Checking Glow Plugs With Test Light - PeachParts Mercedes-Benz Forum

dave300d · #1 06-03-2007, 06:48 PM

If I take the plug off the top of the black relay box. Take my test light and put the clamp on the positive of the battery. Then take the test light and touch each end on the plug that goes to the 5 glow plugs. If my light,lights up,does that mean the glow plugs are good or bad?

300SD81 · #2 06-03-2007, 06:52 PM

Doubt your light gives enough current to test a glow plug.. Maybe a couple 100W high beam bulbs in parallel could pass enough current..

Brian Carlton · #3 06-03-2007, 06:54 PM

Quote:

Originally Posted by dave300d

If I take the plug off the top of the black relay box. Take my test light and put the clamp on the positive of the battery. Then take the test light and touch each end on the plug that goes to the 5 glow plugs. If my light,lights up,does that mean the glow plugs are good or bad?

Meaningless in most cases.

You need to measure the resistance to ground from the sockets at the black relay box. Anything that's approx. 1 ohm is fine. Above 1.6 ohms or open requires replacement.

Even this is no gurarantee. The plugs may not heat properly when aged even with proper resistance.

guage · #4 06-03-2007, 06:55 PM

I clamp my light to the screw holding the fuse in the relay box.
Then pull the 5 or 6 pin connector from the relay and touch each female pin
if it lights plug is good!

barry123400 · #5 06-03-2007, 06:55 PM

It will indicate if any individual glow plug is burnt open. That is the most common failure. Your test light will just not light if the continuity of the circuit is not there.. I see nothing wrong with this as a quick test.

truckinik · #6 06-03-2007, 07:47 PM

G.P.'s are so cheap, why would anyone bother to check them...? Like spark plugs, if the thing seems to be having an G.P. issue, I just assume throw a new set at the thing, and be done with it...

dannym · #7 06-03-2007, 09:11 PM

Quote:

Originally Posted by truckinik

G.P.'s are so cheap, why would anyone bother to check them...? Like spark plugs, if the thing seems to be having an G.P. issue, I just assume throw a new set at the thing, and be done with it...

Well said, from someone making 200k a year.
Me, I test my plugs and only replace them when they go bad.
I don't see any problem with testing them with continuity. Especially if you don't have a meter handy.

If they test good and the car starts properly then your good to go.

Are you having starting issues??

danny

jack avant · #8 06-03-2007, 09:33 PM

I check mine with an amp meter. Get a piece of wire with a male end plug it in to each female thread it through the amp meter and put other end to the 12 volt post at the relay and read amps. No amps means bad glow plug. Hope this helps.

truckinik · #9 06-03-2007, 10:05 PM

Quote:

Originally Posted by dannym

Well said, from someone making 200k a year.
Me, I test my plugs and only replace them when they go bad.
I don't see any problem with testing them with continuity. Especially if you don't have a meter handy.

If they test good and the car starts properly then your good to go.

Are you having starting issues??

danny

Yes, I'm having starting issues with one of my 79 300CD's. I replaced all of the G.P.'s, and still no luck. The G.P. light does not come on, and I know the bulb is good. They seem to be doing nothing for the car, as I need to use Ether, to get it started, once it cools down, or even sits for a little while.

I also believe that my fuel system is air-bound. I can start the car, run it a while, and shut it down, and it won't restart, if the car sits more than about ten minutes. I have tried to bleed the system, countless times, but aparently, something's leaking air. I r+r'd all fuel filters, and replaced a few short sections of fuel line, in the under-hood areas...

Where else is there rubber line, on this car's fuel system..?

After loosening the relief bolt, on the main fuel filter, and pumping away on the primer vigorously, until air bubbles stop coming out, and then tightening it back down. The car will display a brief moment of ZENN, and start. Once it sits a while again though, it's bust for fuel, or start.

I feel that among other things, the car has an air-bound fuel system, and a bad G.P. relay..

Until a few months ago, the car sat for 4.5 years, in the back yard, under trees. One day recently I walked up to the car, Popped the hood to drop in a battery, and turned the key. Amazingly, she started right up, and ran like a..aaa.. a swiss watch.

Problems did not occure, 'til the next day when I went to start it up, to warm it up, for the full, and complete fluids change, and new radiator, water pump, and thermostat..

2.5Turbo · #10 06-04-2007, 02:18 PM

Quote:

Originally Posted by truckinik

Yes, I'm having starting issues with one of my 79 300CD's. I replaced all of the G.P.'s, and still no luck. The G.P. light does not come on, and I know the bulb is good. They seem to be doing nothing for the car, as I need to use Ether, to get it started, once it cools down, or even sits for a little while.

I also believe that my fuel system is air-bound. I can start the car, run it a while, and shut it down, and it won't restart, if the car sits more than about ten minutes. I have tried to bleed the system, countless times, but aparently, something's leaking air. I r+r'd all fuel filters, and replaced a few short sections of fuel line, in the under-hood areas...

Where else is there rubber line, on this car's fuel system..?

After loosening the relief bolt, on the main fuel filter, and pumping away on the primer vigorously, until air bubbles stop coming out, and then tightening it back down. The car will display a brief moment of ZENN, and start. Once it sits a while again though, it's bust for fuel, or start.

I feel that among other things, the car has an air-bound fuel system, and a bad G.P. relay..

Until a few months ago, the car sat for 4.5 years, in the back yard, under trees. One day recently I walked up to the car, Popped the hood to drop in a battery, and turned the key. Amazingly, she started right up, and ran like a..aaa.. a swiss watch.

Problems did not occure, 'til the next day when I went to start it up, to warm it up, for the full, and complete fluids change, and new radiator, water pump, and thermostat..

Have you checked the strip fuse for damage/cracks?

dave300d · #11 06-04-2007, 09:10 PM

The test light worked great at finding the 2 bad glow plugs. Replaced them and light comes on now and starts great.