Power spike while powering up blower fans

In the early Mk III versions, at least, the refrigeration system/compressor was engaged whenever the climate control system was in operation, regardless of mode selection or temperature selection…just as it was with the older Mk II system. All incoming air was first refrigerated then heated when necessary.

As years went by there were some changes. I think the last few model years had a economy or vent feature which allowed the compressor to be turned off.

Some convertibles were wired so that the compressor was always on, period. Even if the mode selector was turn “off”. The idea was to keep the fuel cooling capability active at all times.

Cheers
DD

Sometime in [calendar year] 1986, near as I can tell

My Feb '87 XJS had the Mk III system.

Cheers
DD

Normal operation for the XJS up to a point, yes. Beginning circa 1989 the aux fan circuit was changed; there were various iterations after that. In some cases the aux fan circuit was divorced from the climate control.

Cheers
DD

For power switching they may have used MOSFETs instead of bipolar transistors. Their failure mode is closed, so would be switched on all the time if they broke.

Does this car have a load dump module?

Kirbert, yes it has - with every Bosch 115amp alternator by default. Not sure how that relates to the current spike reaching almost short-circuit values… ? It should work in favour of the circuit design. Guessing you’re suggesting it disconnects the alternator somehow on AC energizing?
Any ideas highly appreciated.

Short update - just checked and aux fan can be exluded. However, there seems to be a relay under lower-right ‘corner’ of the steering wheel that gives fast repeat click even with both fuses from blower motors out. Shitty relay or AC module…?

What happens if you unplug the a/c compressor, I wonder?

Just thinking out loud

Cheers
DD

I know that when it misbehaves, the symptoms are weird. So I’d suggest just disconnecting it and see what happens.

Switching inductive loads causes huge spikes in voltage and current.

At startup, the current drawn is infinite but it drops down exponentially. The coil of wire then acts to keep current flow constant. At switch off, the exact reverse happens.

The typical way to ameliorate this is to switch the inductive load through a transistor, but have the transistor’s control signal have to charge up a small capacitor in the process. This delays the inrush of current because the transistor is turned on more slowly and so the top of the spike is trimmed.

This is the same principle behind the fuel pump circuit board which acts as a points saver for SU pumps, but the current switched for the blower fans is so much higher. See the circuit diagram higher up the thread XK Tranistorized Fuel Pump

kind regards
Marek

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Tonight I undressed myself and walked naked in the night, I saw red stars falling, and I saw burned towns, and I saw crouds of people…

Marek, my guess is - you have 17th Edition and dozen of refreshers. So in other words - capacitor inside of transistor housing ?

I’m not sure I understand any of that.

If the small capacitor in the circuit I posted fails, then the transistor can never conduct. What appears to be happening in your case is that the transistor always conducts 100%, i.e. the switching transistor is bad because it is not accepting the control signal. You need to sketch out the circuit and all will become clear.

A transistor is a “current multiplier”. It switches on when it isw passed a small control current through its base (“B”) terminal and this allows a larger current to flow between the “C” and “E” terminals. The little capacitor is in the “B” leg of the circuit, so it has to be charged up by the low current control signal - this means the main circuit only turns on in proportion to how the small circuit turns on.

kind regards
Marek

Oj Marek Marek…
Let’s try to recall some images from post-mortem:

Going by the probability calculation - it means that each capacitor/transistor (out of four blowers) is busted… Everything is possible with British automotive, however blower motors are also fused with 25amp /each. Let’s try to re-word it:

In well-working AC system - does anyone ever experienced "click!-k!!” while truning the knob from off to low??? This sound is still present even with both fuses off.
My Jag (1989) has nothing like that (blowers going on silently) while the Jag in interest of this topic is doing so… The plan is to trace the sound to the source…

Also, AC compressor electromagnet was disconnected as a first thing to check…

I see you rusted the interior to much the rest of the car? :slight_smile:

Four blowers?
Why do I think there are only two?
Yes, there is a click when turning the knob, even with the ignition off. I guess it must be the micro-switches behind.

It is possible that the click is the high speed relay on each blower; something engaging relay immediately with key on- this would cause large draw on start (50a). Relay is fed from an always hot brown wire if I recall, don’t think the 25a blower fuse prevents high speed but am not sure. If so, than problem is on the control side of relay (module?), otherwise blower would stay on with key off.

So far I can see one very rusty transistor, probably two resistors and a small diode of some sorts. It’d help to know what the writing is on these components and how they are connected.

Perhaps someone with a working example can contribute.

kind regards
Marek

http://www.jag-lovers.org/xj-s/book/acblower.htm

Thank you Kirby.
That appears to cover off everything you’d ever want to know about the circuit.

kind regards
Marek

EDIT - spray the newly constructed board with lacquer so it won’t rust up and short out like the last one did. If you really want, mount the big transistor on a heat sink.

Two pairs of the blowers were tested with one AC module - hence total number of 4.

Rusty photo above was taken during destructive disassembly of the blower. Or rather what was left from it.

Indeed the sound belongs to high speed relay which is hot wired regardless of the fuse status. It is loid as the flaps are jammed half-open :wink:

Both blowers are actually starting for 2-3 seconds, then being disconnected by AC module I guess…

In the same time, with engine warm (heater matrix warm?) - everything goes back to normal after few minutes run on high speed (no fault, all speeds back, with temp sensors disconnected/connected = no difference)

Without semantics - it does look like AC module fault. To be confirmed shortly via quick module swap.

My ‘88 has a functional system (as far as I know), and I have observed the same “high blower for a second or two” on start, but only occasionally; seems to happen when system was in high blower cooling mode on shut down. May be normal under those conditions…
Mine seems to have a temp overshoot issue, cooling on restart when not necessary- I understand that Jaguar corrected in later models by using a in-car sensor with a fan.