[modern] Re: Air Cond Resistor Failure ... Bogus?


You’re correct on this statement … “Failure analysis is VERY interesting!”

First of all, I was never questioning whether the 68 ohm resistor in the
amplifier circuit was prone to failure … I would think it would not be
prone to failure. I was just responding to Dan’s statement concerning the
purpose of the resistor.

Regrettably, I will have to disagree with your statement … “You wont get
any voltage spikes from the engine.”

Its true, the engine itself, will not cause problems. But all the inductive
loads within the car (some are even attached to the engine) create
incredible voltage spikes. These spikes are for the most part, are
extremely low in energy, but they are definitely of sufficient potential to
“pierce” semi-conductor boundary layers. Modern cars (especially Jaguars)
are loaded with voltage spike generators … every time the field collapses
around a coil (solenoids, magnetic clutches, motors, etc.) spikes are

Even in XJ40s, great care was taken to protect integrated circuits from
over-voltage. An example of this protection is the simple relay coil diode
/ resistor configuration in the control circuitry. Another example of this
over-voltage protection in XJ40s is the “load dump module” within the
alternator circuitry. Per Haynes … “The alternator load dump module
protects the electrical circuits from excessive voltage surges. When the
battery cables are removed, large amounts of transient voltage is released
through the electrical circuits. This device diverts up to thirty load
volts of excess voltage to ground by way of a voltage dependent resistor.”
Whether or not Haynes got the actual description correct, its apparent that
this component is a voltage suppression device.

Its no coincidence that with the exponential jump in the use of solid-state
and integrated circuits in automobiles, the use of over-voltage protection
devices and circuitry has also improved and increased in popularity. There
are many knowledgeable technicians which will never use “jumper cables” to
start a car … due to the fear of an inducted voltage transient and its
very detrimental effects upon the microprocessers.

I think you pegged me correctly again when you said …

“The difference between you and me is you tend to believe something that is
published, even if its just someone who wrote up the blower info on the
Jag-lovers web page. I base my info on analysis of the data as well as the
real world indications of the parts involved.”

Your insight into my cognitive abilities and thought patterns is amazing …
you must have been talking with my wife. Its true … I never analyze

John P.