I recently had a similar problem. I had a Lack-of-Braking at Maximum Gain Setting. I Emailed Tekonsha and they sent the Tech Bulletin below.
Issue Date: October 10,
Testing brake controls
Issue:Lack of Braking at Maximum Gain Setting
Tools required: Voltmeter, 12 volt incandescent test light
Verify: Trailer is plugged into tow vehicle. Gain is set to max. Trailer brakes are adjusted according to manufacturers specification. Battery voltage (approximately 12-13 volts) between the Black power wire and the White ground wire at the rear of the brake control. This is the input voltage. With manual slide lever fully applied, measure the voltage at the wheels of the trailer. If you have 11 volts or more, check trailer brake adjustment. If brake adjustment is within specs, check brakes for mechanical issues.
If the voltage at the wheels is less than 11 volts, unplug the trailer. Then connect ground clip of test light to the ground pin in the trailer connector on the tow vehicle, contact the probe of the test light to the Electric Brake pin. With the manual lever fully applied, using volt meter, measure the voltage between the Electric Brake pin and the ground pin. If the voltage at the rear of the tow vehicle is 11-12 volts, repair or replace trailer wiring or brake magnets. If voltage is not 11-12 volts, relocate the ground probe of test light to a known good ground on the tow vehicle and retest. If the voltage is now 11-12 volts, repair or replace ground wire to trailer connector. If voltage is not within this range, check the output voltage at the control.
Please see page 2 for checking output voltage of control.
TECH BULLETIN Page 2.
To check the output voltage at the rear of the control, cut the Blue wire 5-6 inches behind the control. Install the test light between a known good chassis ground and the Blue wire coming out of the control. Apply manual slide fully, using the volt meter measure the voltage from the Blue wire to chassis ground. If the output voltage is within 1 volt of input, control is good, repair or replace Electric Brake wire on tow vehicle. If it is not within 1 volt of input voltage, measure the input voltage again, from the Black wire to the White wire of the brake control, with the manual slide not applied. As you are measuring this, fully apply the manual slide and monitor the voltage. If the voltage drops, there is a voltage supply issue to the brake control. If the voltage spikes, there is a ground issue to the brake control. If the input voltage stayed at battery voltage with the manual slide fully applied and the output voltage is low, please call our Technical Support Hotline at (888) 785-5832.
I completed the above checks and found a bad connection in the Blue wire, behind the brake control. The terminal was loose in the socket(plug).
That wasn't my only problem. After making sure I had a good connection at the back of the brake control, I was getting erratic voltage readings at the trailer brake wiring(near the brake drum).
I started testing with the Gain wheel at it's lowest setting. I held the manual brake lever in the full power position. I then started rotating the Gain wheel slowly toward the high end and watched the reading on the Volt Meter slowly increase.
As the Gain wheel was moved, the Voltage would rise. When it got to about 3.2 Volts, it would then jump to 4.3 Volts( not a slow and gradual increase). Then the Voltage reading continued smoothly until the Gain wheel reached the highest setting. My final reading was 13.2 Volts(with the TV engine running).
I felt the erratic voltage readings were coming from the Gain POT(potentiometer). Sometimes the contact surfaces inside the POT will tarnish when not moved(adjusted).
I moved the Gain wheel back-&-forth in a swift motion several times to clean-up the tarnished area on the contact surface. After doing this, my lack-of-braking problem cleared up. It's been working correctly sense then.