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450 SL, 230 SL
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Discussion Starter #1
My car is a 1975 450 SL with the D-Jetronic electronic fuel injection system. A problem I posted an inquiry about last month has gone from bad to worse. Previously I had reported an intermittent problem where the engine would completely die (tach to "0", complete stall) but only at certain (distinct) throttle positions. It seemed to happen only when the engine was cold and was intermittent. Yesterday, the problem cropped up again only the engine was warm and it happened whenever I took my foot OFF the accelerator pedal. (tach to 0, engine off) If I pressed on the gas, the engine would come back to life.
I nursed the car home & let it sit several hours. Later that evening, it started fine and idled with my foot off the pedal.
I had heard that this problem could be dirty idle circuit, so I removed the housings for the cold start fuel injector and the idle speed valve, cleaned everything and reinstalled to see if this would help. The alternator was moved to replace a belt during the process, but no wiring was disconnected.

Now the car will not start at all. It is hard to believe I could have disturbed something critical to the ignition system, fuel system, or both during this process. Nevertheless, I now have the following symptoms:

1) Car cranks but will not start
2) no spark
3) fuel supply pressure is 0 (pump, filter ?)
4) with ignition switched on, as I move the throttle, I can hear a clicking sound (and feel it) in the fuel delivery system (at the pressure regulator). When I switch the ignition off and move the throttle through its travel - no clicks. What's this ?
5) Tach needle has been seen swinging wildly (unrelated to actual engine revs) on an intermittent basis for the past several weeks. Any connection here ?

Car ran smoothly without problems until this suddenly happened. It has new plugs, wires, rotor, cap, points, fuel injectors, and coil.

Seems like fuel and ignition have stopped working (?!). Does anyone know what to check next ? I assume I need to start looking at things like the ignition control unit, a common relay or a fuse or other connection which I should check out, ballast resistors, fuel pump, other sensors ? I'm stuck ... any advice out there on where to start ?

Thanks in advance for any help or insight on this ...

Chris[:(]
 

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450 SL, 230 SL
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13 Posts
Discussion Starter #2
Re: Help - Now my 75 450 SL won't start at all -- New Information

Update on previous post below:

After sitting overnight (and cooling down) car started fine. Ran for about 5 minutes, then idle start hunting, then stalled. Would not start. Let it cool, started again, then stopped. Seems temperature related. Disconnected and grounded single pole temp sensor plug (which turns on electric fan)and got it to start and run even when warm. But then it stalls out as temperature increases further. I've scoured all the posts describing similar (but not quite identical) warmup related stalling problems Anyone have any thoughts thoughts on what's happening here ? Thanks so much !



My car is a 1975 450 SL with the D-Jetronic electronic fuel injection system. A problem I posted an inquiry about last month has gone from bad to worse. Previously I had reported an intermittent problem where the engine would completely die (tach to "0", complete stall) but only at certain (distinct) throttle positions. It seemed to happen only when the engine was cold and was intermittent. Yesterday, the problem cropped up again only the engine was warm and it happened whenever I took my foot OFF the accelerator pedal. (tach to 0, engine off) If I pressed on the gas, the engine would come back to life.
I nursed the car home & let it sit several hours. Later that evening, it started fine and idled with my foot off the pedal.
I had heard that this problem could be dirty idle circuit, so I removed the housings for the cold start fuel injector and the idle speed valve, cleaned everything and reinstalled to see if this would help. The alternator was moved to replace a belt during the process, but no wiring was disconnected.

Now the car will not start at all. It is hard to believe I could have disturbed something critical to the ignition system, fuel system, or both during this process. Nevertheless, I now have the following symptoms:

1) Car cranks but will not start
2) no spark
3) fuel supply pressure is 0 (pump, filter ?)
4) with ignition switched on, as I move the throttle, I can hear a clicking sound (and feel it) in the fuel delivery system (at the pressure regulator). When I switch the ignition off and move the throttle through its travel - no clicks. What's this ?
5) Tach needle has been seen swinging wildly (unrelated to actual engine revs) on an intermittent basis for the past several weeks. Any connection here ?

Car ran smoothly without problems until this suddenly happened. It has new plugs, wires, rotor, cap, points, fuel injectors, and coil.

Seems like fuel and ignition have stopped working (?!). Does anyone know what to check next ? I assume I need to start looking at things like the ignition control unit, a common relay or a fuse or other connection which I should check out, ballast resistors, fuel pump, other sensors ? I'm stuck ... any advice out there on where to start ?

Thanks in advance for any help or insight on this ...

Chris[:(]
 

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Registered
82 500 SL
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41 Posts
In reply to your problem

I think you may have a problem with the Ignition module that converts the signal from your Distributor to Drive the coil.

These tend to fail regularly as they run very hot.
Dry joints inside are very common or the main Drive transistor blows. This can happen if a plug lead is disconected whilst the engine is running causing the primary coil voltage to increase and damage the Transistor, or just due to running to hot for to long.

A good suggestion for these is to remove them and clean the back of the module of any corrosion and put some silicon grease on the back before bolting it back into position. This will allow for better heat transfer to the car body and the unit will run a bit cooler.

Good Luck

Damian
 

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Registered
350SLC, 500SLC, 300TE, 190E2.3 Sportline
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690 Posts
Your 450SL problem

Tachometer behaviour suggests primary (low tension) ignition system problem. Secondary (high tension) problems would not cause this tacho behaviour so you can ignor rotor, cap, plugs, leads etc. Start with the easy bits first (you will need a multimeter (volts & ohms) and a dwell meter). Check that power is available to the ignition module. You mentioned new points. Check the dwell angle with a dwell meter, relying on measuring points gap really is not good enough. Check the two ballast resistors near the coil, they should measure low resistance (I forget the figures but less than a few ohms). Check the capacitor (condensor) across the points (sustitute with another if possible). Check the pigtail wire at the points and the green points wire running back to the terminal block near the coil and ballasts (these have been known to break). If all these check OK then try coil next (sustitute with a known good one). If still no go then ignition electronic module is suspect. You also mention possible fuel problems. It would be surprising to have both an ignition and injection problem, but it would also be worth checking the separate thin wire to the positive battery terminal as this supplies power for injection system relay and fuel pump relay. This wire has been known to corrode and make bad connection at the battery. This will definitely stop the engine. I would also check the injection and fuel pump relays. The system is not that complicated if you work through each of these methodically.
 

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450 SL, 230 SL
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13 Posts
Discussion Starter #5
Problem Found

Thanks Greg (and Damion previously). After receiving your messages I focused on the ignition as opposed to fuel system (always seemed to be an electrical problem). After step by step troubleshooting with a voltmeter, I found the 1" wire connected to the points (pigtail wire ?) at the (relatively) new point set to be bad. Looked fine, but NO voltage thru that small piece of wire! Had an old point set, cleaned it up, checked for voltage at both ends of the wire, installed it, and the car starts & runs great ! No tachometer problems either. Thanks to both of you for focusing my search.
 
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