[Sean's Jaguar XJS Tech Pages]
Voltage drop when engaging Climate Control on a Jaguar XJS

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Last modified 2005 APR 07 04:37:58 GMT

Yesterday morning (as of this writing, MAR 1999), Ian Southwell posted on the Jag-Lovers XJS list about a voltage drop he experiences on his XJS when he turns on the A/C blower. As yet, an elaboration of the specifics of his problem hasn't been posted, but seeing as I've noted that my headlamps dim for ever so brief a moment, I decided to take a voltage measurement on my 1988 XJ-SC last night.

I don't know if the brief dimming is normal or not -- on the one hand, it doesn't seem right, but on the other, it is certainly not a prolonged dimming - just a brief flick of the lamps -- so brief, I think I might be being retentive just harping about it. It doesn't matter what level the A/C is set to -- this only happens when it is first engaged (though to be honest, I haven't metred the system during several A/C compressor cycles in AUTO climate mode).

Below are three images from my Fluke Scopemetre of the exact same event. The first two are higher resolution representations of the data produced by the PC-based software interface - one is an averaged waveform, and the other is a min-max display. Absolute voltage figures differ from the screen capture from the scope because the screen capture is reporting the figures for the instant at the end of the displayed sample, not an average over the sample as the other two displays report.

The measurement was taken at about 1000 RPM with the lowbeams ON. Note that my specific voltage readings may be rather atypical -- I'm running with a GM alternator mounted with the John's Cars GMALT bracket. At issue is that the voltage differential doesn't represent a large swing nor is it long in duration. The waveforms were fairly reproduceable - no two samplings were identical (we're not dealing with digital electronics here), but they all approximated one another.

Basically, we see here that I have an average voltage of 14.65V (shortly after a cold start), with a dive of about 2.2V (to 12.5V) for just 150 ms (15/100 second) when the A/C is first engaged. On my cat, this brief voltage drop manifests itself in a perceiveable flicker of the headlamps - I think not entirely unlike the dimming of a household lamp right at the moment a furnace on the same circuit might kick in.

When I obtain some other figures (amperage draw on the A/C circuit for instance), I'll see to posting them here. If anyone happens to have data which correlates to this in any way, I'd be interested in seeing it. Better yet, if you're in my neighbourhood, perhaps we can arrange to take a reading directly from your cat (I promise, the process is VERY unobtrusive).


Voltage reading when A/C first turned on (low fan). Averaged waveform.


This is a Min-Max waveform showing the minimum and maximum values for each sampling. Note the spike towards the end of the low spot as voltage is about to return to normal.


Screen capture from the scopemetre for the same event. The spike mentioned above is visible here as well.


This is a trendplot taken a day following the above readings. See below for further discussion of it.

The above trendplot has only a 1 point/second representation, so extremely brief events aren't plotted as a true function of their duration (such as the dips in the voltage due to the Climate control, which from the previous tests, are known to last only 150ms -- here, they are represented as a single sample point between two norms, for an apparent total low voltage of 2 seconds). Because of the sample resolution, trendplots such as this don't present as accurate a picture as you might otherwise get from a realtime plotting for short signals. But, it picks up the extremes of the peaks and dips well enough to correlate with events, and that is what we're using it for here.

This time, I took the reading after returning home and letting the cat sit for about an hour (still reasonably warm), then proceeded with the following sequence of events, which are numbered in the reference image:

(1) 12.25V standing, nothing on or running (not even courtesy lamps).

Insert key, listen to buzzer and turn to engage passive seatbelt harnesses (wait until seatbelts are fully engaged). Elapsed time: 5 seconds. (2) Voltage dropped to a low of 11.90V. Noted that after the seatbelts had engaged, the voltage had only recovered to 12.10V (after a 3 second rise time from the low point). I'm assuming the ECU/injectors and the like are consuming some amount of power at this stage. Left it at this state for about four more seconds waiting to see if it was going to shift and when it did not, proceeded to the next event.

Turn key to START, in about 1 second, (3) voltage dropped to 9.35V, then (4) recovered to a peak of 13.00V two seconds after that, (5) dropped back down to 12.25V a second later, rose to 12.55V where it remained for about 6 seconds, at which time the alternator had apparently kicked in and the voltage climed to 14.00V in 1 second (note: trend sample resolution), then (6) up to 14.20V as the idle settled in.

Let it warm up for about a minute, holding the idle close to 1000 RPM.

Engaged the lowbeam headlamps (US round lamps, not Euro), which surprisingly didn't appear to affect the trend readings (taken at the cigarette lighter, BTW).

I fidgeted with the A/C control while keeping the RPM at 1000 and watching for the headlamps flick. (7) The first dip event from 14.25V was followed by an accidental letting off of the pedal (the gradual lowering of the voltage), which I recovered from, then two more (8 and 9) activations of the A/C, the second one including turning it up through the fan range (except defrost) with no significant trend events resulting.

Each time the A/C was turned on, the electric fan was heard to engage, and a noticeable change in the pitch of the engine occurred (the load of the compressor).

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Sean B. Straw
Post Box 751224
Petaluma, CA 94975-1224 USA

EMail to: Sean.Straw+Jaguar@mail.professional.org


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