The Snakes Alive CMP Wiring Project

[Jacques Reed]

A recent discussion on another forum about the use of Radio Suppression condensers (capacitors) on the fuel senders got me thinking about the design of the fuel sender unit on a Ford.

It was suggested that the condensers served another purpose which was to reduce arcing at the contacts inside the sender as it made and broke contact in operation. This is similar to the use of a condenser at the ignition points. This could be desirable on a device fitted close to fuel vapor; however, I think this was not necessary and the designers, (King Seeley?) didn’t consider it as an issue with a well-sealed unit.

Interestingly the condensers were only fitted “as required”, and not a standard fitment to all CMP vehicles which implies the senders were designed so that any arcing at the bimetal contact is satisfactorily shielded from petrol fumes or liquid. There is also the hot heating wire for the bimetal arm to consider.

I thought I would open up a unit for a close inspection. I had one which I had condemned for a hole eaten through the casing. I made sure there is no liquid inside it by shaking it vigorously as I would be using a Dremel cut off wheel to open it up. Even 50-year-old petrol is flammable.

As the photo shows it has some identifiable parts from the operating diagram. Virtually everything inside attached to the bimetal arm is insulated from earth. There is an adjustment cam for factory setting the starting contact point on the bimetal arm. This cam is insulated from the bimetal arm by what looks like a Bakelite pin. The movable arm on the bellows is earthed as is all the sender casing.

The unit is sealed with a substantial rubber gasket. As far as fumes leaking into the sender, this could only occur if the bellows had a hole in it, or the tank side of the sender had a hole (my condemned one) or there was a hole in the outside of the sender unit and fumes got in, such as in a during refuelling. Ignition should be switched off in any case during refuelling.

So, I think there is a low chance of a fire caused by a sender as long as it is good condition and not suffering from corrosion, or damaged in a way that the internals are exposed to vapor. Having said that, I would have a good check of any fuel sender unit for signs of holes or damage before putting it into service.

Will be interested to hear other comments on this.

Hope this is of some interest.