1973 450sl Stalling Problem

[R Ryan823]

My '73 450SL keeps stalling after about a half hour of driving. Sometimes after it stalls it will start right up. Other times I'm not as fortunate. I did a full tune-up (plugs,wires, etc) but still no luck. If I raise the idle RPM's to about 1,200 the problem goes away. I checked for any vacuum leaks. None found. My next step is to check the timing. I also saw in a previous thread that the idle control module might be the culprit. Before I spend a fortune on replacing that, I was wondering if anyone out there has had a similar problem and might know how to fix this. Any help would be appreciated.

[zeggate]

I have a similiar problem. Car starts up runs fine. I drive car 5-10 miles shut off ,come back 10 -20 minutes later, car stalls shuts off. Did you find the source of your problem?

[ctaylor738]

Your '73 doesn't have any sort of electronic idle control. Idle speed is controlled by the big screw, which increases or decreases the amount of air the engine gets.

Timing could cause what you describe.

[Deltacom]

Here is a testing procedure posted by a Nuts4benz member at benzworld to troubleshooting D-jetronic fuel injection. Hope it helps.

Troubleshooting D Jetronic.

No start and fuel pump not working;

1) Test the fuse, relay, wiring and fuel pump.

No start and fuel pump working;

1) Test the main relay and power from ECU Ensure there is a good ground from ECU terminal #11 to ground.

2) If the ignition system has a good spark, good power supply, ground to the ECU and continuity at the trigger points(cover this later) and injectors fail to pulse, replace the ECU.

Starts when cold and dies when the key is released;

1) Trigger points in distributor lack continuity. When the engine starts, it is starting off of the cold start injector and as you release the key, the cold start
injector shuts off.

Stalls when engine is cold;

1) Inspect the auxiliary air valve. When the engine is cold, remove the hose on the intake side of auxiliary air valve and let air to be drawn into it. If the engine speed increases greatly, replace the auxiliary air valve.

2) Temperature sensor 2 is also critical to cold operation.

Misfire or unstable idle;

1) Ensure that the ignition system is in good shape and also the intake is in good order. These need to be confirmed first as they are more likely to cause this problem.

2) Inspect the circuits and sensors of temperature sensor 1 and 2 and continuity through both sides of trigger points.

3) Inspect the vacuum hose that goes between the pressure sensor and the intake manifold. It should have no kinks and not collapse with vacuum present.

4) If all the above check out, do an injector flow test.

Misfire while driving;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Do a fuel pressure and volume test.

3) If the fuel pressure is constant, test temperature sensor 2, pressure sensor and throttle switch circuits.

Power is suffering;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Most likely this is due to low fuel pressure. Do a fuel pressure-volume test and pay close attention for loss of pressure.

3) If correct, test the circuits of the pressure sensor and temperature sensor 2. If these meet specs, do an injector flow test.

4) If everything checks out, repeat this test.

Stalls when you decelerate;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Inspect the hoses and wiring for good contact.

3) Check your throttle stop adjustment and curb idle RPM's.

Smokes and runs rough;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Do a fuel pressure and volume test.

3) Inspect the circuits of the temperature sensor 2, trigger points, pressure sensor and injectors. If they check out fine, do a injector flow test.

Misfire under Load;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Do a fuel pressure-volume test.

3) Inspect the circuits of temperature sensor 2, pressure sensor, trigger points and injectors.

4) If the above are good, do an injector flow test.


Idle speed too high;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Check and adjust the throttle stop and curb idle speed.

Idle speed too low;

1) Ensure that the ignition system is in good shape and also the intake is in good order.

2) Check and adjust the throttle stop and curb idle speed.


Testing the D Jetronic Components


Main relay;

1) There should be 12 volts at pin # 24 of the ECU.

2) If not, check for 12 volts at pin # 24 of the relay.

3) If 12 volts is not present at the relay but it clicks, replace with a new relay.

4) If there is 12 volts at pin # 24 of the relay, repair # 24 wire between relay and ECU.

5)If the relay does not click, inspect the wire from the main relay pin # 45 to ground.

6) If the ground is good and 12 volts at relay pin # 38 when the engine is cranked, replace the main relay.


Trigger points;

1) With the engine not running and ECU harness disconnected from the ECU, check the resistance between ECU harness 12 and 21, then from 12 to 22. One should show a low resistance and the other should show an open circuit.

2) Rotate the engine 360 degrees and check the resistance. They should have an opposite reading now.

3) If not, perform the same test at the trigger points themselves. This will tell you if it a component or wiring fault.


Temperature sensor 1

1) With the engine not running and the ECU harness disconnected from the ECU, connect an ohmmeter between ECU harness pin # 1 and 13. At an ambient temperature the reading should be 200 ohms.

2) If the resistance is well above 200 ohms, check resistance at the sensor itself to determine if it is a component or wiring fault.

3) Check resistance of all the terminals to chassis ground. There should always be an open circuit.


Temperature sensor 2

1) With the engine not running and the ECU harness disconnected from the ECU, check the resistance between harness terminal 23 and ground. The resistance should be 2000 ohms at ambient temperature.

2) If the engine is near operating temperature, the resistance should be below 1000 ohms.


Throttle switch

1) Test the throttle switch with the key on, engine off.

2) Slowly open the throttle, and as it opens the injectors should alternately click. You should hear 20 evenly spaced clicks.

3) If this is not met, place a 0.016 in.(0.4mm) feeler gauge between the throttle stop and throttle stop screw. Connect a voltmeter to terminal 17 of the throttle switch. The voltmeter should show voltage while the ignition key is in the on position. Remove the feeler gauge and the voltmeter should show no voltage now.

4) If this fails, loosen the screws and rotate the switch until it meets these requirements.

5) If they can not be met and voltage is always present no matter where you position, replace the throttle switch.

6) If the voltmeter reads no voltage, ensure voltage is being supplied to the switch. If so, replace the throttle switch.

7) If no clicks are heard while performing test 1 and 2, check wire numbers 20,17,14 and 9 for continuity end for end from the ECU to throttle switch.


Pressure sensor

1) With the engine not running and the ECU harness disconnected from ECU, check the resistance from ECU harness terminal # 7 to 15.Should be 90 ohms and then check the resistance between # 8 and 10.Should be 350 ohms. If this is not met, test the sensor itself.

2) on occasion, the sensor resistance will check out, but will not hold vacuum. Using a hand held vacuum pump perform this test again (step #1)


Injector circuit

1) With the engine not running and the ECU harness disconnected from ECU, check resistance of ECU harness pins # 3,4,5,and 6. All should have less than 25 ohms.

2) If the resistance is greatly higher or lower than 25 ohms, test the injector itself.

3) If the injectors pass, repair the wiring harness.

Here is a link that will help: http://www.vclassics.com/archive/efi.htm