1983 380sec no power to fuel pump? - PeachParts Mercedes-Benz Forum

dvdguy · #1 10-25-2007, 10:59 AM

I've been trying to figure out what's wrong with my 1983 380sec.
I just replaced the fuel pump, and the fuel pump relay ( the one that says "kickdown")

I have no power going to the fuel pump relay- is there someplace else I should be looking? The car happily cranks over, but obviously never starts.

I checked for +12 at the fuel pump, and I get zero.
I've tried powering the fuel pump from a separate power source- but the car still does not start. arg! Can anyone help?

thanks,
-Matt

vegadonovan · #2 10-25-2007, 12:00 PM

Have you tried to jumpered up the pins to power the pumps manually?

ctaylor738 · #3 10-25-2007, 01:17 PM

You should have 12 volts at Pin 30 at the relay.

If you do, jump 30 to 87 and the pump should run. If it doesn't run then you have a wiring or ground problem.

vegadonovan · #4 10-25-2007, 07:58 PM

31: ground (brown)
15: +12v ignition on (Fuel pump cuts off after 1.5 sec)
50: +12v from starter/crank input
30: + 12 volts from battery (red wire)
87: power to fuel pump
87k: transmission kickdown
87v: power to cold start valve
TF: connects to pin 9 of a 25 pin ECU (no current: will run PFR even when not connected
TD: very important signal current but not 12v
(green with yellow stripe)
from airflow sensor (got the idea from a BMW diagram)
:acts as a fire safety device in case of accidents:
cuts off signal when air flow is in 0 position
when engine dies. if we pull out this wire, PFR cuts out.

RF_126 · #5 10-25-2007, 09:03 PM

Just a couple of extra thoughts building on the above. I am going to assume that your new parts are in good order!

First, some of the pin references above are potentially confusing if you aren't able to consult the M-B circuit diagram (p.107 for this purpose). When you remove your fuel pump relay, the pins you want to bridge are identified in the fuse box as pins 7 and 8. Pin 8 should be "hot" all the time, so test that first. (You did state that you had no power to the relay; perhaps you have already done this.) Connect your voltmeter's positive probe to pin 8 and the negative to a suitable ground; you should see battery voltage without the ignition on.

The next place you can test this circuit is at the 14-pin connector under the driver's side of the dashboard (C104). It takes a while to find at first because of all the other modules in that area, but the one you're after has a fairly thick black, red, & white wire to pin 2. Separate the two halves of this connector and, with pins 7 & 8 jumpered as before, verify battery voltage. The power wire runs from here along the car's floor on the driver's side and then turns in the rear seat area to cross over to the passenger side and enter the trunk near the fuel filler.

The pump's ground connection (G104) is located in the trunk behind the right-hand wheel carpeting, and has a brown wire running to it from the pump. As Mr. Taylor noted above, this could be your problem, especially if you've tried applying another power source and the pump still didn't work. (Did that power source depend on any particular position of the key?) Bear in mind that when all is right, the pump will only energize when the key is turned to the start position. You don't want it on whenever the ignition is on, for safety reasons.

Hope that helps, and good luck.

dvdguy · #6 10-30-2007, 09:01 PM

thanks to everyone for the insights- It's been very helpful so far!

I've discovered that jumping the +12constant and fuel pump pins makes the pump run. cool!

I also put my meter on +12 and ground - good
+12 ignition on - good
+12 cranking - good

I did not check the connector inside the car, assuming all was well.

when I put the relay on- no start, it turns forever, but no fuel pump action at all. aaahh!

I decided to explore the inlet to the fuel distributor. there is plenty of fuel pressure. the inlet filter was not clogged.

I did notice that when I loosened the nut, I got a bit of gas spray as the system depressurized. is that normal, or a sign of a flummoxed fuel distributor?

I thought I'd try starting fluid. - I get nothing.

what could it be now?

thanks to everyone for your help and the pinout of the kickdown relay connector.

RF_126 · #7 10-31-2007, 10:22 AM

dvdguy:

First, have you tried actually starting the car with pins 7 and 8 jumpered? It sounds as if your pump and its circuitry are basically alright, but if it won't start with the fuel pump relay installed then the relay is the prime suspect. When the relay is in, the pump should only energize when the key is turned to start. You say there is "no fuel pump action" in this case, but do you have a helper available who could turn the key while you listen for the pump at the back of the car? If you don't hear it humming with the key on 'start' then your relay is the problem. (You'll have to get your ears pretty close to hear the pump over the starter! A large piece of cardboard as a baffle might help.)

Of course, the ultimate test for fuel flow would be a proper fuel pressure test at the fuel distributor. Or, you could disconnect the fuel supply line to the distributor and verify, with a suitable container and plenty of rags handy, that fuel is being delivered. (When you loosen that connection, it is entirely normal for some fuel to spurt out when there is residual pressure in the system. To minimize this, you can depress the airflow meter plate, which will bleed off fuel to the injectors - and give you a very rich mixture when you eventually try to start! Be sure to use a counterhold on the 17mm nut closest to the fuel distributor whenever you work on these fittings.) If there is a blockage somewhere, it is most likely your fuel filter. I don't know whether you replaced that when you did the pump, but it's always a good idea to do so.

All of this assumes that the car is otherwise sound and that there are no issues with compression or ignition. If you do end up replacing the fuel pump relay, make sure you get a genuine M-B part. The aftermarket units, especially KAE, are notoriously unreliable. Potomac German Auto is a good source for used M-B units if you don't want to spend all that money for a new one.

dvdguy · #8 10-31-2007, 10:57 AM

Yes,I did try to start with the pins jumpered, but with no luck. There is certainly pressure to the fuel distributor, although I have not properly tested it. I guess I should see if there is anything coming OUT of the distributor as well. What's very odd to me is that the car was running fine.. it would idle low and run rough when cold. I drove it about 200 or so miles home from where I bought it, and drove it occasionally during the summer. it would slowly get harder to start, until finally I noticed that the fuel pump stopped working. Because the car did not even cough with starting fluid, I 'm wondering now if I have an ignition problem, or even a greater electrical problem. by the way, I took the cover off my old relay , and it definitely had the scent of toasty electronics. regarding the fuel filter, It looks like I've got a brand new filter attached to the fuel pump.

ctaylor738 · #9 10-31-2007, 11:14 AM

I have learned the hard way that a new fuel filter on a poor-running recently-acquired car is a bad sign. What it usually means is that the car sat and crud accumulated in the fuel system. It could be that it gradually made its way to the FD and clogged it. It can also clog the screen in the warm-up regulator, in which case you will have cold-running problems.

Of course, it could also be spark, or that the plugs are so fouled that they won't fire. My favorite way to check spark is with a inductive timing light. Hook it up and see if it flashes while you crank.