Vintage Cars: 1978 450sl, fuel pump relay, 450sl

Question
QUESTION: 1978 mercedes 450sl. The vehicle cranks but will not fire, just turns over with an occasional sound of a cylinder igniting. the fuel distr. replaced, injector seals, fuel filter, ign. control module, and coil. I have noticed that with the ignition on the coil, new and original one, has 2.5v to 3.5v, depending how long the ignition is left on, to it when the ground is connected. remove the ground and the voltage goes back to 11.00/12.00v. every time the ground is connected i get an arch like theres a short or something.from where the 12v ignition source connects to the first resistor there is 12v and on the other side it drops to 5v to the next resistor and after that one it drops to 2.5v and then to the coil. bought an aftermarket resistor and it changed nothing. i get a spark out of the coil even with the lower voltage but i am thinking it is not powerful enough to ignite the fuel enough to make it run. I do get spark at the cylinders but seems weak to me. At this point it is making no sense how i am losing voltage. All coils i have dealt with have 12v to them to make the car run. with the engine cranking the voltage drops from 3 to 2.5v and the coil is getting hot as well as those resistors the longer the key is on. Any help i would greatly appreciate, i have been chasing this for 5 months.

ANSWER: You are getting fuel to the rails  correct?
One of the main reasons for no start on K jet is fuel pump relay failure
When the key is on  battery voltage goes through 2 preresistors  to to drop it  which means you should be seeing around 3-4 volts there regardless of whether the points are open or closed. IF you are only getting  a SMALL spark while cranking? Could be a bad 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. Are you absolutely sure you have the right coil, and your grounds are good from battery to body.  These cars are coil specific.  Check all lines for bare areas, shorting to ground
Also the Rotor cap and HT wire connection can sometimes be a culprit, check carefully

this may help
http://www.peachparts.com/shopforum/attachment.php?attachmentid=79161&d=12700118



---------- FOLLOW-UP ----------

QUESTION: Yes i am getting fuel out of the fuel distributor, not sure what the psi should be. I do get spark at the plugs. what is the compression rating on these engines if i may ask. I can smell the fuel as i crank as well. When the engine is cranking i can hear a relay kicking on and off as i hold the key in the start position, like its glitching. I am not sure which one is the fuel pump relay as there are about 6 different relays in that lower right kick panel.With the voltage being like it is at the coil and thats the way its supposed to be then now i am back to square one and slightly confused on my next test. I do know that the fuel pump only activates at the same speed as the intake flap opens and closes as it is cranked. And I have, if my memory serves me right, either 65 or 80psi out of the pump.

ANSWER: fuel pump number - 0015450605 6 prong relay
I will need to look up compression and fuel pressure later



---------- FOLLOW-UP ----------

QUESTION: Are you giving me that part number in relation to saying the pump is likely bad, or the relay. Is the pump supposed to canstantly run? or is it normal for it to kick on and off with the motion of the intake? I was thinking so since the injectors are not working off an electrical signal to open.

Answer
just so you can find the relay, as you asked, the fuel pump should spin up and continue to run

pressures
Cold pressure -75 degreesF - 25psi ( engine not running, pump energized)
Warm pressure - engine running 50-60psi

system  pressure -75-84psi

are your fuel filters clean, in tank

here is what you need to rule out and most common cause of non start cranking KJET system ( provided all elctrical systems (igniton) are ok)

lack of fuel pressure ( filters/pump/relay)
jammed or sticking airflow sensor
aux air valve stuck
cold start injector non working
sticking control plunger
restricted injectors