No spark 79 450sl...Pulling my hair out!!! - PeachParts Mercedes-Benz Forum

BPhillips · #1 06-02-2009, 04:19 PM

1979 450 SL, I have no spark. Went through all the tests in the service manual, and according to it, everything test good. Good voltage to coil, good resistances in coil, resistors, transmitter, crank sensor, everything. Ended up replacing the ignition module, which fixed nothing. The boss doesn't want me swapping out parts that I can't prove bad, so I need some advice. Thanks for any help!

mespe · #2 06-02-2009, 06:13 PM

Check to see that the rotor is turning? I'm sure you did, so I would check for corrosion on all the wires and grounding points. I do not know what the manual tells you to test, but have you checked the plugs themselves?

Gurunutkins · #3 06-02-2009, 06:35 PM

if you have volts at coil and good ground at coil then check for spark on the coil to dist HT lead, carries 8x the load of a spark plug so goes bad faster. check the little carbon brush in the center of the top of the dist touches the rotor arm and is not broken or eroded away. check the rotor arm has not earthed itself to the center post of the dist (it happens more often than you would credit) a quick test is to put a glob of grease or silicone inside the rotor arm and push it back on the shaft, isolates it from the shaft for a few minutes usually. check that the end of the rotor arm and the contacts on the dist cap are clean and not corroded. check there is no carbon tracking inside the cap that can run the spark to earth.
if its none of those then start electrical tracking with a volt meter.
cheers
Barri

Takernz_30 · #4 06-02-2009, 07:40 PM

Why not replacing the condenser? I have this problem before but turns out to be busted condenser. The ending no. of condenser is 235. i hope this will help.

MunichTaxi · #5 06-03-2009, 03:13 AM

Don't these models have a Throttle position sensor (TPS) ?

http://en.wikipedia.org/wiki/Throttle_position_sensor

meltedpanda · #6 06-03-2009, 10:14 AM

When you crank the engine - do you get voltage to the plug leads?, if not, crank engine and work backwards until you find the faulty component

Genbiltstein · #7 06-03-2009, 05:06 PM

There is a connector to the left of the distributor. This connector will make you go through fits until you clean the contacts. I know I had to go through it. On my '74 450sl the contacts were corroded enough to make the engine hit and miss. Try it.

dojufitz · #8 06-03-2009, 06:15 PM

Check the metal inside your distributor cap - could be worn out or burnt - check rotor button.

ja17 · #9 06-03-2009, 07:29 PM

Hello,

The transisitor switching unit for the factory electronic ignition may also be bad. Check all the other cheaper items first. If the unit is bad, find a good used one. New ones will be very expensive. Maybe there is an aftermarket ignition control unit for this model?

BPhillips · #10 06-08-2009, 04:47 PM

I have already replaced the switching unit, but it didn't do anything. Checked the signal from my crank sensor, working good. Hooked up the oscilliscope to the transmitter and monitored the output to the switching unit, and it graphed a good looking on-off sine pattern. Ground switched the coil and managed a small spark, so I would think there should be some sort of output, but I still have nothing. What am I missing?

Tomguy · #11 06-08-2009, 05:24 PM

You only got a SMALL spark, testing the coil while cranking? Replace the coil. It needs a heavy enough spark to cross the gap from the cap to the carbon brush, and the gap from the rotor back to the cap, and of course the plug. Try a known good used coil for testing if you dont want to spend $$ for a new one.