4 replies to this topic

[Calidonian]

G’day all

I have been chasing down a fuel pump issue and thanks to many of the great posts on this forum I managed to trace the fault and rectify it, or so I thought?

Well as it turns out I think I only found a symptom of the primary fault, the fuel pump fuse holder (in line type ) had deformed slightly from heat and wasn’t making a

proper connection, hence the intermittent issue.

The fuse holder was replaced with the same type ( before you say anything, it’s all I had at the time) it also had a 25amp fuse fitted instead of the correct, as I now know 10amp one.

Once I put everything back together including the 25amp fuse, it started and ran fine for weeks and around 250km of trouble free driving, until it broke down the other day. Thankfully it was only a short distance from home, and as I could no longer hear the fuel pump I checked the fuse to find the same issue again with fuse holder deformed from heat.

I did a roadside bodge to get home the went into diagnostic mode inspired by the smarter Mini forum gurus.

 

This is what I checked and found based on the assumption I had one or more of the following:

• Bad earth - All the ones I could find checked and cleaned.
• Shorted wire- could not see anything obvious?
• Fuel pump failing - Current draw was 3.5-3.8 Amps and fuel pump motor resistance was 1.6. Ohms

• Removed pump for inspection- All looked clean and no crud in the tank or pre-filter.

The car is a Japanese import (1994 Mayfair 1.3 SPI ) and from what I can tell it had been off the road for around 9 years before I got it, I have changed the fuel filter also and from what I can tell it appears to run as it should? Although mid range acceleration does seem to be a bit flat?

So my question is as I can find no reference, is the fuel pump current draw and resistance in spec or is the pump on its way out?

Your thoughts please.

Edited by Calidonian, 26 January 2021 - 04:44 AM.

[Wazzah]

No one has answered so may I have a go at solving your problem.

Your fuel pump circuit is creating heat which can only occur because of high resistance.

Your current draw and pump resistance don't make much sense to me.

Using Ohms Law I would expect to see a current draw of between 7.5 amps and 8.25 amps which assumes a battery voltage between 12.5 and 13.2 volts across your 1.6 ohm motor.

A 3.5 amp draw through a 1.6 ohm motor indicates a voltage of only 5.6 volts across the motor.

A 3.5 amp draw would not cause the heat you describe.

I've got a spare pump at home so I will measure it's resistance for a comparison.

Whilst I suspect a high resistance somewhere on the positive side of the pump (explained by the voltage drop) it does not explain the low 3.5 amp current flow.

Please describe your method of testing the pump (including test points) so I can better understand what is going on.

The 25 amp fuse holder replacement aspect is bothering me somewhat.

Quite simply you should be experiencing higher current at the pump given the heating symptoms.

Please isolate the fuel pump feed at each end and measure the insulation resistance of the copper wire to earth.

Normally this should be in the many thousands of ohms (megohms in fact)

Then check for resistance end to end of the isolated wire.

This should be a hard zero.

Let me know these results.

Normally I would get you to check all the connector plugs in the circuit for corrosion but you have low current draw at the pump.

With the info at hand at the moment it appears you have leakage to earth upstream of the pump which explains the high current at the fuse end (heat) and low current at the pump. (sum of all current flows in parallel circuits)

I also suspect some conductor damage ( which causes resistance) at the same point which explains the low voltage (drop) at the pump. (sum of all voltages in series circuits)

Check everywhere your wiring passes through a bulk head and anywhere someone has screwed or drilled into your car.

If it was the earth downstream of the pump you would be seeing the high current through the whole of the fuel pump circuit.

Another explanation could be that someone has bodged a poorly connected tee off into the fuel pump circuit as a live feed for some other appliance.

Such an appliance could include a powered speaker or amplifier.

Check the boot and under the rear seat.

Good Luck and I'll watch your posts for more clues.

Regards

Waz

[Calidonian]

Hi Waz and thank you very much for the detailed information and supporting diagnostic advise , and I do apologies for not responding more promptly. Seems I did not set up the notifications from the forum correctly.

From my post you can probably tell my electrical knowledge is basic, and while it may take a few read throughs of your insights, I’ll get on with some rechecking and take some pics of how I’ve measured the draw an resistance.

Since I posted my question I have replaced the in original in-line fuse holder with a blade type and the correct 10amp fuse, with no noticeable heat issues and fuse has not blown.

Cheers

Mike

[Wazzah]

Hey Mike I'm in Melbourne if you ever need anything.

You can get me on 0418 540 773.

Good to see things have settled down up your way.

[Ethel]

Fuel pumps on injection cars are motors. The resistance of a stalled motor can be close to zero. It could be indicative of a faulty pump or obstruction upstream of the pump.