On Thu, 1 May 2003, Noel Cookson wrote:

> Are the ordinary automotive relays able to operate at low voltage? That
> is what is required to make this relay operate more reliably than the
> old solenoid.

The issue isn't so much the voltage as it is the current.  Let me explain.

The starter solenoid has a fairly hefty current drain -- around 8 amps,
IIRC.  This current has to travel through a long run of small-gauge wire
and some contacts of dubious quality in the ignition switch.  That makes
for a high resistance.  Voltage drop is equal to current times resistance,
so if the battery is at 12V you may have only 9 or 10V left at the
solenoid.  (This was an even more dramatic problem on the bay window
buses, where the battery was in the engine compartment.  The ignition
switch wire had to run clear up to the front of the van and then back.)

The reason for using a relay is the relay coil draws a lot less current,
so the voltage drop will be less.  The contacts of the relay then switch
the power to the solenoid through a better current path.  This is why I
don't think a Ford solenoid is such a good idea -- it draws more current
than is really necessary to do the job.  It will probably work, it just
wouldn't be my first choice.

> Also you have to be concerned about exposure to the environment. The
> headlamp relays I am familiar with would not last long on the bottom
> side of the vehicle.

True, and if I were doing this I'd probably mount the relay on the
firewall.  I'm not too certain about the weather resistance of a Ford
contactor, either, since Ford mounts them up on the fender, not down where
they're subject to road splash.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si