Engine cranks but won't start
 

1974 450sl

I replaced the belts and accidently sparked a wrench against possibly the alternator before disconnecting the battery. When I buttoned it all up the engine cranks, fuel pump works, provides fuels from the smell, and spark plug is emitting a spark. It ran for about 2 seconds then died. Never stated up again after that.

The battery is fine. The coil seems to be fine with checking the resistance with a multimeter. The fuel pump comes on and pumps gas. I suspect it might be:
1. a blown fuse - the ones under the dash on passenger side looked fine
2. another fuse box for ignition system - not sure when it is located?
3. Engine Control Unit (ECU) module
4. Electronic Ignition System (EIS) module
5. Fuel Relay switch - no idea where this is located?

I value any feedback and help please?!

Check for power at the alternator battery terminal, loss of power there should not matter but check anyway. If there is power, your problem is somewhere else.

Step 1) Ensure that there is spark at the spark plugs. Remove a lead from a spark plug, insert another plug in the lead, lay the plug on a metallic part of the engine, crank the engine. Is a spark visible at the plug?
If not, repeat the exercise with the coil lead removed from the dist. cap, but still in the coil.

Before any attempt to diagnose the fuel system, it is necessary that there be no question about the ignition.

Thanks for your insights gentlemen. Still not sure what's up with the car?

Yes, there is power to the alternator.

Yes, I checked the spark at the plug and it's there.

Suspect it might be a relay. Is there a way to check if it is operating correctly?

It turns out now that I don't have spark at the plug. I determined the ignition coil was the problem using a simple circuit tester. There was no current through the coil so suspect that I must have fried it by accident.

Thanks for all your inputs!

I would say you shorted out the dynamo or alternator and fried the transistors and diodes .

After replacing the coil with a Bosch part, I checked it with a simple circuit tester and there was barely a dim light indicating not getting much current to the coil.

I will have to look at checking the alternator and dynamo (not sure what this is?). Any ideas on how to check the transistors and diodes?

Grateful for your advice!

Not sure if the voltage regulator is the problem and how to check it?

?? ? ?? A car will run without an alternator as long as the battery has enough capacity.

A bad alternator ( the part that recharges the battery ) won't prevent a car from running.

We need a wiring diagram for the ignition system on this car to determine what to test next.

97 SL 320 is correct. A non-charging alternator will not prevent an engine from starting. The term alternator and dynamo are interchangeable. Dynamo is an older term. Both an alternator or a dynamo produce current to re-charge the battery.

I realize that the alternator is not the issue. Thanks.

Here's a wiring schematic for a 450sl I believe:

https://f01.justanswer.com/ref/http_...52800ae9b8.gif

I have been trying to diagnosis the ignition system using this schematic. The two resistors are good (0.6 and 0.4 ohm). I traced wires to under the drivers side front wheel well leading to a "metal box" (about 3 x 4 inch) that I suspect is the Transistorized Ignition Module or also referred to Transitor Ignition Unit. Any idea how to determine if this is functioning properly?

I have included a few diagrams from Internet surfing.

Thanks for any insights folks!

Transitorized Ignition Module
 

I'd be looking at the green , green / yellow wire running into the distributor. I " think " this system uses am old style contact point arrangement to trigger the ignition module.

Make sure the brown wire at the module is grounded.

There is a thread in the veteran's section about replacing the transistor in the ignition module. It's pretty straight forward with a soldering iron and the part is a few bucks, plus shipping.

The quick test of it is the little green wire at the top of the diagram, hook up a dwell meter to that. Zero dwell while cranking means you have short or a dead box.

BTW, that box must be grounded well; usually the three bolts to the firewall provide that ground. If you snag a replacement box, make sure to actually mount it correctly before firing it up.

-CTH

Is there a resistance check that can be done on the distributor itself? I have a 78 450sl with a no spark condition aswell! I've done quite a bit of reading and thru some tests I feel I've determined the distributor has failed. But I want to confirm it, just cant find a "test" for it. (I dont have a way to monitor dwell). I have battery voltage to ignition coil with key on, 9v while cranking (which means the ignition switch, ballast resistors and everything up to the ignition coil is good) resistance checks on the coil itself check out (.4 ohm and 10.5 kohm, within spec on both), green wire to dist. is brand new, aswell as cap/rotor/plugs/wires, tested terminals 4 and 5 for voltage at diag port (0v like there should be, meaning switchgear/ignition box is good). Which leaves the distributor..... thoughts?

I can provide more detail on what tests were run to determine all of my findings, but its everything that can be found thru google/prodemand(mitchell)

Any and all help is much appreciated!!

Pull the green wire plug from the ignition box, check for resistance between center pin and the shield. 600+/-100 ohms.
Under what conditions was the green wire renewed? What was the cause? By whom performed?

I am still unable to start the engine after all the above diagnostic procedures. Anyone able to offer any insight please?

9V while cranking is too low. Hav you tried with jumper cables form a running car?