Hi Pete

Here is a link to the thread that Bryan mentions (thanks for the kind remark Bryan). The DTC is the same as yours too.

http://members5.boardhost.com/ukautotalk4/msg/1263154567.html

You are right, the frequency that the cycles start & finish is fixed & it is only the ON & OFF ratio that is adjusted by the control unit in it's efforts to aportion the correct amount of current to drive the consumer.

Think of a heating element that, with a constant supply & ground, would draw 10 amps. If you then rapidly switch the circuit (supply or ground) on and off, then you can regulate (modulate) the current flow. If the the width of the rapid ON pulse is for example 50%, then half of the current will flow (5 amps in this 10 amp example). If an ON period of 20% was applied, then 2 amps would flow.

This is possibly over simplifying the subject, as heat is a factor when considering resistance, which is another reason why I like to look at these circuits whilst in operation with the scope.

In your case Pete, I wouldn't expect the PCM to set a heater circuit DTC unless it detected one!

HTH