scudderfish
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 posted on 9/6/14 at 07:59 AM |
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Fuel pump conundrum
I'm posting this as I don't understand what I'm seeing and I hope someone can shed some light on it.
I went to start the car yesterday and the HP fuel pump wasn't running. I took the following measurements:-
1. Voltage at +terminal of pump = 0v
2. Disconnected feed wire at pump, voltage on feed wire +12v
so I assumed the pump had died and ordered another one. However thinking about it last night I realised I had never directly applied power from the
battery so I just did that and the pump ran, drawing about 6 amps.
If the pump is healthy, I don't understand how the feed can have +12v on it and it doesn't even attempt to turn over.
The car is running MS1, and the +12v seen is the 2 second priming pulse on ECU boot. I can hear the relay switching on and off behind the dash, and
the measured voltage on the feed to the pump changes in time with the relay activation. The fuse is sound, and if I put +12v from the battery onto
the feed side of the fuse, the pump runs.
I could probably fix it by swapping things around (relay might be suspect), but I'd like to understand the symptoms I'm seeing. I'd
like to know where the +12v seen in 2. is coming from and why it doesn't spin the pump.
Regards,
Dave
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pekwah1
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| posted on 9/6/14 at 08:13 AM |
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The obvious assumption if you are measuring 12v on the positive is that there is something wrong with your earth.
I actually had the same problem with my low pressure pump not long ago and it was the earth.
I ended up just running another earth cable to the battery instead to make sure it was a good earth.
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Ugg10
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| posted on 9/6/14 at 08:14 AM |
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Off the top of my head an something I guess you have checked - dodgy earth ? Try connecting the -ve directly to the battery -ve and see what happens,
if it runs it is the earth.
Pekwah1 - beat me to it whilst I was typing !
[Edited on 9/6/14 by Ugg10]
---------------------------------------------------------------
1968 Ford Anglia 105e, 1.7 Zetec SE, Mk2 Escort Workd Cup front end, 5 link rear
Build Blog - http://Anglia1968.weebly.com
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daviep
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| posted on 9/6/14 at 08:21 AM |
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Sounds like a bad connection, you see 12v when the pump is disconnected as there is no current flow.
Imagine a garden hose which is blocked and only has a tiny clear path, when the nozzle is closed the whole hose will slowly come up to the same
pressure (voltage in electrical terms), as soon as you open the nozzle the pressure just disappears and there is no flow to do useful work.
Good chance it's dirty relay contacts. Testing with a test lamp is usually quicker / simpler and more reliable than a multimeter, because a test
lamp draws current it either won't light or will be dim if there is a bad connection.
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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scudderfish
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| posted on 9/6/14 at 08:25 AM |
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It's not the earth. I just directly connected -ve on the pump to -ve on the battery and the pump didn't spin when I switched on.
I'll knock up a test lamp and try that.
Regards,
Dave
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HowardB
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| posted on 9/6/14 at 08:37 AM |
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quote:
Originally posted by daviep
Sounds like a bad connection, you see 12v when the pump is disconnected as there is no current flow.
Imagine a garden hose which is blocked and only has a tiny clear path, when the nozzle is closed the whole hose will slowly come up to the same
pressure (voltage in electrical terms), as soon as you open the nozzle the pressure just disappears and there is no flow to do useful work.
Good chance it's dirty relay contacts. Testing with a test lamp is usually quicker / simpler and more reliable than a multimeter, because a test
lamp draws current it either won't light or will be dim if there is a bad connection.
Cheers
Davie
+1
Howard
Fisher Fury was 2000 Zetec - now a 1600 (it Lives again  and goes zoom)
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DW100
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| posted on 9/6/14 at 09:59 AM |
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For something like a fuel pump that draws a reasonable current you could use a headlamp bulb as a test bulb to load the circuit.
Start at the fuel pump end and test step by step back to the pump relay to find where you have a high resistance connection.
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scudderfish
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| posted on 9/6/14 at 01:23 PM |
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I built a test lamp out of a headlight bulb in a jam jar. I've narrowed it down to somewhere between the feed into the relay and the relay
itself, probably in the holder. Now for a slight hiatus as it is a bit of a git to get at.
Thanks for the suggestions.
Regards,
Dave
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