Hi All,

I had some intermittent fuel pump issues so I've done a bit of digging and found some interesting problems...

Issue 1 is easy to solve, the earth wire to the pump wasn't crimped properly (prior to me getting it)


Issue 2 is causing me a headache. The ground wire to the pump is melted. There were two black wires connected together at the pump end


I pulled the fuel pump wiring back into the boot and found that the black ground wires were melted together until the branch to the fuel gauge sender. The ground wire to the fuel gauge sender is melted all the way to the tank. The green/black sender wire is also melted to the sender ground.


So:
The white 12v feed to the pump is fine (not shown in the diagram here)
The black wire from the branch point to the pump is melted
The black wire from the pump to the level sender is melted
The green/black wire from the sender to the branch is melted in spots


Most of the mini wiring looms I can see have separate earths for the pump and sender, but Tony Cripp's 850 book states body harness 5L225 has "12 - Tank unit earth to petrol pump earth" which makes me assume it's original for my car? The earthing point in the loom is near the LH taillight

It seems strange to me to have the pump earthing to the sender, as the easiest path for the current to travel would be the sender rather than up the body loom? I assume there would be no problem with changing these to have separate earths? Is the fuel sender earth connected to the tank body or is it isolated? Is there a specific orientation on the level sender for the earth and fuel gauge wires?

I'm also guessing having a short between the fuel gauge sender and earth wire wouldn't cause any issues, other than causing the tank level to be incorrect? But if the signal wire from the gauge is getting to the earth from the pump this might cause issues?

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The fuel pump and the fuel tank (and sender unit) are both rubber mounted and so require an earth lead each. The wire 12 is joined to wire 3 which is the earth point near the tail light. It's cheaper to put in an extra black wire or two and route to a single earthing point rather than put additional connectors, screws and body drillings for separate earths. The only wire that carries any significant current here is the white pump feed return (-), but if this is OK, then there would appear to be no electrical problem (other than the poor crimping as you've noticed). The melted wires could be a result of them being caught up in the exhaust system or something.

Thanks Tony

The strange part I’m trying to work out is why the only wire that’s melted end to end is the ground from the pump to the sender.

It does have an aftermarket Ecco pump in it & is negative earth, and both these changed happened wel before I got the car. Perhaps one or both of these lead to the issue? (Also worth noting the melted section under the floor is covered internally with dirt, so it’s been like this a while)

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Maybe it melted before with a previous pump for some reason but some dodgy person didn't replace it or maybe even realise.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

Yes it is odd that only the black wires are affected since any short circuit has to surely involve the white feed wire for a path to -ve battery. I think the clue is that those connectors are not factory, and the colours are a bit mixed up with red on the negative white wire and blue on the positive black wire. Perhaps someone didn't realise that the car was postiive earth when the after-market pump was installed and so it was connected the wrong way around at first.

The ECCO pumps have a plastic body so the earth can't cause a short. If the incorrect polarity caused an issue I would expect the pump to be dead. That's why I think the issue could have been before the ECCO pump was fitted.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

For the wire to get hot enough to melt the insulation the copper will show signs of heat trauma. That is it will discolour changing from the usual red colour to a dull pink and it will be fragile having been annealed somewhat.
Looking at yours the copper seems fine so the heat has possibly come from outside the wire.
Having said that the faulty crimp may have caused a high resistance point but the heat would only be local to the joint.
I agree with the others that it was probobly a old previous fault.

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Cheers Dave.

The pump being a motor will not be bothered about polarity, whether tha actual pump would seize going in reverse I do not know.
There will be as much current flowing through the neg as the pos more or less so it could be either wire that fails but the poor connections on the earths would do it.

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Cheers Dave.

There are electronic components in it. They are designed to replace SU pumps and work the same except they have electronics in them just like the electronic SU ones which are polarity sensitive.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

Sorry, I thought it was a standard SU. I should have read your post properly!!

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Cheers Dave.

It seems to me that the only significant source of current in this area is the white wire which comes direct from battery -ve (via the ign sw). The current in the green/black wire is limited to a small value by the gauge and the tank sender unit.
If there is a reduced electrical resistance in the pump (due to reversed electronics), then both white and black wires should be affected by excess current. Those wires are rated at 7A but by the look of the black ones, have been carrying about 20-30A if the problem is electrical – but the white wire is unaffected.
If there has been an external problem, then you’d think the green/wire would also show signs of distress, it being wrapped with one of the black wires.
A high resistance at the black wire connector crimp would reduce current flow overall but for the voltages involved, there would be no more than about 6W dissipated at the connector at 1A or so. Enough to feel with fingers, but not to melt the wires.
There is indeed something unusual going on here, but who knows what’s been connected to this wiring in the past. Could be previous trailer wiring, reverse light, etc?

If everything looks good I would just replace it and assume it was an issue with something previously. I don't think that cloth covered wire is original either, unless they did this for the fuel pumps for some reason?

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

Thanks all!

I think I’ll replace the sender wires as per factory

I’ll change the fuel pump to add a fuse and relay from the battery and use the existing feed as the switching signal

My loom has a couple of the mesh wrapped wires, from what I can tell it’s a factory feature

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The fabric wrapping is factory (and usually painted with lacquer as well) and the white colour signifies an ignition switch controlled circuit (as distinct from a brown wire which is battery feed). I've often wondered why the fuel pump and distributor are not fused. The only thing I can think of is that it must be for safety: if there is a problem, perhaps better to have overheated wires than the ignition or fuel cut out and the engine stop dead.

I had assumed minis were all new enough to only use PVC. I work on some early 50s vehicles which use the cloth covered wire.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis