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MG TD TF 1500 - Fuel Pump Switch

When I rebuilt my TF about 10 years ago I installed a back-up fuel pump (Facet). This has come in handy a couple of times. Under the dash I have switch, single pole double throw (on-off-on) to go from the SU pump to the Facet. It also works as an anti-theft device (sort of!) by leaving it in the off position. The other day I was driving and the engine quit. Thinking the SU pump failed I reached for the switch and found it was quite hot (burning my fingers). There was nothing wrong with either pump so I used a jumper to go from the power cable to one of the pumps (Facet) and all worked fine. Thus far all I have done is to check that both pumps are working and they seem OK. Attached is a photo of my switch. The center power terminal and the terminal for the SU pump had melted. The last thing I ever considered failing was this switch, but as you can see it I was in error. The wires to the switch were not hot and not even warm.
Does anyone have any ideas as to what might have happened? I plan to install another switch but hopefully one of better quality. Any thoughts on a brand switch to buy?

Larry



LD Kanaster

I'm no expert but I don't think a fuel pump should draw more that 4 - 5 amps and that should be a pulsing current. If the points in your SU pump stick it may draw a lot more and, for that matter, when the pump is operating properly it may draw a momentary greater amperage when the points momentarily close.

What was the amperage rating on your switch?

Jud
J K Chapin

Did you have a fuse in each feed AFTER the switch?

Peter
P G Gilvarry

Do you have an extra fuel filter between the pump and the tank?.
If this blocks or the tank does not vent then the pump can stall on the powered suction stroke. The delivery stroke is by spring so a blockage down stream does no damage
Ray TF 2884
Ray Lee

I’m not there to look at it, but most likely the contacts in the switch became corroded. If the resistance across the contacts inside the switch is high, the contacts will heat up, even with normal current going to the pump. Your picture is exactly what I would expect to see. The switch heated from the inside until the terminal melted the case enough to come loose and break free.
If the pump(s) run fine when bypassing the switch, I would think you are fine – although a fuse in the fuel pump power is not a bad idea. It may already be fused in the ignition circuit.
Bruns

I regret that I did not furnish more details as to the installation of the pumps. Fuel flow is not an issue as the fuel flows with no restrictions. I rely on the SU filter and the carb filters and they are always clean when I inspect them. I do have fuses on each wire going from the switch to the pumps. Neither fuse had blown. I do not recall the amperage for the Facet pump but the SU pump runs at about 5 amps and has been rebuilt to solid state.
What amperage switch should I install? The lowest amperage I can find is around 20 amps. I am thinking that after so many years it had to be a switch failure. Perhaps I am in error but I think that if it was something else wouldn't the wires have been heated and probably a fuse would have blown?
Thank you all for your thoughts. It really helps to talk about these things.

Larry
LD Kanaster

If the fuses are after the switch I am going to with bad switch.

I added a modern fuse block under the dash near the switch, uses modern auto fuses, I have them down to 5Amp rating. I am running 10Amp right now enough to blow before the wire melts was my thought.

The FACET is a great little pump, I test mine often to ensure it is working.

Peter
P G Gilvarry

Larry,

The first thing I would check is the current rating of the switch compared to the working current in the circuit. From your photo it seems that the switch is over stressed. Also I can't see how adding fuses in various places would make a difference.

John




J Scragg

The normal SU fuel pump in a T series is NOT fused.

If you add a backup you can then add a fuse AFTER the switch so that a fault in either pump does not strand you on the side if the road.

A fuse before the switch means you need to carry a spare fuse and know something about troubleshooting, or have a recovery service available.

Simple logic.

Peter
P G Gilvarry

Peter- You have an excellent point. That is exactly how I have the system wired, i.e. after the switch. The only thing I noticed when I went through the wires is that the power wire is 16 Gauge and the wires to the pumps are both 14 gauge. I don't see how this can be an issue with this system but what do you think?

Larry
LD Kanaster

As long as you use a 10Amp fuse I think the wire is adequate, I used 14AWG for additional wiring, believing in bigger is better.

The FACET current draw barely deflects my ammeter, but the SU moves it noticeably each time it clicks. But it is not continuous current.

Peter
P G Gilvarry

I should have mentioned, 16AWG is good for 12A up to 15’. SO 10A fuse would be best.

I don’t think you would need more than 15’ to run to rear mounted pump in a T Series.

I bought 14AWG as a good all round wire size, if adding an alternator, which I had planned, I would be using 10 maybe 12AWG.

Peter
P G Gilvarry

This thread was discussed between 22/09/2022 and 26/09/2022

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