Mercedes Diesel Glow Plug Repair

Mercedes Diesel Glow Plug Repair

So you turn the key expecting the glow plug light to illuminate, and nothing happens. So you turn the key again and again, hoping it was just a glitch. But still no light. Well you decide to start the car anyway, hoping the light bulb is just burned out. You crank and crank and crank, and if you have decent compression and its not too cold outside, then maybe your car starts. It blows out some smoke and runs rough for a bit, then settles down to a normal idle. What the heck just happened? Is my car dead? Will it cost a weeks pay to fix? Fear not, its not as bad as it seems.

The glow plug circuit seems to be a mystery, but really is not all that complicated once you understand it. In a nut shell the glow plugs are tiny cylinder heaters that help the car to start when its cold. A car with 400psi of compression or more, will start with relative ease, even with a few bad glow plugs. Unless your engine is new, then the chances of your compression being that high is rare. The lower the compression in the cylinder, the less heat is generated by the piston compressing the air. Hence, the need for glow plugs. They take electrical current and heat the glow plug element, giving the engine a little help when its cool.

Most of the Mercedes diesels have similar glow plug circuits and testing them is mostly all the same. Learn the basics and you can trouble shoot like a $100/hr Mercedes tech.

The following car was in my shop for a "no glow plug light" complaint. The car was also hard to start and made too much smoke when cold. It started great when warm, but the glow plug light never came on. The car in question is a 1986 Mercedes 300SDL. Its Mercedes OM603 6 cylinder diesel engine. The trouble shooting we will do, also applies to the 4cyl and 5cyl diesel with the pencil style glow plug.

Please remember to recycle all your used fluids at an appropriate recycling center. Be mindful to not spill or splash fluids on yourself, others or the ground. Also as a safety tip please remember anytime you are working on, around or under your car, to wear safety glasses and secure the car with wheel stops and approved jack stands!

First please test you battery with your multimeter. You need at least 12.5 volts with the car off and 13 volts plus when running. If this is not the case take care of the battery and alternator problem before going any further. The glow plugs like a lot of juice, and often wont work right if the voltage is low.

Now locate the glow plug relay. It is located on the driver side fender well behind the headlight, A large black box with several wires going into it and a removable top cover.

Remove the top cover and it should look similar to the pic. The large red wire is the main voltage coming into the relay. The silver bridge is a strip fuse. The smaller connector is from the ignition and the larger connector is the power going to the wires that actually go to each glow plug.

Remove the large connector and get your multimeter. Look inside the connector and on the pins for rust or corrosion. Note, there is a number beside each hole of the connector, corresponding to the glow plug cylinder. First we will test the glow plug circuit for continuity.

My meter reads .3 ohms just within the probe wires of itself. So when testing something to get its try resistance value, I subtract .3 from the reading. So in reality the new glow plug is reading .6 ohms which is fine. A more expensive meter may be able to zero out the resistance of its wires and circuitry, but not mine.

Take the negative terminal of your multimeter and connect it to the battery negative. Then take your positive probe and test each socket in the connector. As can be seen here the number 1 cylinder wire and glow plug read .9ohms or .6 ohms without the meter resistance. That is ok. Remember, at this point we are testing the glow plug and the wire to the plug.

Here we test the number 3 cylinder and get 55.3 ohms. Not good. That is way to high. I like the circuit to be around 1 ohms or less. So mark this cylinder for further testing. At this point we don't know if its the wire, the plug or both.

The rest of the cylinders test this value or open. The wire, the plug or both is bad. Chances are it the glow plug, as the wires rarely go bad, unless they broke etc.

Now let test the relay for voltage. To check for voltage, test the large gauge wire coming into the relay. Should have battery voltage and it tests out ok.

No lets test each side of the strip fuse. Both side should read battery voltage. The strip fuse often gets hairline cracks that are undetectable. If the fuse looks suspect, then replace it.

At this point I know all the cylinders have a problem except the number one cylinder. So lets isolate whether the glow plug or wire is bad. On the 4cyl and 5cyl diesels, the glow plugs are easy to get to. On the 6cyl OM603 they are a royal pain. Most people remove the intake to get to them, but I made good use of a 1/4" drive set and u-joint to manipulate getting to the plug. I was able to remove all 6 without taking off the intake. Exercise patience!

So after removing the number 2 glow plug, I will check for continuity. One end of the probe goes on the end of the glow plug wire, the other end goes into the glow plug harness socket. They are numbered at the socket, so make sure you are testing the correct one. As we see the value is .4 ohms or really .1ohms corrected. That is just fine. So we know the wiring is good on number 2.

Now we will check the glow plug itself. One end on the body of the glow plug, the other end on the tip where the wire was. The reading is infinite. This means the glow plug is open internally. If the reading was 0 ohms then it would mean the plug was shorted internally. No amount of power will make it glow. One note is that the only glow plug that worked was the German plug in the number one cylinder. Somebody in the past had put Autolite plugs in the other 5 cylinder and they were ALL bad. Use quality OEM made parts on these cars!

I also like to do a real power test on each plug. I took a jump box and put ground to the body and power to the tip. As you can see the plug does nothing. If it was good it would glow red hot on the tip.

Here is the number one glow plug. Its still good at .9 ohms or .6 corrected, but I will replace it since I have it out.

The good plug is tested on the jump box and as you can see it glows at the tip. Now its possible to have a plug glow at the tip and still not work on your car. I always do a continuity check on the plug to determine if its still serviceable.

Now lets see if the relay is doing its job. Switch back to testing volts and hook up the negative lead on the negative battery terminal, the positive goes on one of the pins. When you turn the ignition key to the on position, you should have battery voltage at each pin. The relay will stay on for about 20 seconds, so work fast and touch the positive probe to each pin. You should have voltage at each pin.

So at this point we know the relay is doing its job properly. The next step will be to install 6 new glow plugs and retest the system.

After acquiring the 6 new glow plugs I needed (OEM Made of course) I tested each of them with the meter to ensure they were all good. It is possible to get a bad one. One important point to remember when doing an ohm test. The value is .6 ohms corrected.

Since its nearly impossible to get your hand anywhere near the glow plug hole to start the glow plugs (since I did not remove the intake manifold). I used an old trick from my spark plug installation days. Get a piece of fuel line, insert over the tip to use in installing the glow plug. You don't run the risk of stripping the threads and makes the job a whole lot easier. I installed all 6 plugs and their power wires.

This is now what happens when I turn the key. Hurray! The car now starts up instantly and runs perfect when cold. The glow plug light will go off in about 10 seconds or so depending on temp. This job saved lots of $$ by doing it yourself and paying yourself the labor. It only took a couple of hours total time, a few sockets and a little patience. But knowing the job was done right, it worth every minute I spent on it.