No start with 1 bad glow plug?
 

My glow plug light just quit working. I tested the relay, its good, then the plugs for continuity, and the number 1 plug is no good. The rest check out and have 12v when glowing. BUT, when I try to start after a complete (30 sec..until the relay clicks off) glow cycle, it just cranks. Takes a long time to fire. When all 5 plugs were working, it would glow for 10-20 secs and start without very much trouble. Wouldn't fire right off, but it would start on the first attempt.

So my question is, will 1 bad glow plug really make that much difference? Is it that hard to start when 4 plugs are good?

Thanks,
-Geoff

EDIT
2nd question: I'm about to order glow plugs. Should I replace all of them at once or just the bad one (what's commonly done)?

I don't think so, however, combined with any other "faults" weak battery, old starter, may give you problems, in cold weather.
When one of mine failed, I replaced all. I bought the "kit" that included a new relay, just in case, you understand!
Depending on where you order from, you may get free shipping.
Yet, see attachment. This is from Star magazine, Sept/Oct 2004.
Your call.

I have heard of GPs testing OK with an ohm meter and still not getting hot. The only real way to know is to remove them and apply 12V through jumper cables or one of those "jumper packs" and see if the tip glows bright orange. Of course, once you go to the trouble of removing them you might as well replace them but it would be interesting to know if your old ones were really working or not. BTW - They should show around .6 ohms to be considered "good" so just getting a reading doesn't mean they are good. What were the actual resistances on each one?

The good ones showed between 0.6 and 1 ohm. My negative lead was on a part of the body near the relay instead of the battery, so I figured that might add a little to the resistance. The bad one showed infinity or overload or something.

Incidentally, I've been trying to start it this morning (forgot to plug in the heater last night :mad:) and I've got a battery charger/jumper connected to help it out, and now my glow plug light is working, but it sure doesn't seem like the plugs are helping. It just cranks and cranks. I'm sure I'm putting a toll on the battery, but I never had any problems cold starting before.

What turns on the glowplug light? I assume its the relay, but how does it decide to turn on the light?

I've got a kit with the 5 plugs and a relay ordered from fastlane now ;)

Let's run down the basic list again..............
Good battery
Properly adjusted valves
Glow plugs in good shape
Diesels use compression to fire, are you using a good quality engine oil?
Once the glow light is off, the relay is off, the glow plugs are off. In other words, crank the engine about halfway through the glow cycle.

What kind of cold weather are you experiencing? How many miles are on your car? Do you have good compression?

What turns on the glowplug light? I assume its the relay, but how does it decide to turn on the light?

There is a switch on the thermostat housing that senses coolant tempature and will adjust the amount of time the relay sends current to the glow plugs.

I I have heard of GPs testing OK with an ohm meter and still not getting hot. The only real way to know is to remove them and apply 12V through jumper cables or one of those "jumper packs" and see if the tip glows bright orange.

Take a look here. http://www.dieselgiant.com/glowplugrepair.htm
The page takes a few minutes to load, you may have to reload it a few times.

Hmmm...actually this is not factually correct. The GPs will continue to glow until you turn the key to the start position - at least they do with the cars not equipped with "afterglow" - on those they continue to glow even after you start. If you don't believe me put a Voltmeter on your GPs and glow...you will see they continue to glow after the light goes out but stop once the starter is engaged. So, you should not start the car until the light goes out...in fact, count to 15-30 or so after that time before cranking when it is really cold to give it extra glow...

Once and for all lol -

Do they go off when you START cranking?

Or do they go off when you STOP cranking and the engine is running independently of the starter?

It's important :D

Good question - IIRC on my 300SD they went off when the starter was engaged. They probably do this so 100% of the power goes to the starter while it is cranking.

It is possible that different models did different things too but they definitely stay on after the light goes out though which was what I was trying to correct. The light is nothing more than a "guide" to help the driver know when to start cranking but if you read the manual I think it says something to the effect that in extra cold conditions you should do two cycles before attempting to crank.

Correct. Unless the vehicle has an "afterglow" relay, the plugs shutdown instantly when the key is turned to the start position.

Thanks!

It was for this reason the other day that I stated that I waited until the relay shut off and then immediately attempted to start the car. I figured (accurately, it seems) that the heat shut off when you hit the starter regardless, therefore waiting for the relay to trip out and THEN cranking had only the effect of ensuring the maximum amount of heat in the cylinder for that cranking attempt.

Somebody was trying to say that the plugs kept running while you cranked, in which case my attempt was in error... thanks for the confirmation.

Someday I'll have the ambition to get out there with a volt meter and figure these things out - but unplugging cold plastic connectors this time of year when they're 21 years old seems to attract problems for me. Half the connectors are falling apart already. :rolleyes:

Glow Plugs
 

1984 300d Turbo 147k
You Have To Ream The Glow Plug Holes To Take The Carbon Out And
See When Your Inside Light Goes Bright Your Glow Plugs Are Done.
Its Time To Start. **************.com Have Reamer Kits With Guide. And A New Battery.autozone $90.00 Plus Core

Okay, on Friday when I first saw the glow plug light problem, I had one plug test bad.

Today, I tried and tried to start with no luck (on Friday it started after a bit of cranking). I tested the plugs again EVERY ONE was bad. I think I'm going to try to increase my shipping to overnight on my new plug kit.

What kind of luck is that?

And what about this reamer kit? Is it a tap to clean the threads?

That would basically be impossible.

To suffer four bad plugs in a space of three days is one in one million.

Furthermore, the data on Friday suggests that more than one glow plug was non-functional...........because the engine refused to start. One bad plug cannot prevent a start.

I would revisit the test procedure and seek out any possible error that caused false data on the initial test on Friday.

My car certainly lets me know when I have one bad plug with temps around freezing. It will go from starting beautifully in less than two seconds and immediately running well to taking about five and being one cranky beast for the first 15 seconds or so. It does, however always start. Check to make sure you have actual voltage on the plugs and if you haven't already placed your order, toss in a few of the 80 amp strip fuses. They can appear good when really they have tiny cracks that will impede current flow.

How are you testing them? I suggest unplugging the relay plug and testing them directly on each plug from the center post to the body of the plug. Something is wrong if you went from 4 being good to none being good. Check your ohm meter too to make sure it is on (don't ask me how I know this can be the problem).