Why won't my hot tub heat up?

This is an old 'OHECO' hot tub...[early 1990s era].

The water on this hot tub is heated by a coil wrapped around the jet motor. The water runs through coil and heats up before returning to the tub.

After doing repair work, [see my other post, 'only runs for 10 minutes...], I started it up yesterday and let it run all night.

This morning it was only warmed up to 70 degrees.

Any suggestions on why this is and what to do?

Thanks,

Keith Pike

UPDATE: for those who might have this problem, it ended up being a 'HI-LIMIT' switch.