What is the purpose of the fender relay on newer trucks with PMGR starters? Why not go directly to the starter solenoid????

 

I looked in the manual, and they describe the starter motor having two coils inside it. One of them is the solenoid coil that pulls the gear out to the starter, and also connects the large battery + wire to the starter motor windings. Then they say there is another coil in there called a holding coil, and this coil holds the solenoid engaged till the starter loses battery power. So that must be reason. The relay on the fender breaks all power to the starter which is needed to release the starter and stop it.

 

Thanks for the information, and for your time in researching. The only thing is, the starter will work perfectly without the fender relay. I think I read somewhere that it is a current limiter, but I could be wrong. I really can't see how it could limit current, but then I can't see a lot of things.... jimd

 

I finally got a chance to go by Ford. The relay is a current limiter protecting against surge currents. jim d

 

I do not buy that at all.
The Power for the starter does not go through it, just coil power.

Here is a diagram of it.

 

The relay is to protect the ignition switch circuit. The starter solenoid winding takes more current than the ignition switch circuit is designed to handle, so the fender relay was retained to keep high currents from flowing thru the ignition switch. jim d

 

That makes sense. I am learning all kinds of stuff on here.

 

I do not know what manual called it a holding coil. After looking it over it does not hold it, it does the opposite of the other coil (although it would help pull it in initially). It disengages the start contacts and helps pull the gear out of the flywheel. This coil would have no current going through it while the engine is cranking, as it would have the same potential at both ends. Once the key is released you would need to have the circuit to the bottom coil opened fast and the arc extinguished hence the starter relay. Now with the relay open and the solenoid still closed the current now flows in the other direction in the top coil pulling the solenoid open. The top coil must be stronger than the bottom coil, I would think it would be wound on the inside and the bottom coil wound be on the outside of it.

Relays not only switch higher currents but also they can break higher currents also. It is much easier to allow current to flow and conduct it but it is hard to stop it. Stopping current is where all the damage is done to contacts, that is why they had a capacitor around the points in point type distributors.

This goes along with what jimdandy posted about the ignition circuit and what Franklin2 read in the book other than about the holding coil.

 

I got the holding coil thing right off a factory Ford DVD disc. I don't quite get all what's going on myself inside the starter coils, but after I saw your diagram it became clear the fender relay just activates the control power for the starter. I missed that while looking at the diagram on the factory DVD diagram.

 

I copied what was on the DVD below.

Starting System
The starting system consists of the:

starter motor
starter switch (part of the ignition switch)
digital transmission range (TR) sensor (automatic transmission equipped vehicles)
clutch pedal position (CPP) switch (manual transmission equipped vehicles)
starter motor solenoid relay switch
starter solenoid
starter drive

The starter motor:
is a permanent magnet, gear reduced, 12 V dc motor.
has an integral starter solenoid.
has an over-running clutch in the starter drive.

The starter motor solenoid relay switch:
controls current to the starter solenoid.
is engaged by the ignition switch.
connects the battery to the starter motor.
The digital TR sensor on automatic transmission equipped vehicles prevents starter motor engagement unless the transmission is in the NEUTRAL or PARK position.

The CPP switch on manual transmission equipped vehicles prevents starter motor engagement unless the clutch pedal is pressed.

The starter drive:
is operated by the starter solenoid.
engages the flywheel to turn the engine.
has an internal over-running clutch to disengage the starter motor when the engine starts.

The starter solenoid:
is an electromagnet.
engages the starter drive to the flywheel.
when energized, the contacts close, connecting the battery to the starter motor. This causes the starter motor to turn the starter drive.
is kept in the energized condition by a hold-in winding until the starter switch is released.

When the ignition switch is turned to the START position, the starter motor solenoid relay switch is energized connecting the battery to the starter solenoid. The starter solenoid moves the drive lever and pin to engage the starter drive with the flywheel ring gear and connects the battery to the starter motor.

Clutch Pedal Position (CPP) Switch

The clutch pedal position (CPP) switch is controlled by the clutch pedal. Depressing the clutch pedal manually closes the CPP switch and allows engine cranking. The CPP switch will also deactivate the speed control system.

 

"is kept in the energized condition by a hold-in winding until the starter switch is released."

They are talking about the bottom coil being the hold-in winding, this is true.