My weird to me glow plug situation . . .
 

Howdy,

On my 82 240D, it has been starting harder lately with temps in the high 50's in the morning . . .

Usually the glow plug light does not come on, so I still leave it in that position while I hook up my seatbelt, then I crank it, for 3-5 sec it will start but run rough, and in about 5-8 sec the last cyl starts firing, and will smooth out, cloud of smoke out the back . . .

I went to Diesel Giant website, and followed his tutorial (well almost! [I did not remove the plugs]) I have 13.7v engine running, 12.8 engine off.
Battery voltage on both sides of fuse strip.
Cyl 1,2,3 good continuity(.001), #4 bad (doesn't move needle!) [I just tested between the wire conn on plug to ground]
I tested the relay . . .I get power to all five pins, and in about 20 sec shuts off.
When I tested the wires it appears that the front three are hooked together! For instance with a lead on #4 glow plug, and I probe #4 socket in the relay harness, that is the only one that will work! BUT if I move to glow plug #3, #2, or #1 I can probe either #1,#2, or #3 socket in the relay harness and get a reading. What is going on?

I know I need a new plug for #4 (I'll buy four new new ones, and keep the old 3 for emergency!) But what else is going on? Maybe only 5% of the time will the glow plug light come on, and only stays on for maybe 5 sec, and still starts hard. Could it be the fuse?

It also appears that maybe somebody at some time replaced a relay from a 617?

http://i153.photobucket.com/albums/s...owplugs001.jpg

http://i153.photobucket.com/albums/s...owplugs002.jpg

Edit to add: All of the wires had full continuity from relay plug harness to glow plugs.

I believe the inconsistent power readings between #1 and the rest are a result of the fact that the self-test feature of the relay compares #1 to the rest. I wouldn't worry about it. Get new plugs, put them in and your problem should be solved.

OK, I was gonna put in an order, but wanted to know if I should get anything else . . .

Thanks!

It took me a couple times of changing out glow plugs before I learned how to properly test resistance on them.

If your light is not coming on, something is amiss. The most likely cause of that is bad glow plugs.

The easiest way to test the glow plugs (for me) is to set it to ohms and put one probe on the very back of the plug and the other probe as close to the engine block as possible.

Even if they all test good, you should pull them as they can test good and be covered with carbon on the tips and not preforming well. Test them with a battery charger/jumper to really see how the old ones are performing.

Your relay is good, so glow plugs are probably the culprit.

I believe all relays are five cylinder capable.

I tested them with one probe on the end where the wire hooks on, and the other was the stud for the valve cover . . . seems funny that it would start hard if only one plug is bad, I plan on reaming them out when I have it apart . . .

I already ordered 4 new ones from Phil + some other stuff that was on the "list":D

If the glow plug light comes on normally does that necessarily indicate that all the glow plugs are working? Or can it light even with a bad plug?

I don't know . . .mine rarely comes on anymore, been starting hard since it doesn't come on . . .

Yes, it's possible for the glow plug light to function and have bad glow plugs. The system compares #1 draw with the rest. I think #1 can go out and the light will still work. It also might be possible that the light works when all the plugs are bad.
A resistance test is not definitive. A plug can still be bad. I've had a plug show good resistance when cold but when current is applied and it warms up, the resistance goes off the chart. The only definitive test is to put 12 volts to them and see if the tip glows as rosy as a known good one. Weak plugs will glow, but not as hot as new ones.

Happy glow plug difficulty season everyone!:party2:

Party on! :vbac47679

My light worked and I had two bad plugs. #3 and #5. But I was having rough starting so I pulled them and replaced them. Then the next week the light stopped working. #1 turned out to be bad. So I pulled it and reamed out all the holes and put the two good ones back in along with 3 new ones.

So it appears GP #1 is the one that causes the light to go out...at least in my case.

I had just installed the new afterglow relay (it was cheaper then the regular one when mine relay went bad). The plugs were about a year old. We'll see how well I like the afterglow now that the cold weather is here.

Yep, that was the case on one of mine. It tested fine, but the tip had a carbon build up and wasn't glowing that well. I tested it by hooking it up to a battery jumper box.

Yes, at first I thought the same thing. But on these old diesels, the compression isn't the same as it was from the factory anymore. Each cylinder needs heat to start promptly. One GP out can indeed cause rough starting.

See my last post. I had two bad plugs with the light still working. Then the next week the light went out with one bad plug.

I was able to swap out all without removing the fuel lines, I did move that one hose over a bit and remove some linkage on the VCV and only dropped two of the nuts!
My dash light stopped working when only #1 tested bad. I replaced 'em all, I was bored. Did not ream or use anit-seize compound. Starts and runs well.

I've never had luck testing the resistance to tell if they were any good. I always pull them and give'm 12v on the bench and see if they glow bright orange or not. NOTE: they get REALLY hot so be carefull when handling them after glowing.

Usually a faulty GP light is the sign of bad glow plugs. I'd pull each one and bench test it.