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Ask Lou P. master tech Your Own Question
Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3914
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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2000 CLK 430JA: Sometimes things that you think will be

Customer Question

2000 CLK 430
JA: Sometimes things that you think will be really complicated end up being easy to fix with Mercedes. The Mercedes Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so he can help you best.
Customer: Low voltage at the fuel pump relay. about 8 volts. not enough to trip the relay to energize the pump. Any ideas?
JA: There are so many things that can cause battery issues. Some simple, some complicated. The mechanics have tons of experience in that area. Will the car start?
Customer: yes
JA: Glad to hear that. How old is the battery?
Customer: I don't know. It has 12.5 volts when the car is not running and high 13 when running
JA: Are you hoping to fix this yourself?
Customer: yes
JA: Great! What have you tried so far?
Customer: nothing other than trying to figure why the fuel pump wont start by measuring voltages
JA: Anything else I can tell the Mercedes Mechanic before I connect you two?
Customer: low voltage at the fuel pump relay. around 8 volts.
JA: I'm sending you to a secure page on JustAnswer so you can place the $5 fully refundable deposit now. While you're filling out that form, I'll tell the Mercedes Mechanic about your situation and connect you two.
Submitted: 9 months ago.
Category: Mercedes
Expert:  Lou P. master tech replied 9 months ago.
Hello, can you tell me how the issue started and what led you to checking the fuel pump relay activation signal?
Customer: replied 9 months ago.
The car hesitates to start. It will start only but only after cranking for 4 or more seconds. I checked the shrader valve on the fuel rail with the key in the on position and there was no drip of fuel. I am also having fuel trim codes and engine misfire codes when the accelerator is pressed more than about halfway.
Expert:  Lou P. master tech replied 9 months ago.
ok thanks, ***** ***** car ever crank, catch and then shut right off?
Customer: replied 9 months ago.
the issue has been going on for a while but since it eventually starts I didn't worry about it too much.
since there was no fuel pressure with the key on I checked the relay. the relay clicks with 12 volts applied and completes the circuit. when installed there is only 8volts and that doesn't trip the relay
Customer: replied 9 months ago.
the cranks and catches but does not quit. it keeps running.
Expert:  Lou P. master tech replied 9 months ago.
so you know the ME-SFI control module (engine module) will apply some voltage to test the circuit in order to trip a fault code or not. that could be why its 8 volts. it also depends on if the relay is ground activated or positive activated. i would have to check
Customer: replied 9 months ago.
I don't know what any of that stuff is
Expert:  Lou P. master tech replied 9 months ago.
ok, gotcha, so the issue is mainly at times you have extended crank . on this engine there was a issue with rust on the flywheel so lubing the flywheel will take care of the scenario i mentioned with the cranking, catching and dying right away. what any of what stuff is?
Customer: replied 9 months ago.
it always cranks and finally catches but doesn't die. It runs fine as long as you don't push the accelerator more than about halfway
Expert:  Lou P. master tech replied 9 months ago.
ok, so far sounds like a failed fuel pump that cannot produce enough pressure, do you have a pressure gauge or a parts store nearby where you can rent/purchase one?
Customer: replied 9 months ago.
it seems to start immediately when cold but has extended crank when hot or recently driven
Customer: replied 9 months ago.
I do not have one but there is a store nearby
Expert:  Lou P. master tech replied 9 months ago.
ok that seems also more and more like a fuel pump. i think a fuel pressure test is the best thing to do, if they rent tools then rent a fuel pressure gauge and a noid light. the noid light will go in the injector connector and tell us if the fuel injectors are getting a signal pulsed to spray fuel, also for a quick test, unplug the mass air flow sensor and see if the car runs better with it unplugged.
Customer: replied 9 months ago.
ok. is there anything else i should try?
Expert:  Lou P. master tech replied 9 months ago.
not at this point, fuel pressure test is the biggest thing. that will tell us a lot.