72 280 SE 4.5 Fuel Delivery - PeachParts Mercedes-Benz Forum

Fisherman · #1 07-06-2007, 10:25 AM

Hey Guys,
Off the diesel kick and now picked up a gas powered MBZ (actually 2) and now I need help.
I have ordered the manual for the 108 but am trying to troubleshoot and finding myself at a lack without the manual.

I cannot get power to my fuel pump on this car. It wants to run but without fuel well I guess you know.

When I turn the key on it appears that no power is even getting to the fuel pump. I have checked the fuse (number 4) and replaced it and all is good there except there is no power to the fuse with the key on. Is this controlled by the central ECU in some fashion? Do I need to just start tracing power from the battery all the way through the ECU to the fuel pump? It does not appear to have a secondary fuse according to the drawings I could find for the electrical but perhaps it runs through a secondary relay before getting to the fuse?

Any help/advice you can give would be greatly appreciated.
Thanks,
Mitch

david s poole · #2 07-06-2007, 11:31 AM

i think there is a fuel pump relay and the fuse supplies the relay.

Tomguy · #3 07-06-2007, 04:01 PM

There is a small wire that attatches to the (+) terminal on the battery.

This feeds the fuel pump relay's power - and I believe the entire D-Jet ECU etc. I am not sure if it powers the fuse... but if the wire is off, well... no way will it run!

Fisherman · #4 07-06-2007, 09:18 PM

Yeah I thought of that. I'll trace it to the connection and make sure it is doing it's job but I am pretty sure it is okay.

Any other ideas?

Fisherman · #5 07-09-2007, 07:38 PM

Okay,
Had to trace everything from the pump all the way to the relay.
Here is the layout if anyone ever needs it.

The wire on the 1972 280SE 4.5 is black with opposing red stripes kind of like a large swoosh. There are two wires very similar but it's the number 12 not the heavier (10 or 8) one. It runs from the pump under the rocker panel to a large (12 pin) plug on the drivers side footwell just in front of the fuse panel.
It then goes through the firewall, across the back of the engine and under the battery tray (wire wrap had disintegrated and the coloring was black with cream stripes) to a relay (this and the master relay are both rectangular with 4 prongs and integrated mounting brackets) that is buried way down and to the outside of the ECU on the passenger side headlight.
I mean this baby is buried. I thought I had all the relays identified until I traced this wire.

The secondary wire off the battery supplies power to the "main" relay (mounted right next to the pump relay) which in turn feeds the pump relay. The fuse powers on the relay when the key is turned.

My pump was frozen (or so I thought). I took the cover off of it after dumping the 8 gallons of lacquer thinner that the gas had become. Man, what a rotten smell. When I pulled the cover off the pump most of the rollers in the pump assemply where gummed up so bad that it was stopping the pump from running. I cleaned them all with a wire brush and stuck them back in and the pump runs fine on the bench.

Now I just have to replace the high pressure hoses, put in some good fuel and try that relay again to see if it is burned out or if it was just the pump being frozen that casued it to shut off power. Hope it is the latter cuz even at Fastlane it is 80 some dollars, dealer wants 126 for the relay!

I could have sworn I pulled a couple off beaters at the graveyard but darned if I can find them in my shop now.

One other item of note. By the firwall and right next to the battery tray there are two relays that look very, very similar and are most likely the same just for reference.
Thanks for your help...
Fisherman

Tomguy · #6 07-09-2007, 09:17 PM

I could have possibly saved you time tracing! Sorry I didnt see that post sooner.

280SE 4.5L 1972 Where is fuel pump relay?

Fisherman · #7 07-10-2007, 09:29 AM

But obviously I didn't read it too clearly. If I had only noted that he stated that he found 2 relays under the ECU I would have dug deeper.

Any idea of a guy can switch the AC relay (by the battery) for the pump relay? It looks like the same relay but mine are so old you cannot read part numbers anymore on them. I'd like to try this so that I can test the pump.

Actually there are two relays by the battery and I do not know what they power. On that other thread there was mention of 1 for the AC but what is the other one doing?

Thanks,
Mitch

Tomguy · #8 07-10-2007, 09:58 PM

There are 4 relays.
1) The aux fan
2) The CSV
4) The D-Jet system relay
5) The fuel pump
http://benz.axiasoft.org/resources/relays.gif

Fisherman · #9 07-11-2007, 04:15 PM

Yeah I checked the other relays and they are not the same so I guess I have to deal with this one and see what I can discover through testing.

Can I ask a little Relay 101?

It is my undertsanding that a relay like this receives a "hot" feed into one pin. Then there is a ground pin. Then the other two are "power on" (usually from a fused connection) and the feed circuit which it is sending power to, in this case the fuel pump.

So based on this the fuel pump gets it's raw power feed from teh master relay which if working properly should supply the raw power feed as soon as the key is turned on. Then when the key is turned or the signal from the fuse turns on the relay it switches closed the feed circuit connector allowing the raw poer feed to pass through the relay and out to the unit it is powering.

Am I correct or at least on the right track with this analysis? The reason I ask is that when the key is on the master relay is supposed to provide the raw power feed but it is not. So even if this relay were working properly there is no raw power feed to supply electricity through (not to) the relay to the power circuit and hence the fuel pump.

If this is true then I could also trace the raw power connection pin, the ground pin, the power on pin and finally if supplying twelve volts through the power on pin and grounding the ground pin I should be able to test for continuity through the raw power and feed circuit pins to see if the relay is functioning properly right?

Thanks for your patience but sometimes electrical stuff eludes me...

kpb · #10 09-01-2007, 10:44 PM

Gotta post here becuz the fix was so simple.

Just got the 108 back on the road after a deer hit almot 18 mos. ago.

Friday, 8/24, puttering into a convenience store 35 miles from home -- DIES, will not restart, lots of spark, fuels seems lacking. Towed home, assume (original) fuel pump finally died.

Putz around, find the lower ignition ballast resistor (blue bracket, original ignition system) is broken and crumbling. Knowing nothing about how my ignition really functions, replace it and car seems A-OK on short hops around town.

Friday, 8/31, driving into work at 80 MPH -- DIES, pull off interstate, refuses to restart, towed home.

Waiting for the tow I realize I cannot hear the fuel pump energize prior to start. Putz around later and find juice to fuse (#4) and no juice at pump. Had been assuming bad pump and cannot find one for sale online, but then realize "Maybe there is a relay." Dig up wiring diagram, find relay, check online here and find valuable threads, most in which Tomguy is repeatedly saying "check the hot wire from battery + to fuel pump." Still thinking, damn, now I gotta source a relay and get the sucker outta its hiding spot by the ECU.

BUT, decide to check the wire...

VIOLA -- feed to fuel pump relay was corroded through at the battery. A zero dollar fix!! Now running like a champ.

Thank you forum and Tomguy.

Tomguy · #11 09-01-2007, 10:50 PM

kpb: That wire bit me once, and I never forget things that bite me! My Benz was my DD while at college and it never needed a tow. It ALMOST stranded me twice - once when the carbon brush in the cap cracked (limped it back), and the other when that wire was loose. Fortunately, my MB pliers (originals!) came in handy, and I pulled the loose wire out of the loop and clamped it in the terminal. Stopped at Radio Shack, got a new loop (or whatever that crimp connector is called) and fixed it!

Fisherman · #12 09-26-2007, 09:54 AM

I put everything back together this last weekend after installing a new (used) fuel pump and getting the car started.
After putting back in the battery tray and realy bracket on the firewall tried to start the car but no fuel pump running prior ot start.

So I started pulling relays and I found two wires, one in the master relay and one in the fuel pump relay that had broken off at the soldered connection inside the plug. If you start pulling these plugs around to test them with a test light be careful...

I soldered them back up and all is running as it should.

Now I still couldn't figure out why the car was running on only two cylinders and I pulled the injectors expecting them to be extremely fouled, the fuel I removed was lacquer thinner... Well they are all shooting a decent fine cone shaped mist so I ruled out the injectors. I then pulled the plugs and although black with soot I cleaned them up and reinstalled them. Still no change.

So after standing there scratching myself I decided to just do some maintenance work and invested the whole $16.00 in some new NGK plugs. All of a sudden I have eight cylinders firing... woo hoo!

Now I have a few more things to fix before I go much further. The bushings in the control linkage are completely shot so I have ordered them and then I need to set the dwell and timing since it now runs enough for me to do so.

Still have a bit of a rough runner and heavy indication of a too rich mixture (black out the tailpipe) but I have not checked all the vacuum lines or sensors so that is the next thing.

Can someone tell me if the sensors are like the diesel versions in that when at operating temp they go closed to ground or are they open (no connection) or closed to power?

Thanks,
Mitch