It never fails! Previously in a Driving Old Cars article, I wrote about cleaning up your driver. After doing this, I took the Mercedes 300D out for fuel. When I got back in the car, it wouldnt crank or start. After pushing it back out of the way, I turned on the headlights and had my wife watch them as I tried to start the car. They dimmed. The battery was dead.
Knowing that it is a NAPA commercial battery and only a little over a year old, I thought it must be the alternatorand it was. I called for a ride home to bring back another battery. When it was installed, the car started right up and drove trouble-free until I got home. Then out of habit, I tried to re-start it and it would crank but not start!
Having owned Mercedes diesels since 74 (a 190, 240, 2-300 Ds) you get to know them. First I checked the grounds. These are at the battery on the right side, and on the left side, a strap running from the engine block to lower body. They were fine. I put a volt meter on my battery (-) and (+), and got over 12 volts. I checked my relay. is what sends current to the glow plugs.) I used a test light
to see if the fuse was good. It gets 12V all the time from a heavy gauge wire that comes in on the right side of the fuse. The fuse is a thin strip of metal with 12 volts coming in on the right side and crossing to the left if it is good . Mine was good!
Next, I checked the middle (smaller) plug pins using a test light with the key on in firstposition. You only have 20-30 seconds then the relay cuts off power to the glow plugs. The test light didnt light up. I then checked the bottom (large) pins on the relay. There are 5 pins that supply voltage to the 5 glow plugs. They did not light the test light. So I had 12V coming in at the fuse, but 0V going out. I disconnected the battery. I found a tested relay online from a low-mileage car (cost $55.), put the relay on, reversing how it came out, and hooked up the battery. Got in and turned the key to heat up the glow plus and the 300D started! If your relay works
but the car still wont start, then its time to go to the glow plugs. Here is a previous article on glow plugs:
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 10,000 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. in parallel so that if one fails, the others can still function. TESTING THE GLOW PLUG SYSTEM:
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
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 con- tinuity, 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 toget
It is important to test the car's glow plug system occasionally. A faulty plug will cause the engine to crank, but not start.
GLOW PLUG SYSTEMDescription
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 which should show ze- ro. If it doesn't, the meter's calibration is off. Some meters can be adjusted back to zero. Mine is a low-buck dig- ital and can't be adjust- ed. 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 cor-
rected value test shows 1.0, I subtract .6, giving a corrected reading of .4). With the ignition OFF 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 with- out 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 mag- net also helps. With all the glow plugs out and discon- nected from the wiring, I checked each wire for conti- nuity by placing one probe in each of the harness sockets and the other end on the glow plug end where the wire con- nects. 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.
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 nega- tive lead to the battery's neg- ative 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, bad relay 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 visit- ing 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 sup-
port in keeping our loved
cars on the road. Keep 'em driving!