72 280 sel 4.5 - PeachParts Mercedes-Benz Forum

johnamc · #1 07-21-2015, 04:13 PM

hi guys.

new to the list. im trying to get my benz going. it sat for 2 years, before it ran awesome. im looking for details on the wiring harness from the computer to the fusible relays on the firewall. i have a brown wire from the voltage regulator to the alternator that the casing has completely burned off. from tip to sternum. cant seem to get power to my injectors. it will start with gas in the throttle body but wont stay running. tons of compression and great spark....

any thoughts?
cheers,
john
laval,qc

meltedpanda · #2 07-22-2015, 11:06 AM

take one step at a time. Are you getting fuel to the fuel rail and cold start valve?
If so are the injectors clean , able to spray?

Brown is ground in Mercedes
When you say computer , you mean the ECU under the dash or the ignition control module on the fender ?

68_280SE · #3 07-22-2015, 02:44 PM

The brown wire between the voltage regulator and the alternator is a ground wire. I had the exact same thing happen to that brown wire on my 68 280SE. Turned out the problem was the main ground cable between the firewall and engine bell housing had corroded. When i would try and start the car, the current would flow through this small wire between the alternator and voltage regulator to ground. The high current from operating the starter melted the insulation on this small ground wire. I had to cut out the burned wire an install new wire. I would definitely check the main ground cable between the firewall and bell housing. It is located on the drivers side firewall. It looks like a large braided cable with ring connectors on both ends. Remove it and make sure all of the connections are clean and tight.

When you turn the ignition key on you should here the fuel pump at the back operate for a second in order to pressurize the fuel system. If you don't hear the fuel pump come on, then i would check on the condition of wire that provides power to the fuel injection system. This wire runs from the positive battery terminal down to a relay in front of the ECU. Make sure the connection at the battery terminal is clean and tight.

Also I would also check the the ground connection for the fuel injection system. The connection is located behind the battery near the firewall. You will see 8 brown wires screwed down there. Make sure the connection is clean and tight.

Hope this helps
John

johnamc · #4 07-22-2015, 03:06 PM

@meltedpanda
there is fuel getting to the injectors, just no power. the computer is the one on the passenger side fender at the front. as for wiring im just not sure which relay on the firewall controls which part.

@68_280se
i will check the ground wire, THANKS. it wouldn't surprise me if that's the issue. i am definitely getting power to the pump. opened the rails and pumped out a ton of old yellow gas.

cheers,
john

meltedpanda · #5 07-22-2015, 03:32 PM

sounds like your fuel pump is working, so onto that ground, replace that , and then let us know. As mentioned on thing at a time, you will see why, many interconnected systems on these ol gals
next step distributor and the trigger poitns

68_280SE · #6 07-22-2015, 04:17 PM

Melted panda is right on track. The ECU controls the operation of the fuel pump and since you know the fuel pump powers on for a few seconds when you turn on the key, you know the ECU is getting power. The next things to check are the main engine ground connection and the fuel injection ground connections. Once the grounds have been cleaned and checked, then onto checking and cleaning the trigger points.

Another easy check to verify the operation of the injectors is to turn on the ignition, but don't start the car. Remove the air filter housing and then open and close the throttle a few times. You should hear the injectors clicking because the throttle switch actuates the injectors to enrich the fuel/air mixture during acceleration.

John