I would start by confirming that all electrical connections from the battery to the various places are clean and mechanically sound.  This would include the battery terminals, the ground on the engine, those on the strap, engine and frame for the ground between the engine and the frame, those on the latching relay, the starter solenoid and the starter.  Do you also have a ground between the engine block and the firewall?  If not, add one.  If you do, make sure those connections are also clean and have good mechanical connection.

Verify that all the cable ends are firmly attached to the cables.

If that does not help bypass the latching relay and go directly to the solenoid from the battery.

Confirm that you have a Motorcraft solenoid.

Let us know what you find out.
 

Last edited by John Ha (9/28/2018 4:22 PM)

Sounds like your battery is low or bad.  Disconnect it from the car and test the voltage.  It should be close to 6.0 volts.  Testing while running only tells you that the alternator works.  Clean the battery terminals and cables too while you are at it.  Bypass the latching key too.  Perform a continuity check on both sets of cables.  Resistance should be very low.

My WAG is the plug on the firewall.

Update on my electrical problem:  all the grounds and positive connections removed and checked and have continuity through them all.
Have a ground cable from the motor to the firewall and have continuity there.

Battery checked out and had load test done on it  checked out ok

I don't have a connection plug through the firewall.  I totally rewired the vehicle with a Haywire electrical kit.

The latching relay is rated at 110 amps.  I didn't bypass the latching relay yet.  I want to road test it before I do and see if it is fixed before I do something else.

My next step is to replace the starter.  Thinking maybe the armature is grounding against the poles internally for some reason. 
I will road test tomorrow and see what happens.  Trouble is, it doesn't do it all the time.

Thanks to everyone for the help.  Jerry

retired 65 wrote:

Update on my electrical problem:  all the grounds and positive connections removed and checked and have continuity through them all.
Have a ground cable from the motor to the firewall and have continuity there.

Battery checked out and had load test done on it  checked out ok

I don't have a connection plug through the firewall.  I totally rewired the vehicle with a Haywire electrical kit.

The latching relay is rated at 110 amps.  I didn't bypass the latching relay yet.  I want to road test it before I do and see if it is fixed before I do something else.

My next step is to replace the starter.  Thinking maybe the armature is grounding against the poles internally for some reason. 
I will road test tomorrow and see what happens.  Trouble is, it doesn't do it all the time.

Thanks to everyone for the help.  Jerry

You said in your original post that "The interior lights are real dim and light on the radio is barely lit ... and voltmeter on car didn't register voltage"  If you were not trying to make the starter spin when these symptoms were present then it seems more likely that the issue is in something else, unless you have a starter that is wired directly to battery.  I would still try to bypass the latching relay first, since it is in the line of supply to every other electrical device in the car.

Load test the battery like Steve said.

That’s quite possible. I’m a electrician and I’ve seen stuff that if I hadn’t witnessed it myself I would have doubted was the problem. A coworker of mine years ago was a fire inspector. He said one day he walked into his living room and saw smoke coming from either a sofa or table. He said his wife had a crystal hanging in the window. The sun was hitting it and it acted like a magnifying glass. Everything was just right for the light to be concentrated to burn. He said if he hadn’t seen it he wouldn’t have believed it.