Working on a 1988 300SE. I have lost power to the fuel pump. Pump and engine run with a by-pass power supply to the pump.



Anyone have a link to a schematic of the fuel pump circiut?

The fuel pump relay (FPR) is located on the firewall, left of center. The OVP relay is the one with a clear top over 1 or 2 fuses. The FPR is next to the OVP.
Remove FPR, jumper sockets 7 & 8, pump should run. If so, the FPR is defective.

Thanks for the info, but which pins are #7 & 8?

It has a new fuel pump that runs with a jumper wire. Can I assume there are no switches or relays between the FCR and the pump?

Thank you for sharing your knowledge.

The individual pin sockets/sleeves are marked (flashlight), and 7&8 correspond to pins 30 & 87 on the relay.
Nothing between the relay and the pump.

You turn the relay upside down once you pull it out ,,and view the pins on the bottom and molded in to the case are numbers you then wire 7 and 8 with a piece of wire .Important after this test remove the wire link from the pins 7 and 8 after ,,,dont use it hot wired .

Had to really look for the numbers on the plug-in base. The new relay solved the issue.

Only to reveal another issue. This car sat for seven years. It cranks and runs, but after it is up to running temp, If I shut the engine down and try to restart the engine, it acts like it jumps timing. Let it sit overnight and it started right up. Comes up to temp and then it will not restart.

Classic faulty distributor cap behavior on a m103.

Why OK cold start, but not hot? What happens to the dist. cap when hot that is not occurring when cold?

https://w124performance.com/docs/mb/M119/M119_ignition_misfire_whitepaper.pdf

"Whitepaper", or snowjob? If condensation and/or tracking in the cap is the culprit, cleaning & drying should provide at least a temporary remission.
A K-jet system left unused for seven years will also exhibit the symptoms being experienced by the OP. All the cold-start provisions will provide sufficient richness for an initial start, but partial clogging of metering valves and nozzles may cause a lean condition when warm.

Just got a new flywheel sensor and the problem still shows up.

If you could point me out to any articles about cleaning the fuel injectors or would injector cleaner be the first start?

Did you replace your cap and rotor? During a warm NON restart, if it cranks slow then fast then slow again like the ignition timing is off, it's your cap and rotor. We've literally replaced 100s of these on the 103s and 119s for this very problem. Even when the cap looks OK and there's no moisture or anything growing inside.

Well, Pelican had the injectors in stock, so they are on the way.

I would never imagine that the cap and rotor could be an issue. Not with the cold start being so good. I just happen to have both in hand new,, so they will be in this afternoon.

Thank you to everyone responding. I'll post what the final outcome is.

Before you replace your injectors, break each individual fuel line loose on the fuel distributor one at a time while the car is running and lightly tap on the line near the fitting with your wrench. If rusty gas comes out, do this until it turns clear.

Mr. Roberts, Spot on. There was moisture inside the distributor cap. I can hardly believe the engine even fired off. New rotor and cap installed. Ran the engine until it reached normal operating temps. Cranked it three times and had no issue.

The fourth time, no fuel pump. If I jumped the relay out, the fuel pump ran fine.

Does anyone have a drawing of where the relay gets the control power or know what terminals to check? Relay terminals 31, (base term 12), relay terminal 50 (base terminal 11) relay terminal 15 (base terminal 10), and IIRC relay terminal TD, (base terminal 9) all show power when the ignition is hot.

Jump base terminals 7 & 8 and the fuel pump runs