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Old 12-02-2001, 04:45 PM
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Question Glow plug(s) or pre-glow timer relay problem?

The glow plug light stopped working a couple days ago, but the car still starts (hard) after waiting for the normal several seconds.
So yesterday after doing a search on here and reading the Haynes manual, I went to check the glow plugs. The Haynes manual and previous posts said to unplug the plug connector at the relay, connect the ohmmeter to ground and each of the numbered glow plug terminals. I should get a 12V or infinity reading, but I got zero for all five . I checked the strip fuse and it has current on both sides. Just for the hell of it I connected each of the terminals to one of the screws that holds the strip fuse in place and got readings for all but one of the terminals (middle one of the left row). With the glow plug connecter at the relay unplugged the glow plug light goes on and off like nothing is wrong. Is this normal? I didn't try to start the car with the connecter unplugged figuring it wouldn't work. Any help would be greatly appreciated .
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Old 12-02-2001, 04:58 PM
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We can help you better if we know what year and model you are working on.

Have a great day,
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Old 12-02-2001, 09:02 PM
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Sorry about that. It's an 1982 300D Turbo.
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Old 12-03-2001, 08:36 AM
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Thanks. This is a pin type glow plug engine. In many cases, even though the plugs continuity checks okay, they still are bad.

I would suggest that you replace all five glow plugs. In most cases this will make all the difference in the world. While you have them all out, check your harness by checking continuity between the disconnected plug at the relay to each of the eye terminals while disconnected from the plug. There should be continuity between each of the eye terminals and only one of the pins at the plug.

Good luck,
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Old 12-04-2001, 07:18 PM
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Thanks for the reply Larry. I'm getting the glow plugs tomorrow. I sure hope it's not the relay that needs to be replaced.
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Old 12-05-2001, 09:01 AM
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From my experience, odds are heavily in your favor. If you replace all glow plugs, and you have good compression it will start like a new car.

Good luck,
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Old 12-05-2001, 09:13 AM
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Changing all the plugs definately fixed my cold start problems. If you haven't, be sure to do a search on how-to re: changing plugs. You just about have to take off the metal fuel lines to get at the little devils and be prepared for some tedious extraction. Use 10W40 to smooth the way.

There is a special tool that helps with removal of the fuel line (if you have a real good friend that owns a mercedes shop).

This project can be done by most anyone as I succeeded.

'85 300CD
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Old 12-05-2001, 09:35 AM
Registered Diesel Burner
Join Date: Aug 2001
Location: Northern Virginia
Posts: 2,911
I was able to narrow a glow plug problem down to the specific component using some of the following steps. I was not sure if the problem was the relay or glow plug(s).

1) Remove the cover on the glow plug relay connectors.

2) Visually examine the fuse strip. In my case I actually removed it for inspection - probably not necessary.

3) Verify battery voltage at the incoming heavy battery line to the glow plug relay using a multimeter. This happens to be one side of the fuse strip. Also verify battery voltage at the terminal on the other side of the fuse strip (helps check electrical continuity of fuse strip).

4) Pull the larger plug connector from the glow plug relay. The wires in this connector go individually to all your glow plugs (talking about the newer pin/parallel systems).

5) Turn the key switch over to ON as if you were warming the glow plugs. You should now have battery voltage at each terminal on the glow plug relay that you have exposed by removing the connector in step 4). Voltage should be removed and you should hear a click after the glow plug relay times out.

6) Inside the connector, each wire going to the individual glow plugs should show a low ohm connection. (set meter to read resistance, of course).

7) Here's the harder part, because it requires a DC ammeter that can handle some current. I happen to have one that can handle 20 Amps. (If you don't have this capability, I'll describe a seat-of-the-pants alternative approach.) Measure the current that each glow plug draws by connecting the DC ammeter between the battery supply line on the glow plug relay and the individual wire connection inside the removed plug. Each glow plug should settle down into a current draw somewhere around 10 amps.
[You might also want to run this test using the glow plug relay and the appropriate pin the glow plug connects to - in order to insure the glow plug relay is able to handle the current load.]

8) A less precise alternative to 7) above is to use a piece of wire and visually gauge the spark drawn when connecting the glow plug. [You have to be careful doing this, and must absolutely know what you are connecting - you don't want to mistakenly ground the battery supply through a small wire - the glow plug has to be in the circuit.]

I had one glow plug that drew considerably less current than all the rest. It also seemed to heat only halfway up the length of the plug - not at the tip of the plug. It drew less than 2 amps and visually produced a much fainter spark when connecting to the battery supply.

I suppose if you don't have a multimeter, you could use a 12-volt bulb to test the glow plug relay output voltage.

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