One of my favorite "drivers" is a 1982 Mercedes 300D, a 5-cylinder diesel that I've owned for 10 years.  We are the third owners and have put many of the 430,000 miles that are on the car.  The 300Ds were made between 1975-1985.  A turbo was added in 1981 providing more power and a higher rear end gear ratio, giving it more pep in town and the ability to cruise easily at highway speeds.  It's a car that you either love or hate, and it does have its little idiosyncrasies, such as needing a block heater in the winter, and its unique way of starting consisting of turning on the ignition key and waiting until the dash indicator light goes out before cranking the car.  There is not a lot of maintenance, but valve adjustments are necessary every 5,000 miles, and the two fuel filters must be replaced twice a year, requiring bleeding the system with an inline hand pump.  If these things are routinely done, you can depend on the car for many, many miles of service.

        It is important to test the car's glow plug system occasionally.  A faulty plug will cause the engine to crank, but not start.  



        The fuel/air mixture in a diesel engine self-ignites as it is sprayed into a hot, highly-compressed combustion chamber.  Since this high compression and heat are not present when starting a cold engine, a pre-glow system using glow plugs in the combustion chamber is used to start ignition.  When the ignition switch is turned on, but not cranked and the engine is cold, voltage is sent from the battery through a relay to the glow plugs, igniting the fuel/air mixture until the engine reaches normal operating temperature.  Our 300D has pencil type glow plugs that are wired in parallel so that if one fails, the others can still function.


We first checked the battery.  These cars require a large, 1000 cold cranking amps battery to turn over the high-compression diesel engine.  Using a multimeter set to DC Volts, our battery showed 12.84 volts with the key offan excellent reading.  Next was to check the glow plug relaya 2 1/2" x 3 1/2" black plastic box located on the left side fender well just behind the headlight.  The cover just snaps on and off exposing the wiring.  With the cover off, we located the 80-amp silver strip fuse at the top of the box.  The red wire is the incoming voltage wire.  The smaller wire is from the ignition and the multi wire connector (black plug) that provides power to each glow plug.  With our multimeter still set on DC Volts, we checked each side of the fuse strip, running the multimeter ground test lead to battery ground (this is a negative ground car) and the positive lead to one side of the fuse and then the other.  Our voltage was 12.84 (the same as the battery).  This showed that the fuse was good.  Then I checked the voltage into the relay (the screw on the right, under the fuse strip) by keeping my ground and touching the screw with the positive lead.  Again, it was 12.84 volts.  To test the connector (this supplies the glow plugs with voltage) I unplugged it from the five pins, checking the pins and connector holes for corrosion.  The pins tested at 12.69 volts, which is okay.

        Next was to test the wiring from the connector to the glow plugs for continuity, to make sure we didn't have any broken wires.  For continuity, the multimeter is set to Ohms at the 200 value.  To test the meter itself, I touched the (-) and (+) leads together, which should show zero.  If it doesn't, the meter's calibration is off.  Some meters can be adjusted back to zero.  Mine is a low-buck digital and can't be adjusted.  When the leads are touched together it shows .6 ohms, so I have to correct my test readings by subtracting the .6 reading I got by touching the two leads together.  (For example, if my lead touch corrected value test shows 1.0, I subtract .6, giving a corrected reading of .4).  With the ignition ON and the meter set to Ohms at the 200 value, we tested each connector socket hole by inserting the positive lead into each hole, which are numbered indicating which glow plug each represents.  For each socket the reading was 1.1, or .5 corrected.  All wires should test at 1 ohms or less.  Bad readings would be infinity, indicating an open circuit, or zero ohms indicating an internal short in the circuit.  All of ours were good.  This test was made with the glow plugs in the engine block, so the plugs and wires were being tested as a unit.  Next we tested the wires and plugs separately.

        To test, we first disconnected the battery's negative cable and removed each individual plug from the block.  This takes time and can be done without removing any lines, linkages, or anything else around them.  I used a set of metric ratchet wrenches, 1/4" deep socket set, a swivel extension and a sheet of plastic under the car to keep the glow plug wire nuts from getting lost when they fall.  A mechanic's magnet also helps.  With all the glow plugs out and disconnected from the wiring, I checked each wire for continuity by placing one probe in each of the harness sockets and the other end on the glow plug end where the wire connects.  All wires showed 1.0 ohms (or .4 corrected).  All is good!  We then checked each glow plug.  With the multimeter still on Ohms, I put one lead on the body of the plug and one lead on the plug's wire connector end.  The corrected reading was .5, indicating good plugs (New Bosch plugs show .4).

        As a final glow plug test, I connected a battery charger (12V), the negative clip to the plug body and the positive lead to the wire connector end.  The glow plug end should glow cherry red, so be sure to position the plug away from grease, fuel or your skin, and be aware that it takes time to cool downtouching it could result in a serious burn!  All the plugs showed around .5 ohms corrected.  The true condition of a glow plug can only be known if both tests (battery and continuity) are done.  It is possible to have a weak plug that will still glow with the battery test, but weak plugs won't start the car.

        As a final test, we checked the voltage at the relay to the plugs.  With the key ON, and the multimeter set to DC voltage, the negative lead to the battery's negative post, we checked each of the five pins in the relay with the positive lead.  They all read battery voltage of 12.84.  The relay only stays on about 20 seconds, so you have to test the pins quickly, or turn the key off, then back on to continue testing.  All of our pins tested good, and there were no problems found in the system.

        If a diesel will crank but not start, the usual reason is bad glow plugs.  However, low compression, valve adjustment and timing can also cause the same problem.  Owning one of these old diesels can be a lot of fun, but it's really a car for people who like to do their own work.  There are fewer and fewer shops that work on them, so buying a Mercedes and a Haynes motors manual is a must!   Making friends with other owners and visiting internet forums is also helpful.  Everybody wants to offer help, it seems.  When I'm driving my 300D and pass a fellow 300D driver, we always give each other a thumbs up!

        With each project I am always pleased and amazed at how helpful our car community is to provide knowledge, help and support in keeping our loved cars on the road.  Keep 'em driving!