Ignition Issues- 450 SE - PeachParts Mercedes-Benz Forum

addwestern · #1 03-20-2007, 10:30 PM

Hi all- I appreciate the help everyone has been in sorting this issue thus far. I have a 75 450 SE. I recently acquired it and has had some strange ignition issues- granted I am new to Mercedes- so it may be correct.

I acquired the car with the distributor out and it was missing the transistor box. I have replaced the box with a used one, and aligned the dizzy and replaced it. She wants to start, but is not. It comes just short of igniting and firing. I have good spark, but only seem to have 6 volts going to the coil while cranking. Is this correct?

It has also been recommended to replace the points with a solid state Pertronix or Crane electronic ignition- however, I can't seem to find the proper set for it, and Pertronix claims they do not supply one for this model. Has anyone installed one, and if so, what model/type did you use?

I appreciate the assistance- I am on my last-ditch efforts before I trailer it over to the local mechanic.

thanks- Nate

Tomguy · #2 03-20-2007, 10:52 PM

Did you try timing it while cranking? Having 1 person crank while you aim a timing light can help fine-tune it to the point of running.

I'd get it running with the stock ignition before replacing it with anything else.

addwestern · #3 03-20-2007, 11:02 PM

Yep- no joy. Do you know if the voltage is right to the coil?

psfred · #4 03-21-2007, 12:00 PM

There are two different switchboxes, and I cannot tell you which one is which -- they are wired up differently. Do you have a correct wiring diagram? The coil is grounded by the switchbox, not supplied with power with the correct switchbox.

Pertronix listed the replacement system last year, check again (MB m117 engine, will fit anything from the 1970-72 280 SE 4.5 to the 1976 450 SE -- not later, as the factory system was breakerless).

Crane makes one too, although I plan to install the Pertronix.

Peter

ctaylor738 · #5 03-21-2007, 05:46 PM

If you are getting good spark from #1 (right front) cylinder at approximately TDC, the engine should at least fire. You did check that #1 was at TDC - up on compression, not on exhaust - when you aligned the distributor? Plug wires are correctly hooked up? Sorry, have to ask.

If all this is right, I would start checking for fuel supply. I am no D-Jet expert, but a couple of simple tests are that the pump is running when you are cranking (put your hand on it while someone cranks), and you should hear the injectors clicking when you open the throttle.

Tomguy · #6 03-21-2007, 10:20 PM

If you're sure your spark is dead on - and not 180° off (by the rotor - 360° off by the crank), and you're getting a spark @ TDC, then I would look at the fuel system. A 4.5 will start and run on ANY 4 cylinders or less (trust me, I've done it). Pull a plug, because if it's wet, you're flooded - and a 4.5 that's flooded can be a bear to start, what with the CSV dumping fuel into the engine with each attempt...

GaryF · #7 03-22-2007, 10:50 AM

Quote:

Originally Posted by addwestern

It comes just short of igniting and firing. I have good spark, but only seem to have 6 volts going to the coil while cranking. Is this correct?

How are you measuring the coil votage, with a AVO meter? The voltage will be pulsing from 0 to 12(ish) Volts while cranking so you're probably seeing an average. If your measuring it with a scope and its peaking at 6 then your ignition circuit may not be wired up right. While cranking the ballast resistor is bypassed to compensate for the starter load on the battery and you should get the full battery voltage.

addwestern · #8 03-22-2007, 10:54 AM

Quote:

Originally Posted by GaryF

How are you measuring the coil votage, with a AVO meter? The voltage will be pulsing from 0 to 12(ish) Volts while cranking so you're probably seeing an average. If your measuring it with a scope and its peaking at 6 then your ignition circuit may not be wired up right. While cranking the ballast resistor is bypassed to compensate for the starter load on the battery and you should get the full battery voltage.

Yep- with a meter.

ozawa · #9 03-23-2007, 08:38 PM

addwestern,

i believe your switchbox should be #0 227 051 024. assuming this, here's what the factory manual has to say:

terminal 1 on coil with points open: battery voltage

terminal 1 with points closed: .7 to 1.5 volts

terminal 15 on coil with points open: battery voltage

terminal 15 with points closed: 3.6 to 4.6 volts

if you've got another number on your switchbox, let me know. i've got the OE manual and can cross reference your chassis number to find out exactly what box you should be running. the different boxes have different values.