PTC Heater Revealed

It was May of 2015 when I took these scope snapshots. If I’m remembering correctly, here’s what all the channels are probing:

Blue: The voltage at the blower, I think. I suspect that I turned the blower control to the second position which is why you see a little hump in the curve.

Red: The heater request signal, just like Carl described, going from 10% duty cycle (to request no heat) to 60% duty cycle to request heat.

Green: The run signal– probably from the key switch circuit.

Gold: An amp clamp around one of the high voltage wires to the PTC heater.

In this first image you can see that the car is running during the entire event. I happen to turn the blower on co-incidentally at the start of a heater-request pulse, and on the next pulse it has changed to 60% duty cycle. It’s about 1.5 seconds until the PTC heater actually starts drawing current, and because of its PTC nature, the element starts off cold– drawing a lot of current– and then warms up and draws less current.


In the snapshot below, I turn the key off while the heater is still running. You can interpret the green signal going to 0 V as the instant when the key is turned off. The voltage at the blower tapers down, and the heat-request signal goes away very shortly after key off. But the heater continues to draw current until the contactors open under load, and then the heater discharges the capacitor in the PCU.


The time from key off to the contactors opening is that awkward 1.5 seconds of silence when you wonder why you haven’t heard that familiar clicking that normally happens almost instantaneously when you turn the key off. There’s no way to tell how long the heater might go on drawing current if the contactors didn’t open. It definitely continues to try to draw current until after the contactors have had a chance to close again. It’s also hard to say if the CDCM was sending the heat-request signal at 1000x its present frequency if that would stop the heater from drawing current after key off. So is it a problem in the CDCM? Maybe. Is it a problem in the PTC heater control circuitry? Maybe– or at least you would think that you ought to be able to fix this problem by modifying the PTC heater.

Here’s the PTC retrofit instructions in case you’re interested: