Weak Braking w/ Tekonsha P3

tom496 wrote:
I am just getting back to this problem after a few days. Thanks for the comments I received in the meantime.

When I placed a compass next to each wheel and engaged the controller, the needle turned a full 90 degrees on three of the wheels and didn't budge on the driver's side rear wheel.

No magnetic field = no braking - on that wheel. But total failure of one brake should not prevent the other three from having enough force to lock the wheel. Total brake performance will be less, though.

I inspected the wires and saw nothing suspect. I pulled the wheel and inspected the magnet and, although grimy inside the drum, there was no grease or any heavy rust anywhere.

I removed the magnet by cutting the wires. Here's what it looks like:

Looks fine - but that doesn't mean much. :)

I performed the tests recommended here: http://www.etrailer.com/faq-testing-trailer-brake-magnets-for-proper-function.aspx using a 12v battery and multimeter. I was outside the specs:
My reading: 2.9 amps
Recommended reading: 3.2-4.0 Max amps at 12-13 volts
Resistance - my reading: 4.7 ohms
Recommended reading: 3.0-3.8 ohms

It is outside the spec. - but just. The compass should still swing around with that much current flowing. Therefore, the problem must be lack of supply power to that magnet.

I also put my multimeter on the cut ends of the trailer-side leads to the magnet. With the controller manual lever pushed all the way to the left, I read 4.5 volts. (The reading on the Tekonsha was 6.0).

That is not a very trustworthy measurement. Your meter has will read fine if there is a connection so poor that only 1/4 of one 28 AWG strand in the cable is still connected - but the brake magnet won't have enough power to do any work. I'm also puzzled because in an earlier post you said the P3 reported 12.1 V with the manual lever fully applied.

The compass test clearly showed that one magnet was not doing it's job. That should be fixed. However, based on the P3 readings you posted earlier, I suspect there is more problems with your brake system than just the one non-contributing magnet. I would want to check the mechanical parts of all the hubs.

Also, I would expect the P3 to generate 13.6 V - or at least the 12.1 it did before - when the manual lever is fully applied. Since I have never used one, I can't advise you much, but it sounds to me like maybe there is a max-power adjustment that is "turned down".

Regarding further tests... I would measure the voltage at the magnet connection, comparing magnet-connected and magnet-disconnected - both while applying the manual lever on the P3. It should be a little less than when the magnet is not connected. If it is a lot less, then there is wiring troubles - probably corroded or damaged connections. You could also use the compass, though if there is current though the magnet there will be a magnetic field.

I would also check the ground connection through the trailer-to-tow-vehicle connector. If it's bad, the ball and coupler will often supply a ground path - but not a very dependable one.