not, nothing in between.

So, what are the plugs doing with the voltage, do they actually glow, or not? A resistance check, or removing them to apply 12v and watching for a glow, is about all that is left in the way of testing.

I had a relay issue, at least I thought I did. Anyway, turned out to be a missing ground wire behind the dash console (where the speedo etc are). It doesn't sound like that is your issue, but you never know, bad grounds can cause all sorts of weird behaviour.

The way I checked my relay was working was by

1. carefully touch the fuse that goes from the relay to the plugs after the ignition preglow is turned on. It should get hot.

2. If it doesn't get hot, take the relay out (There are only a few screws to get it out) and look inside. You'll see how it works. Remember to remove the negative cable from the battery when moving this stuff around so you don't short out something. Now reconnect everything and have someone turn the key while you watch that the contacts close. Also, you could put an ammeter to one of the glow plugs to check it is getting about 10amps. Be sure your multimeter can handle it before you do it though - mine can take 10amps and it showed about 10 amps to the one glow plug. You can test them all like this. Just cos you're getting 12v doesn't mean you're getting 10amps. You could also check the voltage across the battery when the preglow is turned on. It should drop noticeably.

That's my 2c.

Do you think it's possible that the injection line heaters are draining the battery and causing lower than usual amps? I will try disconnecting the lead to the heaters and see what happens.

There is a procedure described in the Haynes manual and mentioned countless times before on this board about how to properly check the glow plugs. Please do a search. Basically it involves checking the glow plug resistance with a multimeter through the relay connector that powers the plugs.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Cool idea.
8amps/100watts should not drain the power enough to hurt glowplug operation.

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Ok, I took a glow plug out, and it immediately started to smoke, so it's not the glow plugs. But while I was in there, I noticed the vac hose for the turbo overboost had come off. Would it be possible for that to make it hard to start? It seemed to run fine after it was warmed up. I guess I'll know the answer when I go to start it in the morning.

Adjust the valves, it makes a huge impact on starting and performance. Replace the fuel filters if they are old. Try giving it 1/4-1/2 throttle while cranking. Get a compression test (Or check for lots of blow-by right after you get it running).

I always give it a little extra fuel when starting. And yes, there is a lot of blowby, but it's been like that since I bought it. If an engine does have low compression, what is the proper way to remedy it? Is it a matter of rebuilding the whole engine? I don't think that is my problem though, becuase it started perfect up till about 2 weeks ago. I'm really hoping it's just because that hose was disconnected. I'll let you all know tomorrow.