I’m having a fuel delivery problem with my ’89 300SEL. Every time I start the car, it runs for about 3 or 4 seconds and then dies. The fuel pump relay is known to be good. Jumping power directly to the fuel pump causes the fuel pump to run, allowing the car to start and stay running. Also, jumping between slots 7 (which feeds power to the pump) and 8 (which is always hot) of the socket in which the fuel pump relay is seated causes the fuel pump to run (just like jumping to the fuel pump directly).

With the fuel pump relay in place I checked voltage at each slot of the fuel pump relay receiving socket, in each key position. Here are the results:

slot 2 - 0 voltage with ig. switch in off position; 1.69v in all other positions. (This one seemed strange to me)
slot 4 - 0 voltage in all positions except on cranking/start, then 12v.
slot 6 - 0 voltage in all positions except on cranking/start, then 12v.
slot 7 – 0 voltage in off position, 12v for a second or so in run position prior to start, 12v on crank, 12v in run but only for 3 or 4 seconds then drops to 0v.
slot 8 - 12v in all positions.
slot 9 - 0v in off position, 12v in all others.
slot 10 - 0v in off, and 5v in all other positions.
slot 11 - no voltage in any position (believe this is the ground)
slot 12 - 0v in all positions except crank/start, then 12v.

I would certainly appreciate any advice.
Thanks.
-Brett

If I'm reading my schematic right, pin 10 is the RPM input. It should not be a fixed 5 volts DC. If the relay does not see the engine rpm, it shuts the fuel pumps off. The rpm signal is generated by the crankshaft position sensor, and looks something like this on a lab 'scope:



It's an AC voltage that varies in amplitude and frequency as engine speed changes. The wide trace in the middle is what the ignition module looks for when firing the coil. In addition to the ignition module, the signal is also used by the tachometer, A/C compressor control unit, the CIS control module, and the fuel pump relay.

Since the car runs when it has gas, we can assume that the crank signal is OK when it gets to the ignition module. Somewhere between there and your fuel pump relay is where you'll find the problem.

The fuel pump relay pins are as follows:

1 Not used
2 Temperature input
3 Not used
4 Cold start injector
5 Not used
6 Transmission kickdown
7 Fuel pumps and O2 heater
8 B+ always
9 B+ in Start/Run
10 RPM input
11 Ground
12 B+ engine cranking

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1989 300 SEL that mostly works, but needs TLC

"Jumping power directly to the fuel pump causes the fuel pump to run, allowing the car to start and stay running"

Are you sure the Relay is good ? This is just a thought, but from reading your post, it almost sounds as if the relay might not be good. Can you try a different ( borrow..)FPR in it's place to see if the problem goes away? As you know the FPR issues can be either intermittent or plain difficult to nail down because of miniscule solder cracks.
Good luck with it.

Sure sounds like the relay to me too. I see what is being said about the RPM signal though. If you jump the relay (and start the engine) is the tach working OK?
Gilly

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I didn't consider that the relay could be the source of the DC voltage on pin 10 without washing out the crank signal and causing everything to quit, but I guess it could be doing that very thing.

If you jumped pins 7 and 8, start the engine, and measure voltage between pins 10 and 11, it could tell a lot. A steady 5 volts DC would indicate a wiring problem, while an AC voltage that varies with engine speed would be a pretty good indicator that the relay was at fault.

Good catch.

EDIT: 5 volts DC could also indicate another faulty component on the RPM signal line.

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1989 300 SEL that mostly works, but needs TLC

I am certain it is not the FPR. I swapped it with the one from my 190E. The problem with the 300SEL remained constant. The 190E (with the 300SEL FPR) is still running perfectly.

I haven't checked the tach to see if it is working. I'll do that.

With jumping between 7 and 8, voltage between 10 and 11(ground) is a constant 5v DC, except in the 'off' position where voltage is 0.

So if I'm looking at a wiring problem, where do I begin my search?

Thanks.
-Brett

Over by the circular plug with the cap on it on the left inner fender, there is a few wires that all come together under a phillips head machine screw, this is where the TD signal is shared.
Gilly

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There were three wires connecting at that spot. The connections were a touch oxidized, but not bad. I cleaned them up, and there was no change (and tach was not working). Then I traced the wires back. The green w/yellow stripe wire (which runs forward from this connection) was severed about a foot away from the connection. I spliced it back together and the car is running like new.

Thanks to all who helped out on this.
-Brett

Well that's good news! Now I'm curious as to where the 5 VDC came from. I may have to experiment a little.

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1989 300 SEL that mostly works, but needs TLC