I have learned how to jump the fuel pump relay pin #30 and #87 (thanks to the forum).

I am using a 12 gauge solid copper wire as a jumper. Is there any certain size I should use or is that one ok? Can I blow or burn anything up if I leave that jumper in the relay for an extended period of time? Would I have time to check if the fuel pumps are getting 12 volts at the pump as Arthur Dalton suggested to another poster?

I don't have anyone who can help me so can this be a one person job?

Thanks to anyone who can help!

1986 420 sel Sorry forgot to include what it is

The jumper is just a circuit by-pass to test the pump/s.. You can jumper and then just put your hand on the pump..if it is running, you will feel it..
You can also make sure you have 12v at #30 first.....if NO , then the pump won't run regardless of jumper and the problem is elsewhere ..[ like the fuse]

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A Dalton

Hi Arthur Thank you for your quick response.
If I understand you correctly I can put the jumper in the relay and then take my test light and run to the rear of the car test for the 12 volts and touch the pumps and won't damage anything. It's ok to leave jumper in the relay that long?
Thanks to you and your posts I already did check the voltage at the jumper with test light and have power there.
Yesterday a friend stopped by and I had him cycle the ignition a couple of times and the fuel pumps were working but no fuel to the engine.

If you wouldn't mind looking at the post "FUEL AND START PROBLEM WITH 86 420 SEL" title you will find the whole story and see if I have followed the procedure in the order that you would have.

Thank You!!!

If you get 12v at the FP with the jumper, that does not mean the pump is running..it simply means it has power...that is why I ask you to feel the pump..if it runs , it will have a whine sound and you can feel it as a frequency vibration..if NO, then it is most likely gummed up.
The jumper was just to asssure you get 12V to pump. It is a First test only.The jumper is just an easy way to test the pump with Direct Battery V, just as if you had taken the pump OFF and did a 12v Bench test on it...same deal, but this jumper just makes it easy cuz you don't have to remove anything but the relay and install the jumper to get direct Bat V to the pump in situ..that's all the test does..don't assume you have a good pump just cuz you brought 12v over to it..and if you get 12v to the pump and you do not hear/feel it run , disconnect the jumper cuz you have a pump problem.


If you have an Amp meter , you can replace the jumper with the amp meter across 30 and 87 and see what the FP amp draw is...if it is normal, it should be working..if it is High, it is stuck [ .. if NO draw , the circuit is OPEN , electrically.]
That chassis should use a single/dual stage pump, so the Normal current range will be 7-10 Amps.............................
Simply put, if your jumper makes the pump run, then you can start looking Upsteam from the relay and feed circuits, b/c you now know you have a good pump..If still NO pump w/jumper, you know you have a problem Downstream from the relay [ pump/connection/wires/etc]

You seem to have little experience on these matters , so be very carefull when working with gas/electric combo...Dangerous, to say the least.

You can also place a 15 A in line fuse in your jumper if you do not have an amp meter...that will protect the circuit and if the pump is frozen, the high Amp draw caused by a stuck pump should pop that 15A fuse. A stuck pump may also feel warm with the feel test.

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A Dalton

Thank you once again Arthur,
Actually I do have some mechanical experience. I have worked on my old 57 Chevy for over 30 years and my old 69 Mopar for over 20 years, however when it comes to electrical and fuel injection I am basically illiterate. Fortunately nothing has ever gone wrong with this MB thus no experience even though I've owned it for 15 years.
I will learn how to work on this thing by reading the threads and asking questions providing that you and other forum members can tolerate my ignorance.
This car has the dual pumps both of which are running. When I test the amp draw of the dual pumps should the range still be 7-10 amps or would that change? What kind of amp draw would I look for if the pumps are gummed up?

Thank You so much!

Dual pumps are 4-8 amp.

You are asking for an amp draw on a gummered up pump..there is no way to know that ...
You are now saying the pumps are runnin g w/o telling me the conclusion to the test I posted.
So, if the pumps have fuel on the suction sde , are running, and no fuel is comming out the high side, you have a gummed up or mechanically failed pump. So check if you have fuel flow.

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A Dalton

Arthur,
I have a DMM and I will go do the test and report back ASAP.
I jumpered the relay, went to the back and grabbed the pumps and they are both vibrating and sound as though they are working. Will have to report later as to fuel flow.

Thank You for being patient

You don't need the DMM now ...you know the pumps run, so we are beyond that test..the test I wanted reported was that you grabbed the pumps and they are running..that's all...so now you want to know if they actually pump
So, flow is the next test.
There is a screen in the tank and there is a check valve in the pump outlet...so, flow is the test to find out if they are gummed up or the tank screen is plugged.

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A Dalton

Thank You Arthur
I got it, but will have to report on the flow and check valves tomorrow.
I'm happy that the DMM report was not necessary b/c I just bought the thing and I couldn't figure out how to get the amp. reading. (YET)
I will be posting as soon as I have your answer and if you have the time to keep up with this thread I will surely appreciate it.

Thank You and have a great evening. Dale

OK


Your main concern will be to see if you have flow from the tank to the pump inlet b/c there is a screen in the tank to catch all the crap, so they are always suspect if the pump is running and you have no flow. That is easy to check by clamping the line and unhooking it at the pump and open it into a container. If it flows freely, you know you have fuel to the pump. Some guy blow a little air in if it's clogged and that usually clears it out. Look in the PARTS section at top of page and you will see a pic of that screen.
I see you have other threads on the same complaint, so it is getting hard to follow where you are, so I will just wait on an outcome ...rather bothersome to have to jump around to other post.

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A Dalton