How do you use the low range ammeter recommended for trailer inspection? What tool would you recommend and where can you purchase this?

Post date: Dec 19, 2013 4:17:17 AM

You bring up a very good question. Trailer inspection stations are indeed require to have an "ammeter- low range type". There is no stipulation on what this tool must look like and there are several varieties that a technician could use. The ammeter is needed to perform this check listed in Subchapter G:

"The maximum amperage on the electric brakes is more than 20% above or more than 30% below the brake manufacturers’ maximum current rating."

In order to check this, one must know the manufacturers' maximum current rating for the electric brakes. I found a few good sources online, one being this link that does a good job describing electric trailer brake maintenance and inspection:

http://www.rvdoctor.com/2002/02/electric-trailer-brake-maintenance.html

I also found this chart from a manufacturer recommending a maximum amp draw per axle (number of brakes):

As you can see, the acceptable amps draw is going to vary by the size of the brakes and the number of axles. This means that the maximum draw (taking into account the 20% allowable) will range from 6 to 21.6 Amps. The technician will need an ammeter capable of measuring up to 25 amps, making most ammeters built into multimeters insufficient for this task.

As far as the wording, "ammeter- low range type", it appears that any ammeter that measures 100 amps or less is considered to be of the low range type. For efficiency and ease of use I would recommend a clamp-on ammeter such as is available from Craftsman ( http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P?sid=IDx01192011x000001&kpid=03482369000&kispla=03482369000P ) or EETA308D available from Snap-On ( http://store.snapon.com/Probes-and-Adaptors-Probe-Low-Amps-P642210.aspx) (No matter what brand you may use, make sure your inductive probe can measure DC Amps)

There are various ways you can perform this test, but I will provide my recommendation for what I see to be the most simple and accurate test method. If a technician were to measure the amperage at the trailer connection it would require some type of adapter, plug or breakout box where one could isolate the wire feeding the electric brakes while keeping the trailer connected. This starts to get complicated and would require different adapters for different plug styles.

An easier way to measure the total amperage draw of the brakes is to locate the brake controller in the cab of the vehicle, usually under the dash. On the back of the brake controller the blue wire is the wire that runs to the electric brakes and by clamping the inductive probe around that wire only, the technician will be able to measure the current draw to the brakes. (Remember, current draw is consistent throughout the circuit so it makes no difference where one measures it.)

Once the amps probe has been connected (and with the trailer properly connected to the vehicle) the technician can apply the brakes using the brake pedal or the slide/button on the brake controller. This should provide maximum brake application and allow the technician to measure maximum current draw. (Due to safety concerns I recommend performing this test with the Key On, Engine Off.) The technician can now compare this result to the maximum as recommended by the manufacturer. For ease of reference here is a chart showing the PA limits based on the earlier manufacturer recommendations:

If a problem exists the technician can further isolate it by measuring the current draw at each individual brake magnet to locate the problem. If the current draw is between the minimum and maximum, no further testing is needed and the technician can move on to the next item required in the inspection.