Andrew Cornwall
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THERMAL INSTABILITY OF MY TEKONSHA VOYAGER BRAKE CONTROLLER

Andrew Cornwall

Copyright

December 2001




Picture of controller in truck
controller mounted
under dashboard
Nature of the Problem

This article is about a problem of thermal instability that I experience with my Voyager trailer brake controllers, model 9030 made by Tekonsha, when pulling my 5th wheel RV trailer. By thermal instability I mean that, all else being equal, the amount of electricity sent to the trailer's brakes varies with the ambient temperature of the controller. When the temperature in the pickup truck is cool the braking is reduced; conversely when the temperature is warm braking is increased. The controller is so sensitive it even responds to the air conditioning cycling on and off in the cab. As a result, I have to readjust the controller many times a day. (I have become quite proficient at this). I've used two of these controllers, purchased several months apart and in two regions of North America, and they both have this problem. My Voyager brake controllers are not a 'set it and forget it' kind of device.

This problem extends back to 1999, more than two years ago, when I purchased my first Voyager. After similar results with the second, I wrote to Tekonsha, and Mr. Scott DeLoach, Technical / Customer Service Manager, responded courteously. He doubted that the Voyager brake controllers were at fault, and he sent a video tape on how to correctly install the unit and set the gain and level controls. Indeed, the video tape verified that both of my units had been installed and adjusted correctly. I was left to adapt to the thermal instability. But the problem continues to nag, so I recently put one of the Voyagers through a series of tests to determine the extent of its thermal instability. The nature of the tests and their results are explained in the rest of this article. In conclusion, my Voyager brake controllers are very sensitive to changes in temperature.

In my case part of the problem may be due to the relatively light weight of my 5th wheel trailer, a 4,000 lb. Ultralite, making it more responsive to changes in braking force than heavier trailers. Such as many 5th wheel trailers, mine has two axels and four brakes.

If you suspect that you have a problem with thermal instability with a Voyager, or other controller, you may benefit from learning about my measurements. Even if you don't have a problem, you may want to find out more about how this kind of controller works, which is also described below. The Voyager 9030 is still being made and sold.


How the Voyager Brake Controller Works

The Voyager brake controller is mounted in the cab of the tow vehicle and wired to the vehicle's electrical system and to the trailer brakes. Depressing the brake pedal, which turns on the brake lights, activates the controller. The controller senses the rate of the vehicle's deceleration by means of a an accelerometer inside of the device. At the heart of the accelerometer is a pendulum, which swings forward as the vehicle slows down. More rapid deceloration causes the pendulum to move forward further. Within the limit set by the gain control, the position of the pendulum determines how much electricity is sent to the brakes. There is a spring-loaded "Manual Override Slide Bar" at the front of the Voyager which at any time can be moved by hand to send various amounts of electricity, up to the gain limit, to the trailer brakes.

The Voyager has two controls, gain and level:


Amount of Electricity to the Brakes - Pulse Width Modulation

Oscilloscope View of
Controller's Output
at Various Angles

scope: zero degs. and zero volts
zero degs. / zero volts
scope: 15 degs. and 2.9 volts
15 degs. / 2.9 volts
scope: 20 degs. and 6.3 volts
20 degs. / 6.3 volts
scope: 35 degs. and 12.1 volts
35 degs. / 12.1 volts
scope limitation does
not show voltage
going to zero

The Voyager varies the amount of electric energy going to the brakes by pulse width modulation. The controller provides electricity to the trailer brakes in square-wave pulses; with my unit the pulse rate is nominally 3,100 Hertz (i.e. cycles per second). Apparently this frequency is high enough so the inductance-laden trailer magnetic brakes are applied smoothly and otherwise do not buzz. The instantaneous voltage from the controller is either zero or (approx.) 12 volts. (Peak voltage is a small amount less than that of the vehicle's electrical system; with my unit the drop is about 0.6 volts.) The proportion of the pulse during which (peak) voltage is present varies from 0%, no electricity going to the brakes, to 100%, maximum braking at full gain. When measuring the output of the controller we can still think of volts, but in an integral sense. If 12 volts is sent 25% of the time, then the integral voltage during the pulse is equivalent to 3 volts; likewise 50% would be 6 volts, etc. There is a good design reason to regulate the amount of electricity going to the brakes by pulse width modulation instead of the alternative of variable resistance. I will not go into it here, but suffice it to say that other brake controllers also use pulse width modulation.

The oscilloscope pictures show how pulse width modulation works. The center of the scope screen is zero volts, and the peak wave goes to 13.1 volts (I was using a 13.7 volt regulated power supply). The pictures also exhibit how tilting the controller (front up) varies the pulse width and consequently the amount of electricity going to the brakes. For these measurements the Voyager's gain control was set to maximum. The first picture depicts the controller at zero degrees (level) delivering 'zero volts'; next 15 degrees delivering 2.9 volts, then 20 degrees for 6.3 volts; finally 35 degrees for 12.1 volts, which is 92% of the highest power possible. If I had set the gain control to one-half, the power at 35 degrees would be about 6.6 volts (very similar to the 20 degree scope picture).

Measuring Pulse Width Modulation 'Voltage' with a Digital Voltmeter

While conducting these experiments I determined that it is possible to use a digital voltmeter to measure pulse width integral voltage. Initially I was unsure that I could do this, because the digital voltmeter repeatedly samples the circuit and then interprets the result in the display. I could imagine a hit-and-miss situation as the digital volt meter sampled the pulse when it was either zero or maximum; resulting in readings that flick from 0 to about 12 volts - or maybe something else in between. But this did not happen, at least with the 3,100 Hertz frequency of the Voyager. I checked the digital voltmeterís readings with various pulse proportions seen on the oscilloscope, and they seemed to match. I made similar measurements with another digital voltmeter and with an analog voltmeter; they were all in agreement. With this background I felt confident in using a digital voltmeter to take voltage measurements for this project.


Taking Measurements

Test Rig for Tilting the Voyager
and Making Voltage and
Temperature Measurements


test rig, front view
test rig, front view
meter on left measures temperature
meter on right, output voltage
box at rear is power supply
test rig, back view
test rig, back view
temperature probe is under polyester wad
test rig, angle pointer
test rig, angle pointer
Three kinds of measurements are needed to investigate the thermal instability of my Voyager controller:

I constructed a test rig, shown in the pictures, consisting of my Voyager controller strapped to a wooden platform with a hinge so its angle can be changed and measured. An automotive tail light bulb substitutes for the electrical load of the braking system. At maximum controller output of the light draws 2.55 amps. This is less than the approximately maximum 11 amp draw of my trailerís brake system, but I doubt that the difference would distort the kind of measurements I am making. The switch, next to the light bulb, serves the function of the brake pedal; switch up for brake 'on' and down for brake 'off'.

A temperature probe is lashed to the side of the controller. The probe is insulated with a wad of polyester to help isolate it from the effect of nearby air temperature. It is important to note that the probe measures the temperature at the side of the controllerís case, this is not the same as measuring ambient temperature but it is the closest I could come. Another problem with temperature measurement is that the controller is mounted on a board which partially insulates its bottom from changes in ambient temperature. This creates a lag effect, where the bottom of the controller does not change temperature as quickly as other parts of the device including the side of the case.

Output voltage was measured with a digital voltmeter. Refer to the description in the box to see how I verified the reliability of this approach.

Before taking a series of readings I would turn on the controller, with the 'brake' off, for about 5 minutes to let circuit stabilize. When measuring voltage with respect to angles I took a reading every 2 degrees once the voltage went above zero, e.g. 16 degrees, 18 degrees, 20 degrees, etc. When measuring voltage versus temperature I took a reading every two degrees Fahrenheit. Just after turning on the brake switch it often took a few seconds for the voltage reading to reach a level, sustained value, especially when the controller was cold. Therefore, I waited about 5 seconds after turning the brake switch on before recording a voltage, then I turned the brake switch off while preparing for the next reading.


TEST RESULTS

Output Voltage Measurements
at Various Angles and/or Temperatures


GRAPH 1
graph of output voltage at various angles and temperatures
output voltage at various angles for
five temperature ranges (see text)
gain and level controls each set to one-half
GRAPH 2
graph of output voltage from 48 to 99 degrees Farhenheit
output voltage for temperature
range of 48 to 99 degrees
gain set at one-half
level control set at neutral (see text)
I conducted two sequences of measurements to determine the thermal characteristics of my Voyager controller. For both, the gain knob was set at its mid point (notch up). The first, shown in the graph 1, examines the characteristics of the controller by angle, 16 through 48 degrees measured from lowest to highest angle, for five different relatively narrow ranges of temperatures, 47-51, 58-64, 71-75, 84-89, 95-100 degrees. Several minutes were needed to take readings for a temperature range, and the starting temperature invariably migrated upward during the process. Graph 1 shows that the Voyager behaves differently for different temperatures. In general, the hotter temperatures are associated with higher output voltages. At all temperatures, however, they maximum voltage converges to about 7.3 volts. This implies that the cause of temperature sensitivity is related to how the level control is being interpreted by the Voyager. There is an anomaly in graph 1 because the line for 84-89 degrees temperature is generally above that for 95-100 degrees temperature. I recalibrated the gain and level settings before taking the 95-100 degree readings and could have skewed these results. Never the less, the effect of rising temperature on the Voyager is clearly demonstrated. It is most pronounced when comparing the lowest and highest temperatures. At 32 degrees of angle, for example, the following output voltages were recorded:

Temp. F.
47-51
58-64
71-75
84-89
95-100
Voltage
3.83
5.01
5.25
6.58*
6.06*
*see text


I believe that output voltage differences of this magnitude are sufficient to markedly change how the trailer brakes react when applied.

Graph 2 shows the second sequence of measurements. It consists of a single series of output voltage measurements while the controller is simply allowed to 'warm up', going from 48 to 99 degrees Fahrenheit. The Voyager rested flat (i.e. zero degrees) throughout. The level setting was adjusted, when its temperature was 48 degrees , so the LED was a pale yellow within 5 seconds of the brake switch being turned on . The 'pale yellow' setting is approximately the recommended neutral position of the level control, to be set when the vehicle is stopped and the brake pedal engaged. The result of this test is consistent with that above. The Voyager exhibited considerable thermal sensitivity, with output going from 0.59 volts at 48 degrees temperature to 2.20 volts at 99 degrees. The sensitivity is most evident below 60 degrees and above 82 degrees. Unfortunately, I often encounter these temperature ranges while pulling my 5th wheel trailer, i.e. from morning to noon to evening during a drive, when the truck has been parked in the sun, and when driving on hot days when the air conditioner sends cold air to the lower portion of the cab.

When conducting the second test I noticed that the output power pulse frequency went from 3,100 Hertz at 65 degrees Fahrenheit to 3,500 Hertz at 99 degrees. I just happened to measure the frequency at these temperatures, but I do not know of the significance of the change.