Try a new fuse.

Measure DC voltage (to battery ground wire) at both top test tabs of installed fuse 31. Do this immediately after removing and installing the fuse (when pump primes) and later when the pump won't prime.

The metal frame will work.

 

Gave fuse 31 a volt reading, and on both tabs, and while it was priming and after priming, it read just under 12 volts.

I'll give the other ones you've mentioned a try.

 

Was voltage at both tabs still around 12V when the pump later fails to prime?

What is voltage across the two battery posts? How much does it differ from the test tabs of fuse 31?

 

OK, fuse 24 has readings of above -12 on each tab.

A23 and A24 is 0.007 volts

Battery reads 12.24 volts. It differs by about 0.5 volts.

It did, but I'll try again just in case.

 

This result reveals a problem. Verify it.

Then answer this question:

If under all conditions, both fuse 31 test tabs measure battery voltage while A23 and A24 have only very low voltage, my recommendation is that you test your resoldered main relay.

 

Was this test done with the key in ON(II)?

 

I think so, let me try again. I have a spare main relay if that's the case.

 

I'm measuring the same, I'll give another main relay a shot.

 

Measuring the same with new relay. 11.92V on fuse 31, and measuring 0.011V on A23 and A24.

How can I test whether the main relay is bad or not?

 

Key in ON(II)?