Trailer Brake Adjustment?

Troubleshooting is really the same thing as good experimental design. I've had some some great troubleshooters work for me the past, and the very best ones all have multiple ways to get at solving the problem at hand. Worth their weight in gold, and in my opinion, one of the things that made American science and engineering the best in the world but is now being lost with overspecialization and the separation occurring between hands on technical and scientific training. How many young engineers do you know these days who have ever rebuilt an engine? 

First, I did pull off and inspect my old magnets, way too worn to consider reusing. I did test them both electrically though and they are actually ok.  In the process I found that my theory that buzzing indicates a bad magnet was dead wrong. Its normal when they are connected to the brake controller, which must be putting out a pulse width modulated voltage to do its control thing. Buzzing disappears when connected directly to a 12V dc source.

Next I found that the magnetic fields generated by the magnets themselves were sometimes confusing the readings on my dc clamp on meter.  DC clamp ons use hall effect sensors which are pretty sensitive. So, I cut the wires and went back to using an old fashioned current shunt to do those measurements.

End result is that both the new magnets are likely within specs and don't exhibit ground faults. The passenger side one has about 5-7% higher resistance than the drivers side (hard to measure exactly because they heat up a lot when on and copper resistance increases with temp so the current goes down). I also have about another 2-3 percent loss in the longer wiring going over to the pass side. So overall the current through the pass side magnet is about 8-10% lower than the driver's side.   

Nest I reconnected the pass side brake, left the driver's side disconnected, and attempted to burnish/burn in the pass side brake. My thought was that I had never really been able to get the pass side heated up and burnished before because aggressive trailer braking resulted in the drivers side getting plenty hot while the pass side was barely warm. 

That seems to have improved braking action on the pass side so I reconnected both sides and did a few test stops to measured brake temps. I'm still quite a lot hotter (say 200 vs 140 degrees F) on the driver's side but its better than before. 

So my conclusion now is that the 8-10%-ish side to side magnetic force difference must have been enough to keep me from being able to properly burnish the passenger side brakes. It surprises me that 10% would make that much difference so there certainly could be something else still going on. 

Since most of the difference is in the magnets themselves not the wiring, I'm thinking just rewiring as the etrailer tech suggested won't in itself correct things. So I'm thinking of going with one of GlueGuy's suggestions and adding a dropping resistor to the drivers side circuit to see if balancing the currents between the two sides balances the braking action. 

BTW, back on the original question on this thread, the etrailer tech also told me that the p2 max braking level adjustment should be near to the high end for a travel trailer and not to expect a TT to lock up brakes when setting the controller. He said that was more for a utility or car trailer in an lightly loaded condition. 

He also said that the boost setting does not effect the max voltage output of the p2 but it does increase the output during light to medium range braking. He thought I should be at least at a boost level of b1 for my rig, and that would help a lot to keep the rear brakes of the TV from overheating on grades, and that b2 might be a better selection for long fairly shallow grades. b3 is reserved for rigs where the trailer weighs more than the TV. 

 

I now get smooth even braking and drum temps are within 10-20 degrees of each other side to side even after a series of aggressive stops using the trailer brakes only. So I have to conclude that the seemingly minor 10% difference in magnet current side to side was my problem and kept me from even getting the passenger side brake burnished in originally.  

I read in some of the Lippert documentation that it could take as many as 50 hard stops to fully burnish/seat/burn in a set of new brakes. So, I temporarily disconnected the drivers side brake and did 20 stops (with cooldown in between) using the pass side only. Then I reconnected both and did a couple of stops and things seem to now be very close between the two sides.

I called tech service at the manufacturer of the Prodigy controllers who confirmed to me that there are no operational differences between the electro/hydraulic mode and the normal electric mode in their controllers. The only differences are in fault testing, mostly that the "not connected" resistance mode threshold is at a lower resistance and the inductance test is eliminated. They also confirmed that the brake buzzing is due to their PWM controller. 

So, I cleaned up and dressed down my trailer brake wiring and am at this point planning to leave my brakes in series rather than putting them back in parallel and call this fixed.

Now, back to my original question which was why my rear TV brakes are getting hotter than my front brakes. The reason is apparently because Highlanders like many other modern vehicles have a subsystem of ABS called electronic brakeforce distribution (EBD). This basically acts as an electronic version of an old school front/rear hydraulic proportioning valve, sending more brakeforce to the wheels with higher weight loading. So, initially under panic stop braking before the weight distribution shifts forward, the rear wheels can carry more brake force before lockup and they get that via the EBD system. The problem is that under light braking when towing the system apparently misinterprets the higher rear loading from the trailer as a sign that it should send more force to the rear brakes than it really ought to. Nothing to do about it I guess because the programming can't really be modified so I just have to live with more wear on the rear brakes and try to keep brake usage low on downhill grades so I don't overheat the rear brakes. At least now I can get both the trailer brakes to do some of the work which they weren't before. This also explains mcarter's observation that rear brakes sometimes wear out faster than normal when towing.