Help with fuel pump power.
 

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?

"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

Tried it again this morning, cold engine. No GO. Is there any kind of fuel pressure switch in the system or is the pump constant run?

If you can always get running by jumping the relay, the fault will be with the relay. All to often "rebuilt" relays will exhibit intermittent operation. The usual problem is cold solder joints on the relay circuit board; a careful hand with a soldering iron may solve the problem. When the relay closes the pump runs.

no power to fuel pumps
 

Another possibility is ovp relay , and or td signal to fuel relay may not always be there , monitor the td signal , when a freinds sl560 had similiar problem i used
two volt meters , one to ezl ignition module to check voltage to fuel pump relay
and other volt meter to monitor fuel pumps voltage , also check amp draw of pumps , if one pump fails the other may be erratic

The fuel pump relay on the 103 is another part that we replaced left and right back in the day. The late 117s too. We use to stock them like air filters until they went to the mas relay in the late 103s. Just buy a relay. You can get a rebuild from programa or a K.A.E. or a MTC brand. Just don't buy a Euro brand unless you buy three of them and keep two in your glove box.

So your relay needs replacing if you have to hot wire the F.P relay at no 7 and 8 to get the pumps working...you need a new relay .

Just wanted to be sure there was nothing upstream of the pump relay.


I have two new relays cominging that should be here Thursdy.


I am more than ready to get her back on the road. Love the way she rides and drives.


Thank you for sharing your knowledge.

Still no joy. I have a new MB relay and an aftermarket one. Neither relay will operate the pump.

I found a pin out on this forum of the relay and the pin from the ignition goes hot when the key is on. Am I correct in my thinking that this is what trips the relay?

Now, even with the relay jumped out the engine is a no run. What would kill the ignition system?

if the relay pin number 30 and pin 87 are jumpered it has to run the pumps if not you have wiring or pump short or wire short , for the pump to run with relay in you need a couple of pins with power, crank signal, and key on signal and grounds

When it wont run, do you have spark from the center distributor cap coming from the coil?, it should jump a 1/4 gap, care full dont shock yourself

Poking around the engine today and found a vacuum line loose. One line goes to the ignition unit. The other runs under the body of the jetronic body. Where does that line hook up.

I've one fuel pump
smithj6779@gmail.com
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Thank you for the offer, but I put a brand new Bosch OE fuel pump right from the start.

Well, it got dark enough to check for ignition spark. It's got spark out the wazoo.

I tracked down the loose vacuum line and still can't find where it goes to. The unit it goes to is mounted on the side of the fuel distributor. Standing at the front of the car this unit is forward of the fuel distributor with one pipe fitting to the top of the distributor and another out the side toward the fender. I can see discoloration on the hose from a rubber vacuum fitting but can't find anything it would attach to.

The unit in question is the fuel pressure regulator. Where does the vacuum hose in the end of the fuel pressure regulator go?

Well I think I found the answer on the pressure regulator. It's a vent line, not intended to be hooked up anywhere.

Ok just to be clear, when the key is turned to run the fuel relay will turn the pump on to prime the fuel system, then shuts off. Then while cranking and getting the crank signal allows the pump to continue to run while engine runs.

I have four pump relays. One from Mercedes that lasted about four minutes. Two brand new unbranded pump relays have done nothing right out of the box. So no, at this time I can not turn the ignition on and get the fuel pump to run.

If I dump half a shot glass of fuel down her throat, she will run. Jump out the FPR and the pump runs but not the car. I just installed new BOSCH injectors with no go.

Tomorrow I pull one of the injectors and see what kind of fuel delivery I'm getting.

ok so you have ruled out likely not ignition, it will run with shot glass of fuel , next as soon as key turn to on the pumps should ren for a couple seconds , your not getting that?

Working by myself, so I can't tell if I'm getting the pump to build pressure or not. The hazard warnings drown it out. I did have fuel in the delivery tubes and injectors when I changed them out.

What shuts down the fuel pump when pressure builds? I can't imagine the pump runs continuously. Even in your question you indicate the pump will come on for a short period of time then shut off. What controls that shutdown?

I have not checked the O2 sensor yet. If it were bad, would this cause a crank/no run failure?

OK, so I just went out and powered up the car. After I shut the system down, my fuel distributor has and holds fuel pressure. If I break the nut on the fuel return line fuel comes out under pressure.

I have the fuel line to cylinder No. 1 removed and when I operate the intake, nothing comes out of the fuel line. Shouldn't fuel come out

Went and pulled the fuel line completely. I can blow air thru it with no issue. Also found and pulled the fuel distributor filter and it also was clear.

I had fuel under pressure when I took off the filter. So I'm getting fuel to the fuel distributor but I don't seem to be able to get any out. So I'm going to guess it is the fuel distributor needs a rebuild or replacement..Or could it be the pressure regulator?

The fuel relays is what controls shutdown, only primes system until it gets crank signal then keeps running as long as it's getting a signal,if you want to see if your getting pressure bridge relay circuit 87 and 30 and depress air inlet Plat briefly, it will chirp at injectors. Which indicate enough pressure to squirt fuel into engine

fuel pump
 

looking up your car , i found a wiring diagram 126.024 your car designation s class with a 6 cylinder reading via pin id of fuel relay= pin 4 = starting valve ie when car cold it gets the starting valve injecting additional fuel , pin 8 =12 volts from x35 which is a terminal block which in turn receive power via overvoltage relay, pin 7 goes to output power to pumps and o2 sensor heating element , pin 11 is ground side of cold starting injector, pin 10 is td signal a connection from ac control unit n6 to pin 2 of n6 ac control unit , and goes to cis n3 fuel injection brain , pin 12 is crank signal from starter circuit 50 or = started solenoid, pin 2 is from brain cis n3 unsure what signal that is , pin 6 is to kick down switch, usually to disengage ac compressor under full throttle , pin 9 is key on to overvoltage relay ending at overvoltage pin 3 one leg and fuse f1 fuse 7 from ignition switch, f1 is front fuse box

Sorry to be slow in responding. Thank you very much for taking the time to track that all down. Excellent information.



This is just a matter of keeping finding a problem, fixing it, and trying again. The engine control on this car is new to me, even thou it's over 35 years old.