W203 Fuel pump not priming

[carlh2269]

The car is terrible to start so I've looked at the connector at the fuel pump.
There is no volts at the pump connector during the 10secs so the priming does not happen.
I've swapped out the existing timing relay and the fuse which prooved to be servicable.
The odd thing is, when ignition is switched on, the relay shows no signs of life almost as if there are no volts to the relay coil.
This would be the reason why the volts are missing at the pump plug.
The question is where does the relay coil supply come from to initiate the timing sequence?
There are only 3 wires to the pump but the sender unit works fine as proven by the diesel indicator on the instrument panel.
Any ideas of my next move would be appreciated.

 

[mersum1es]

What is model year of your car? 2008????? it is not W203 then... W204 has totally different fuel system, and common faults are totally different.

 

[Steve@Avantgarde]

Could you post the first six digits of your vin number after the first 3 letters. I would know what model and engine code you have. I can then advise you further.

 

[carlh2269]

Could you post the first six digits of your vin number after the first 3 letters. I would know what model and engine code you have. I can then advise you further.

WDB2037062A
The connector to the pump from the car is fully populated but the pump connector uses only three of the wires.
One wire is ground, the other is used for the sender unit, which works fine and I assume the remaining wire is the dc supply to drive the pump.
I hope this throws a little light.
Regards

 

[Steve@Avantgarde]

Right, W203. I am not sure why you are looking at the fuel pump to the rear, I will assume there are no fault codes and you have checked all diagnostic possibilities at the engine such as injector hub value and leak off, correctly fitted pipes and made sure all pressure sensors are reading?

The fuel pump gets its feed from the Rear SAM unit once the drive authorisation has been completed. As soon as the front SAM recieves the information from the EIS that the key is correct it triggers 12v at the starter and sends a CAN message to the rear sam to prime the fuel pump. The fuel pump is fused in the rear SAM also.

So the simple test here is:

Do you have 12v to the pump at ignition pos 2? If no, go back to the rear SAM, check its fuse. If fuse is good you will need a wiring diagram to test the continuity of the wiring to the rear SAM from the pump. If that is good, check for 12v into the rear SAM. If that is good chances are the rear SAM is duff.

If there is 12v at the pump you need to measure the fuel pressure coming out of it. Spec is 3.8-4.2bar. You will need to connect a fuel pressure tester in line to test accurately. If it doesnt maintain that value you know the pump is weak.

If it passes those tests you know you are looking at something else on the engine side you have missed or possibly haven't thought of or tested.

These 646s are pretty easy to fix. As long as there is good fuel pressure and injector spray pattern along with good compression they should start. There is no over complicated electronic system which will prevent it.

 

[carlh2269]

Right, W203. I am not sure why you are looking at the fuel pump to the rear, I will assume there are no fault codes and you have checked all diagnostic possibilities at the engine such as injector hub value and leak off, correctly fitted pipes and made sure all pressure sensors are reading?

The fuel pump gets its feed from the Rear SAM unit once the drive authorisation has been completed. As soon as the front SAM recieves the information from the EIS that the key is correct it triggers 12v at the starter and sends a CAN message to the rear sam to prime the fuel pump. The fuel pump is fused in the rear SAM also.

So the simple test here is:

Do you have 12v to the pump at ignition pos 2? If no, go back to the rear SAM, check its fuse. If fuse is good you will need a wiring diagram to test the continuity of the wiring to the rear SAM from the pump. If that is good, check for 12v into the rear SAM. If that is good chances are the rear SAM is duff.

If there is 12v at the pump you need to measure the fuel pressure coming out of it. Spec is 3.8-4.2bar. You will need to connect a fuel pressure tester in line to test accurately. If it doesnt maintain that value you know the pump is weak.

If it passes those tests you know you are looking at something else on the engine side you have missed or possibly haven't thought of or tested.

These 646s are pretty easy to fix. As long as there is good fuel pressure and injector spray pattern along with good compression they should start. There is no over complicated electronic system which will prevent it.

Many thanks for your help.
The fuse is good but there are no volts at the pump connector. The timer relay doesn't operate at all but I've proven it's serviceable.
As I thought I've to troubleshoot the wires but as a thought I'll take 12v to the pump to see what happens first.

 

[Steve@Avantgarde]

What timer relay are you talking about? Do you mean the glow plug relay? If so it has nothing to do with the fuel pump.

If you have no volts at the fuel pump but the pump is good you need to check if the rear SAM has voltage and if there is voltage at the fuse. Check the wiring to the pump from the rear SAM. If the answer is yes then the rear SAM is likely goosed.

You will need 12v at ignition position 2. Its not a permanent live to the pump.

 

[carlh2269]

At the moment I'm making comparison checks between my good w203 08 plate facelift and my bad w203 03 plate. In my good 08 the top relay switches the volts to the pump then times out which removes the volts. Click can be heard.
I've the same relay in my 03 plate which doesn't seem to do anything at the moment. The glow plugs function as normal as indicated on the dash.
One of the odd things is that the 20amp fuse is good.
As you say I'll need to troubleshoot the SAM and make further comparisons.
This'll have to wait till the weekend unfortunately

 

[mersum1es]

Its because 2008 model year car has primer pump in tank, 2003 model year (diesel engine) doesn't. Suction aux pump front of the motor. Transfer system between saddles in tank. And 2008 c-class is W204...


Edit: W203 fuel system is changed somewhere 6/2003, older has no pump in tank.

 

[carlh2269]

This must be the older type. It would explain why there's no heavy guage wire used at the pump connection.
The Mercedes garage has also given me the wrong printout even tho I gave them the chassis number.
I'll check later under bonnet to check what's there.

 

[carlh2269]

At the front there's a black unit with diesel pipes fitted. Haven't taken much notice of this item till now.
I'll nip into Mercedes to see if I can get some technical info.
Can't do much now till the weekend.

 

[Steve@Avantgarde]

The 03 will be running CDI2 with a lift pump driven off the camshaft with a Venturi effect swirl chamber in the tanks to pump the fuel from left to right.

The 08 will run CDI3/4 and will have an electronic pump in the tank to deliver fuel to the engine.

So which car has the non start fault, the 03 or the 08?

 

[carlh2269]

The 03 will be running CDI2 with a lift pump driven off the camshaft with a Venturi effect swirl chamber in the tanks to pump the fuel from left to right.

The 08 will run CDI3/4 and will have an electronic pump in the tank to deliver fuel to the engine.

So which car has the non start fault, the 03 or the 08?

All sorted now thanks.
Turned out to be the HP pump driven from the camshaft.

 

[abdellatif]

Bonjour à la communauté de ce forum.
Je me présente Abdellatif

Je possède une Mercedes avec 203 cdi 220 année 2004 OM 646 963
j'ai d'abord changé un joint de culasse à ce jour la voiture refuse de démarrer le défaut de la pompe de surpression le carburant n'atteint pas le filtre (BASSE PRESSION) passe en sécurité
J'ai changé beaucoup de pièces de rechange pour la pompe de surpression arrière sam ect ....... le relais de pompe SAM derrière la borne de commande 85 a réagi et n'est pas arrivé même pas une deuxième étape de transfert.
organes testés
1) alimentation sam. Arrière 12 volts.
2) fusible de pompe n 4 bon
3) pompe de surpression avec un bon connecteur à 5 fils
J'ai vérifié tout pour l'absence d'anomalies électriques pour le moment.
seuls les feux arrière qui s'allument, seul le ventilateur est froid lorsque tout le régime moteur est sûr.
steve @ Avantgarde Membre dans son post # 12 pour parler de quelque chose d'intéressant sur une motopompe et un bras de came.
J'ai également changé un joint de culasse à cause d'un boulon d'injecteur cassé.
mes questions.
sur monte la culasse je n'ai rien fait de bête sur les pointes des pignons pompe haute pression HP ou chaîne de distribution ou pompe à vide.

J'attends vos réponses.
merci pour toute aide de votre part
Merci.

 

[mersum1es]

Kovaa ajoo vaan!