Ford Mechanical Voltage Regulators


My 70’s Ford truck uses a mechanical voltage regulator to adjust the alternator’s field coil current depending on the engine rpm & how much battery charging needs to be done. No problems with it, just curious how that gadget works. Anybody here monkeyed with those things?

It’s very simple in design, has two relays in it, the field relay, and the voltage adjuster relay. Inputs are the battery voltage, and the voltage from the ignition switch. The only output is the field voltage. It seems like when I turn the ignition switch to “run”, that always causes the field relay to close. The voltage adjust relay looks like it determines the field output voltage, and seems to switch only between three selections: the battery voltage, a reduced battery voltage (but always the same %, determined by resistors), and 0 volts (for when alternator output isn’t needed presumably).

How frequently does the field voltage from the regulator switch to a different value? Every few seconds? Or just occasionally, like every few minutes? Or is it constantly adjusting the field voltage w/some means I haven’t discovered?

Edit: Here’s a link to a photo of the schematic


It’s been quite a while since I last opened a voltage regulator but a Ford had one electromagnet tickler to control voltage and a relay to operate the dash warning light for alternators. On models with an amp meter only the regulator was installed.


The inputs are wired up a little differently depending on whether the vehicle has an amp meter, or a dummy light for the alternator. With an amp meter, one input isn’t used at all.

What do you mean by “tickler”? That seems to imply the output voltage to the field coil is rapidly changing, not kept at one voltage for a while, then changing to another. Is that what you mean?


It was a mechanical version of PWM.
The regulator relays’ moving contact vibrates back and fourth connecting and disconnecting the field coil.
The better designs use a “freewheel” diode to reduce contact arcing.


My experience with voltage regulators goes way back to about 1960 hanging around at a battery shop where anything to do with charging and starting was serviced. The shop owner was trained by the army in WW II and his terminology stuck with me. I guess that the voltage regulators contact moving between the two fixed contacts was considered “tickling.” There were 2 vacuum tube rectifiers about 2 feet tall that were also called ticklers. Their output was 120v DC and batteries were connected in series and parallel to keep them at full charge until sold.

My friend built his business in the late 40’s because the owner of the old battery shop in town refused to work on cars with radios. He insisted that the radios overloaded the system and cut the life of batteries and generators making him seem incompetent. How does that compare to some of the debates here?


The field assembly in an alternator is like a big inductor. To charge that inductor you apply voltage until the current comes up to the correct amount. The current produces the magnetic field that passes into the stator poles. As the field assembly rotates, a varying flux is produced in the stator poles. The stator windings around these poles produce the alternating voltage that is applied to the rectifier diodes. The rectified current is applied to the DC side of the car’s system.

The first relay in the voltage regulator connects the field windings to the battery. This is activated by the ignition key circuit. If the engine is not running, the field winding will get the maximum current that the resistance of the windings will allow. With the engine running the rectified voltage will supply the car’s system needs and charge the battery. When the battery voltage rises to the voltage regulator’s set point the second relay will open the upper contacts, disconnecting the field winding from the battery. The field current will still flow because of the effect of the inductance. It will flow through the resistor connected to the battery. If the battery voltage goes high enough the current will flow through the lower contact to ground. In either case the field current ramps down as the inductive energy is dissipated into resistive heat either in the field windings or the internal resistors of the regulator. If you watch a regulator in operation you will see the second relay contacts making and breaking i.e. sparking. This is the tickling referred to above.


Interesting. Is it correct to say then that the two contacts at the upper left are just an on/off switch for the voltage regulator; i.e. always on (connected) when the ignition is turned to “on”? And disconnected otherwise? And the three contacts at the lower right rapidly switch (multiple times per second) between the upper two contacts connected, and the lower two connected, the % of time each pair are connected determined by how much current the car’s electrical load and battery are needing? Or is there also a % allocated to neither pair being connected?

Why don’t the lower contacts quickly burn out from the back emf when they disconnect from the field coil?


Yes the upper left contacts work as an ON/OFF switch supplying current to the field winding via the regulator relay on the right as long as the ignition switch is turned on… When the engine is not running or not running very fast, the moving contact between the upper and lower stationary will be making contact with the upper stationary contact, supplying current from the battery to the field winding. The alternator will provide as much current as it can. As the battery and load allow the B+ to rise to the voltage set point, the moving contact will pull away from the upper stationary contact; the field current will start to diminish through the voltage divider (10 ohm/50 ohm; B+ will drop; and the moving contact will touch the upper again (in a tickling fashion i.e touch-break-touch-break-etc.). As the engine goes faster the time the moving contact stays in the middle ,no contact, space, will increase. If the voltage becomes excessively high, the moving contact touches the lower contact that is connected to ground. Now the field current decays at a faster rate determined by the winding resistance and the inductance of that big field coil/metal structure. When the B+ voltage returns to the set level, the moving contact will again return to the middle allowing the field to draw current from the voltage divider. If B+ goes higher again the moving contact will retouch ground. Now you are getting lower excitation ticking.

Hope this helps you understand this system.


Thanks @researcher, yes that improved my understanding of how it all works. Still a little confused how come the lower right contacts don’t burn out pretty fast, as you’d think every time during their “tickling” dance, when they disconnect from the field coil there’d be a big spark…


If you look at the circuitry diagram of the regulator, the 10 ohm and 50 ohm are current paths when the moving contact is separating or separated from the stationary contact(s). This will limit inductive kick to around 30 volts and the shunting current path would carry self induced current away from the contacts so the resultant arc is minimal. In practice voltage regulator contacts go tens of years before they need to be riffle filed.

Just for kicks google “kickback diodes”. I found an instructive explanation of using diodes across inductive loads in DC circuits to protect solid state drivers. At the end of one article there was a point brought up about putting a resistor across an inductive load to limit the inductive kickback even in an AC circuit.