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.

[m606paz]

The Ford Solenoid are ground activated, ok?
But modern replacement solenoid no says are ground or voltage activated... how buy?

[Jacques Reed]

Hi Mariano,

The direction of the magnetic field in a solenoid is determined by the current flow in it. if you swap the positive and negative terminals in a solenoid the direction of the magnetic field changes direction 180 degrees.

So, if a positive earthed solenoid (early Ford) was replaced with a negative earthed (later) one the magnetic field, and thus the force, on the armature, the centre part that closes the contacts between the battery and the starter, would be reversed.
This would cause the force on the armature to be away from the contacts and thus no closing of them to supply current to the starter would occur.

Search "Right hand Rule for Solenoids" for more info.

Also, as current flow by convention is positive to negative, perhaps "voltage activated" refers to a positive voltage applied to the solenoid in a negatively earthed one.

Hope this helps.

Cheers,

[m606paz]

Hi Jacques, thanks for your reply.
I say voltage activated according this diagram for Tractors

[Jacques Reed]

Hi Mariano,

I just put my multimeter on two old Ford starter solenoids. There was no continuity between the cases and the starter button terminals which indicates to me they are ground activated.

I also checked the continuity between the battery terminal (negative) on the solenoid and the starter button terminal and there was continuity there. This is the same at shown in the ground activated diagram.

There is no voltage at the start button. Only when it is pushed (earthed) does current flow from it, (positive) to the battery side of the solenoid, (negative)

Cheers,

[m606paz]

Hi Jaques
Thanks for the explanation, it has been very useful and a learning class for me.

So....
It's getting hard to get the right solenoid to work with the positive ground. All the ones I get here are postwar with negative ground.
Now, on the late electrical circuit data plate, it indicates that vehicles destined for India were negative grounded. What changes do I have to make to make it work with Negative Ground? Is it only to change the polarity of the battery and the instruments and electrical elements do not need changes? Only the solenoid is changed according to...?

Regards

[Mike K]

Originally Posted by m606paz

Hi Jaques
Thanks for the explanation, it has been very useful and a learning class for me.

So....
It's getting hard to get the right solenoid to work with the positive ground. All the ones I get here are postwar with negative ground.
Now, on the late electrical circuit data plate, it indicates that vehicles destined for India were negative grounded. What changes do I have to make to make it work with Negative Ground? Is it only to change the polarity of the battery and the instruments and electrical elements do not need changes? Only the solenoid is changed according to...?

Regards

Hi Mariano

The generator polarity needs to be changed if you are changing the battery terminals over.

The field coils in the generator are wound around iron cores, the cores have a small amount of residual magnetism in them that controls the polarity of the coils. It is easy to change the residual magnetism by quickly connecting a battery through the coils, it only takes a fraction of a second , the iron cores re-establish themselves with the opposite polarity .