electric trailer brake voltage?

[dlb]

we got a new-to-us truck (a 95 diesel f-250, for those interested) and i'm going over a few things before we put it into regular service. the trailer wiring all seems good except for the electric brakes. when i put the volt meter on the terminal for the electric brakes and the brakes are NOT depressed, i get 12.2 V. when jenn steps on the brakes, it goes up to 12.8 V. squeezing the slider and playing with all the other dials on the controller has no effect on the voltage.

i don't have a lot of experience with electric trailer brakes but i assume that there should be no voltage until the slider is squeezed or the brakes are depressed. can someone correct me if i'm wrong?

thanks gang.

[keith]

It depends on how the electric brakes are controlled, how you are measuring the voltage and what type of voltmeter you are using. If the electric brake controller has a rheostat (variable resistor, similar to the one that measures your fuel level in the gas tank) and you are using a good analog voltmeter AND you are measuring at the trailer connector with it disconnected, then you can get the voltages you are seeing.

To get accurate readings, the trailer connecter has to be hooked up so you have a complete circuit, then you probe the connector or wire with your meter lead.

The rheostat has a very high resistance at one end, but not a complete open in most cases. The solenoid in the brakes has a very low resistance to DC. In a complete circuit, very little current gets through the rheostat so very little voltage is dropped across the solenoid, all the voltage is being dropped by the rheostat.

When you disconnect the trailer harness, you now have infinite resistance in series with a high resistance. If you used a digital voltmeter, you should get full battery voltage at the terminal of the connector as digital voltmeters usually do not load the circuit. Analog voltmeters do have some circuit loading, the better the meter, the higher the loading so you will get some voltage drop across the rheostat in its max resistance side and as you press the brakes, less voltage is dropped by the rheostat and more by the voltmeter.

[splatterdog]

When I don't have a trailer to test the vehicle, I use a headlight bulb as a load testing test light. It should get brighter the more the trailer brakes are applied. Off when not. It's possible the brake unit sends power to tell if there is a trailer there or not.

Headlights also make a good general circuit load tester. One strand of wire will light up a meter or standard test light. Got burned once on a fuel pump job. Had a bad pin on an obscure connector. Connector at pump lit up the test light nice and bright. Not enough amps available though...

[dlb]

i like that idea, SD. how do you wire the headlight bulb to the trailer plug?

[splatterdog]

Just some jumper wires to any sealed beam. Polarity doesn't matter. If you have a spare trailer side connector sitting around, you could use that too. Don't use a chassis ground for testing. Use the big(usually white) ground wire for the trailer wiring. Typically the blue wire for t-brake positive.

Never, ever ground your trailer through the ball...

[keith]

This sounds to me like you are troubleshooting a problem you don't know if you have yet. Why not hook up the trailer and see if everything works OK. You haven't said whether you have tried the trailer yet.

[dlb]

just gathering ideas, keith. i was away this weekend so i only started looking further into this today.

the first thing i did before posting here was hook the trailer up and go for a test drive. couldn't get any braking at the trailer whatsoever. that was when i started testing stuff with the multi meter and got the readings that i initially posted here.

today i hooked the trailer up and re-checked the voltage at the electric brake wire. no longer any voltage while no brakes applied, and can't remember what it was when brakes applied but it was low. so i checked for voltage at the controller wires and found the ground and brake light wires good but it was only getting about 6 V at its battery wire. went upstream and found one of the circuit breakers was the culprit. removed it and cleaned up the corrosion but the V still drops by .5 V on the other side of it so i picked up a new one for $8 on the way to work today. will throw that it in tomorrow and retest.

[dlb]

hooray, the $8 circuit breaker did the trick. i put it in and the V downstream was better so for the real test, i applied max braking power to the trailer while trying to pull it with the truck, and the trailer wheels locked up immediately.

i love cheap, easy fixes. i thought i was going to have to spend $60-$100 on a new brake controller.