Bad fuel pump or immobiliser?

100dollar124

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W124 300E M103
Hey all,
New to Mercedes scene so if this is a stupid question excuse me. Recently picked up a W124. All electrics work and it turns over fine and has spark. So I checked fuel pump and I can feel it buzz when the ignition is switched on however neither of them do anything when I go to turn it over. It had an aftermarket alarm system that was acting up. I only got the valet key with it. Could it be something to do with that? Or is it just a buggered fuel pump?
Cheers
 

bembo449

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i would concentrate on getting fuel to the engine first and as fuel pumps aren't to expensive , put a new one on it and se if it springs to life ? the fact that you have ignition and spark suggests the alarm immo is a red herring
 

LostKiwi

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The sequence goes:
Ignition on.
Pump runs to prime the system.
Pump stops.
Cranking begins.
Pump restarts only if crank position sensor detects crankshaft rotation.


So if pump runs and generates pressure it's probably ok.
If it doesn't fire at all then it's likely the CPS has failed.

An aftermarket immobiliser can screw this up too depending how it's connected (and being aftermarket god only knows how it's setup).
As always if you can get codes read then you will soon see if there is an error.
 

ajlsl600

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Disable alarm.Does it even try to fire up? Is batt in good order? If so take air filter out ..with eng turning mist some fuel in there does it try to fire up ? Yes.look pump /pipework/wiring .no look cps.
If you have access to star or icarsoft you can get answer that way.
 

LostKiwi

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Disable alarm.Does it even try to fire up? Is batt in good order? If so take air filter out ..with eng turning mist some fuel in there does it try to fire up ? Yes.look pump /pipework/wiring .no look cps.
If you have access to star or icarsoft you can get answer that way.
Icarsoft won't work on a 124. That will have either a 38 pin plug or the earlier 14? 16? Pin plug.
 

DREAMER NO2

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W124 2.6E M103 1989
If you have the M103 in your 300E hot wire number 7 and 8 on the fuel pump relay, just to see if it will start, after the start up remove the wire --important you do so /
 

ajlsl600

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Icarsoft won't work on a 124. That will have either a 38 pin plug or the earlier 14? 16? Pin plug.
Corrected. But star will? I think my kit incl 38 pin.as per w208 .
 

LostKiwi

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We already know the pump is trying to run (OP said it buzzes) but we don't know if it's generating pressure. We also don't know if it's getting a signal from CPS to tell it to start running again during cranking. Removing a plug lead and using a sparkplug against the engine will show if the CPS is working - no CPS, no spark, no fuel.
If we get spark we must have CPS signal so check fuel pressure (crack a line and look for pumping). No fuel then check if pump still runs and if still no fuel when running change pump.
 

d215yq

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Both my newer w124s (e220s) with factory immobilizer would turn over but not start when immobilized. This confused me the first time as I thought immobilizers immobilised the starter motor... So it's possibly the issue here.
 

DREAMER NO2

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Yes it was the same as my W124 M103 260E .But i had an aftermarket alarm and immobiliser on mine,, but it would spin the engine over,, but no spark at the plugs .I changed every part i could most all the electrical system i had spares for ,, even EZL But in the end i did not know what else to do,, so i had to call in the auto electric guy wou found the alarm and immobiliser fault. He took 2 wires off the distributor to get it up and running.
 
OP
100dollar124

100dollar124

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W124 300E M103
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Hey all,
Sorry for not responding sooner. Cheers for all the suggestions. It’s an M103 Lostkiwi. I’ll attach to my profile. So far I’ve tried starter fluid straight down the throat which got it to kick and run briefly. Correct me if I’m wrong but is the fuel relay located behind the battery? I think that might be my next best bet. If that doesn’t work I’ll replace the fuel filter and pump assembly. I did take a quick fuel sample and it looks pretty grotty so I could just be over thinking it as well. Cheers again for all the suggestions
 

DREAMER NO2

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So pull it out if you have a M103 the fuel pump rel;ay will be at the rear of the battery ..Pull off the plastic type cover to find it .You might need some shims pushed down inside the holder to release the fpr . Mine was a bersteward to get out of the holding bracket ..This is for the M103 engine ,, also check fuses on ovp relay are good .
 

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LostKiwi

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Before messing with relays and pumps are you getting any fuel?
Disconnect the main fuel pipe under the bonnet and feed it into a bottle. Turn on ignition.
No fuel then pump or relay is next port of call. If you get fuel you can forget the relay and probably pump (though if the engine is carburettor fed it should run even with low pressure). If injected it needs more pressure so if the pump can't deliver pressure it won't fire up. To test the pump you need a pressure gauge.
 

DREAMER NO2

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There is an easy way to check if there is fuel in the distributor . Remove the air filter box, and then gently push down on the throttle plate. If it stops down there will be no fuel in there = no pressure .
 
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100dollar124

100dollar124

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W124 300E M103
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Back at the car again today to put things into motion. Took fuel line off to see if it pumped anything and there was a tiny spurt of filthy fuel (see pic). That was fuel after ignition on wait for prime and Turning over. I wasn’t able to tell if it pumped more fuel in between attempts. I’m no mechanic but i’d say thats not nearly enough. Next i’ll jump relay and then drain and clean out fuel system from all the filth. Let me know your thoughts.

Cheers
 

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