• 2019

On the Hot Seat

Heated seats are made for driver comfort. Here’s how to keep them working right.

If you live in a cold weather climate, heated seats are one of those comfort options everyone loves. In fact, a cold leather seat is just as uncomfortable in the winter as a hot leather seat during the summer months. Fortunately, heated seats can take the chill out of the leather or cloth covering relatively quickly, making your daily commute or trip to the store quite comfortable when the temperatures outside begin to dip.

 

THE HEATING OF THE SEATING

A heated seat is a normal seat with a heating pad, or grid, inserted under the seat cover (see Figure 1). The heated seat system operates on battery current received through fused ignition switch output (run) circuits. That means the seats will be heated only when the engine is running. Turn the engine off and the heated seat system turns off automatically.

The heated seat module (HSM) is used to control the heated seat system. The HSM responds to the heated seat switch and ignition switch status inputs and controls the 12-volt DC output to the seat heating elements through integral solid-state relays.

One end of the heated seat element is connected to ground at all times through a splice under the seat. Battery current is directed to the other end of the heated seat element by the HSM. The heated seat module will energize the heated seat element when the heated seat switch is depressed in the LOW or HIGH position.

As electrical current passes through the heated seat element, the resistance of the wire in the element disperses some of the electrical current in the form of heat. The heat produced by the heated seat element radiates through the underside of the seat cushion and the seat back trim covers, warming the seat cover and the driver or passenger sitting in the seat.

All the heated seat elements consist of multiple heating circuits that operate in parallel throughout a carbon fiber element and a wire lead (and connector). The heated seat elements are captured between the seat cover and the seat cushion. If one or more of the individual circuits malfunctions, the remaining operational circuits will continue to provide heat.

The amount of heat produced is proportional to the amount of current passing through the heated seat element — more current, more heat. The operation of the heated seat element is similar to the operation of a heating pad that one might put on their sore back.

Vehicles equipped with the heated seat system have two heated seat elements. One heating element is used for the seat bottom cushion and the other for the seat back cushion. Larger vehicles may even have heated seats in the second row. Another option is an automatic on system. In this type of system, the heated seat system will be activated if the ambient temperature falls to a certain level.

NOT SO COOL ISSUES WITH THE HEATED SEAT SYSTEM

The heating element used in the heated seat system is a grid of wire. When someone sits down in the seat equipped with a heating element, the element moves and flexes. It also flexes when there is movement in the seat (adjusting the sitting position, reaching over to the glove compartment, turning around to talk to someone in the back seat, etc.). This occurs whether the system is on or off.

Added strain can often be placed on the heating elements by objects in someone’s back pocket (phone, wallet, etc.). This strain is most often seen on the element in the bottom seat cushion and it can often cause broken circuits. If enough circuits break, the passenger sitting in the seat is going to notice that the seat is not warm enough.

If there is a problem, it has to be verified. Often, a Diagnostic Trouble Code (DTC) will not be stored, but your customer will tell you that the seat is not getting warm enough. If this is the case, check to see if the problem is isolated to only one seat by operating all the heated seats in the vehicle.

If none of the heated seats operate, there is an issue with the heated seat system. More often than not, there will be a problem with just one seat and it’s usually the driver seat because that is the seat used more than any other.

A simple continuity test can help. Locate and disconnect the heated seat electrical connector. When checking the element for continuity, be sure to move the heating element. This can be done by moving around in the seat. Doing so will eliminate the possibility of an intermittent open in the element when it is in a certain position.

Check the continuity by placing the ohmmeter probes on the “in” and “out” circuits of the electrical connector (this is shown in Figure 2). Typical resistance numbers for a front seat bottom cushion range from 4.7 to 7.0 ohms. Check the service manual for the specific vehicle being serviced. If there is no continuity, the element must be replaced.

GETTING WARMER: REPLACING HEATED SEAT ELEMENTS

In order to replace a bad heated seat element, the seat and seat back must be removed from the vehicle. Before servicing the heated seats, disable the airbag system. Disconnect and isolate the negative battery cable, then wait 2 minutes for the airbag capacitor to discharge before beginning any service.

For this particular discussion, our subject vehicle is a 2012 Chrysler Town & Country. Procedures will vary from model to model. Check the service manual for the specific vehicle being serviced.

Remove the seat from the vehicle. Then remove the seat back from the seat. Remove the armrest and headrest. Disengage the five J-hooks that secure the top and sides of the seat back cover to the frame. Disengage the seat back cover from the two metal tabs located near the bottom of the frame, and then remove the seat back cover and seat back cushion from the frame as an assembly. Using a hooked tool, disengage the retaining strips from the clips that secure the seat back cushion cover, then remove the cover.

Before replacing the heating element, note its position on the cushion. Remove the heating element from the cushion by carefully peeling the element away from the cushion. Try not to remove any excessive foam from the cushion. On the front passenger seat cushion, remove the occupant detection sensor (ODS) and discard it.

Install a new ODS onto the cushion. Peel off the adhesive backing from the new heating element. Position the element to the foam cushion directly to the location of the original heating element. Firmly press the element onto the foam cushion and make sure the element is fully adhered to the cushion.

Install the cover onto the seat bottom in the opposite order in which it was removed. Install the seat bottom, then the seat back into the vehicle. Reconnect the negative battery cable and check the operation of the heated seat.

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