1990 300CE won't stay running - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 07-31-2015, 01:49 PM

I recently got this car, and it will not run for more than a second or so. I read a couple threads, but can't find the exact problem listed. What is the best way to check for fuel pressure? I suspect it is a fuel issue, not ignition. It has a full tank of gas even though the low fuel light is on, and the gauge says empty (can you smell the gas on me from over filling the tank? :-). Where is the fuel pump on this car? I am mostly a Diesel guy, but I got this car with a wrecked 300D 2.5 (the PO was going to put the 2.5 Diesel engine in the CE). I am sure (at least I hope) it is something simple. Any guidance would be greatly appreciated. Thanks, Rich

Frank Reiner · #2 07-31-2015, 02:34 PM

Roller:

Sounds like fuel pump relay. Pull relay out of holder, jumper sockets 7 & 8.
You should hear the pump run continuously. Pump is mounted close to right axle shaft.

ROLLGUY · #3 07-31-2015, 04:16 PM

I will try that. Where is the location of this relay? (remember, I'm a Dieselhead)

Frank Reiner · #4 07-31-2015, 05:10 PM

Roller:

Relay location varies from model to model; in that 124 it should be behind the battery, and may be integrated in a larger package that has other control features. The relay itself should have an RPM marking on it; it serves as a rev limiter by shutting off the pump at the marked RPM.

ROLLGUY · #5 07-31-2015, 06:25 PM

Quote:

Originally Posted by Frank Reiner

Roller:

Relay location varies from model to model; in that 124 it should be behind the battery, and may be integrated in a larger package that has other control features. The relay itself should have an RPM marking on it; it serves as a rev limiter by shutting off the pump at the marked RPM.

Thanks, I will look tomorrow....Rich

ROLLGUY · #6 08-01-2015, 04:31 PM

The problem is definitely no fuel, as the pumps are not running. There is no power to the pumps. I checked the fuses on the OVP, and they are fine. I attached photos of the area behind the battery, and the fuse panel. Can someone tell me where to find the fuel pump relay?

ROLLGUY · #7 08-07-2015, 12:41 PM

Well, I pulled the component marked MAS and jumped the two large terminals toward the front of the car (there are 2 large terminals in the rear as well), and the fuel pump comes on. However, the engine will not start. It wants to, and I think the battery may be low. After charging the battery, I will try again. I took apart the box, and re-flowed the solder joints. After the sealant dries, I will try the unit in the car. If it does not work, I can only assume the part is bad. Anyone have any other suggestions as to what might be wrong?

pmckechnie · #8 08-07-2015, 04:16 PM

Rich, Have you checked for spark at the plugs? The reason I ask is that the fuel pump will not start until it has a tach signal from the ign control system. That will also explain why it won't run with the fuel pump power jumpered.

PaulM

ROLLGUY · #9 08-07-2015, 05:52 PM

Quote:

Originally Posted by pmckechnie

Rich, Have you checked for spark at the plugs? The reason I ask is that the fuel pump will not start until it has a tach signal from the ign control system. That will also explain why it won't run with the fuel pump power jumpered.

PaulM

I tried it a bit ago with the MAS unit after re-flowing the solder joints, and it sorta wants to run. It will start and idle/stumble and then die. That tells me it has spark. I can't really hear the pumps running though, so I suspect the MAS unit is still bad.

Frank Reiner · #10 08-07-2015, 10:55 PM

Quote:

Originally Posted by pmckechnie

Rich, Have you checked for spark at the plugs? The reason I ask is that the fuel pump will not start until it has a tach signal from the ign control system. That will also explain why it won't run with the fuel pump power jumpered.

PaulM

Roller:

There are two triggers that will close the fuel pump relay: 1) the cranking signal that is supplied when the key is turned to "Start", and 2) as cited by pmc, a signal from the ignition after engine start.

When you are cranking the engine the pumps are running on the "Start" signal, and enough fuel is supplied to partially fire the engine.

On the subject of pumps, consider the possibility that only one of the pumps is operating; the symptoms you describe are consistent with low fuel pressure. Try the jumper again with one, and then the other pump ground wire disconnected.

ROLLGUY · #11 08-10-2015, 03:18 PM

Quote:

Originally Posted by Frank Reiner

Roller:

There are two triggers that will close the fuel pump relay: 1) the cranking signal that is supplied when the key is turned to "Start", and 2) as cited by pmc, a signal from the ignition after engine start.

When you are cranking the engine the pumps are running on the "Start" signal, and enough fuel is supplied to partially fire the engine.

On the subject of pumps, consider the possibility that only one of the pumps is operating; the symptoms you describe are consistent with low fuel pressure. Try the jumper again with one, and then the other pump ground wire disconnected.

How exactly would I check that system? I just got through replacing the MAS unit with a wrecking yard one (I assume it is good), and the car still will not start. That makes me believe that the original one is fine. Again, it wants to start and run a little, but only for a second or two. If the relay is not getting the proper signal after start, what would cause this to happen? Is there an easy test I can do?

Frank Reiner · #12 08-10-2015, 03:39 PM

Quote:

Originally Posted by ROLLGUY

How exactly would I check that system? I just got through replacing the MAS unit with a wrecking yard one (I assume it is good), and the car still will not start. That makes me believe that the original one is fine. Again, it wants to start and run a little, but only for a second or two. If the relay is not getting the proper signal after start, what would cause this to happen? Is there an easy test I can do?

1) Are both pumps running? Remove MAS, jumper pump sockets, check that both pumps run.

2) If both pumps run, install MAS, run long jumper from battery to pumps, try engine start. If engine will now run, suspect either a fault in connections of the run-trigger signal [called TD], or a lack of that signal from the ignition controller [EZL]. If the engine runs, does the tachometer work? The tach is driven by the TD signal.

ROLLGUY · #13 08-10-2015, 04:04 PM

Quote:

Originally Posted by Frank Reiner

1) Are both pumps running? Remove MAS, jumper pump sockets, check that both pumps run.

2) If both pumps run, install MAS, run long jumper from battery to pumps, try engine start. If engine will now run, suspect either a fault in connections of the run-trigger signal [called TD], or a lack of that signal from the ignition controller [EZL]. If the engine runs, does the tachometer work? The tach is driven by the TD signal.

Thanks for this info, it is helpful. However, I am not much good with gassers, so I will need specific instructions when troubleshooting. I jumped the one socket that made a pump run, but how do I know which one is running? Are they both on a different socket? I just assumed that if I heard a pump run, the engine will run. Also, does the - side of each pump go directly to ground, and it only needs + to run? If so, it should be easy to hotwire the pumps. I just hope there is no problem back-feeding power into the electronics....Rich

Frank Reiner · #14 08-10-2015, 04:56 PM

Quote:

Originally Posted by ROLLGUY

Thanks for this info, it is helpful. However, I am not much good with gassers, so I will need specific instructions when troubleshooting. I jumped the one socket that made a pump run, but how do I know which one is running? Are they both on a different socket? I just assumed that if I heard a pump run, the engine will run. Also, does the - side of each pump go directly to ground, and it only needs + to run? If so, it should be easy to hotwire the pumps. I just hope there is no problem back-feeding power into the electronics....Rich

The pumps are wired in parallel; when you jumpered the sockets that caused a pump(s) to run, you were powering both. By alternately removing the ground wires (brown) from each pump you can determine if both are operating. Yes, the ground wires are always connected to ground. No problem with powering the pumps directly from batt.

ROLLGUY · #15 08-11-2015, 10:29 AM

Quote:

Originally Posted by Frank Reiner

The pumps are wired in parallel; when you jumpered the sockets that caused a pump(s) to run, you were powering both. By alternately removing the ground wires (brown) from each pump you can determine if both are operating. Yes, the ground wires are always connected to ground. No problem with powering the pumps directly from batt.

Cool! that is exactly what I need to know, thanks. If one of the pumps is not working, would that cause it not to run?