[xj] A/C amp reverse engineering - update

OK, progress! I breadboarded up a version of the circuit I
had traced out from under the epoxy. After a couple of
head-scratching episodes while I doped out what some of the
component values were, I finally got it to spring to life!
I’ll be driving a couple of hours with it installed today,
so we’ll see how it does. My original A/C amp would never
work properly for two hours straight, so this will be a good
test of the circuit and a verification that the amp was what
was wrong with my system. I’ll be putting this all up on a
web page when it’s all done but here are some pictures to
show you what I’m doing.

Here is the circuit tapped in instead of the old one:

Here is how I plan to drive around with it so I can keep an
eye on it:

I added LEDs to the motor drive outputs so I can tell what
state it’s in. Other than that it’s an exact copy of the
original circuit, except for the relays which I could not
source precise replacements for (these work the same though.)

Here is how it sits most of the time:

When it needs to get the car hotter, it changes to this:

When it needs to get the car cooler it changes to this:

I have a different (bigger, more expensive) relay to try,
too. I may try to do some quick-n-dirty lifecycle testing
on them with a real servo motor to see if the cheaper one is
good enough. Once I get all the details sorted out I’ll
probably lay out a circuit board through one of the online
proto board places and build a nice looking version. Though
my ultimate goal is to design an improved solid state one,
so maybe I won’t put that much effort into the original design.–
Duncan Brown 1985 SIII XJ6
Grayslake, IL, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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Good work Duncan!–
Alex
79xj6L SII (BRG + wires)
86xj6 SIII (Black)
61 Sprite MkII (Red)
Menlo Park, Calif.

frobozz wrote:

OK, progress! I breadboarded up a version of the circuit I
had traced out from under the epoxy. After a couple of
head-scratching episodes while I doped out what some of the
component values were, I finally got it to spring to life!
I’ll be driving a couple of hours with it installed today,
so we’ll see how it does. My original A/C amp would never
work properly for two hours straight, so this will be a good
test of the circuit and a verification that the amp was what
was wrong with my system. I’ll be putting this all up on a
web page when it’s all done but here are some pictures to
show you what I’m doing.

Here is the circuit tapped in instead of the old one:
http://www.backglass.org/duncan/jag/cc/breadboard_1.jpg

[clip]

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In reply to a message from Cannara sent Wed 6 Jul 2005:

Holy crap that is the coolest thing EVER done to the xj’s
climate controll system. If it works, you should order a
whole bunch of prototype boards and sell them for cheap
(please?)!
Excellent work!
Chris–
SOLENT BLUE IS PEOPLE! SOLENT BLUE IS PEOPLE!
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In reply to a message from Chris Diamond sent Wed 6 Jul 2005:

Holy crap that is the coolest thing EVER done to the
xj’s climate controll system. If it works, you should
order a whole bunch of prototype boards and sell them
for cheap (please?)!

Well all it is, is the stock A/C amp laid out where you can
see it (and where you can plainly see it’s not worth what
Jaguar charges for one…) Plus the LEDs. You could
trivially add LEDs to the existing circuit with a little
splicing if you really wanted to. They’re just monitoring
the red and purple motor wires that come off the amp.

The far cooler thing will be the solid state replacement,
which will be smaller, cheaper, and should last a lot
longer. LEDs would be optional on that one too.

Well it performed flawlessly across my 2 hours of driving,
other than the annoying hot-perfboard smell! I had cranked
the current way up on the LEDs so I could see them in the
daytime, figuring who cares if I burn them out, but I hadn’t
counted on the heat they’d pour into the perfboard below
them. Gag. So I cut the current to the green ones and
disconnected the red ones; they were just there to monitor
things until I got the board working right.

Having watched this thing in action for a couple of hours,
I’m going to restate my concept of how it works: instead of
returning to dead center on the servo, it instead returns to
a ‘‘stasis point’’ (still fairly near the center), where it is
putting out the exact blended temperature of air requested
by the dial. When the system first comes on, and/or if you
make a large change to the temp dial, the system swings the
servo far past that stasis point, to help change the air
temp rapidly. It usually overshoots a bit and fairly
quickly corrects back in the other direction just a tad…
then slowly walks back to the stasis point as the cabin air
temps get changed to match the requested temp on the dial.

Once it gets to that point, it tends to make very small
corrections back and forth to maintain that temp. How
frequently it makes those corrections is dependent on how
mismatched the requested temp is from the outside temp.
Setting the dial to 70 on an 80 degree day like I was doing
eventually ends up with the amp not needing to move the
servo for minutes at a time.

The trick with the solid state version will be to match the
behavior of the stock one, in terms of making an initial
large servo correction, then increasingly smaller ones, and
eventually very infrequent ones. It would be all too easy
to design a ‘‘twitchy’’ circuit that responded too instantly
to changes on its inputs.–
Duncan Brown 1985 SIII XJ6
Grayslake, IL, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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Duncan! Impressive!

Mike Vaughn
Oviedo, FL
87 xj6 SIII> OK, progress! I breadboarded up a version of the circuit I

had traced out from under the epoxy. After a couple of
head-scratching episodes while I doped out what some of the
component values were, I finally got it to spring to life!
I’ll be driving a couple of hours with it installed today,
so we’ll see how it does. My original A/C amp would never
work properly for two hours straight, so this will be a good
test of the circuit and a verification that the amp was what
was wrong with my system. I’ll be putting this all up on a
web page when it’s all done but here are some pictures to
show you what I’m doing.

===================================================
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Is the ‘stasis’ point constant as seen on the servo cams, Duncan - or it
is variable - dependant on settings/ambient temps, or other factors…?

And again; impressive and useful work!

Frank
xj6 85 Sov Europe (UK/NZ)

frobozz wrote:>Having watched this thing in action for a couple of hours,

I’m going to restate my concept of how it works: instead of
returning to dead center on the servo, it instead returns to
a ‘‘stasis point’’ (still fairly near the center), where it is
putting out the exact blended temperature of air requested
by the dial. When the system first comes on, and/or if you
make a large change to the temp dial, the system swings the
servo far past that stasis point, to help change the air
temp rapidly. It usually overshoots a bit and fairly
quickly corrects back in the other direction just a tad…
then slowly walks back to the stasis point as the cabin air
temps get changed to match the requested temp on the dial.

Once it gets to that point, it tends to make very small
corrections back and forth to maintain that temp. How
frequently it makes those corrections is dependent on how
mismatched the requested temp is from the outside temp.
Setting the dial to 70 on an 80 degree day like I was doing
eventually ends up with the amp not needing to move the
servo for minutes at a time.

===================================================
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In reply to a message from Frank Andersen sent Thu 7 Jul 2005:

Is the ‘stasis’ point constant as seen on the servo cams,
Duncan - or it is variable - dependant on settings/ambient
temps, or other factors…?

Let’s take a setting of 70 degrees. I believe on a specific
day you could mark a point on the cam where the blend of air
coming out of the vents is exactly 70 degrees. So once all
of the air in the cabin is also 70 degrees, the servo would
end up at that position and sit there, only ever needing to
change when the outside air leaked in enough to change the
inside air temp, requiring an adjustment. BUT! That point
on the cam is going to be different when the incoming air
is, say, 100 degrees… that air needs to be cooled more to
make the blend 70 degrees, and it will be battling more heat
ingress from outside. So the ambient air sensor tweaks that
70 degree position more to the cool side of the servo cam.

So for any given set of prevailing conditions, there is a
stasis point on the cam for each requested temp… but that
point scoots around depending on prevailing conditions.

If you read the original patent, they also planned to have a
photocell, to tweak the behavior even further if it was
night time! Man, am I glad they didn’t follow through on
that…

Duncan–
Duncan Brown 1985 SIII XJ6
Grayslake, IL, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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frobozz wrote:

Lets take a setting of 70 degrees. I believe on a specific

day you could mark a point on the cam where the blend of air
coming out of the vents is exactly 70 degrees. So once all
of the air in the cabin is also 70 degrees, the servo would
end up at that position and sit there, only ever needing to
change when the outside air leaked in enough to change the
inside air temp, requiring an adjustment. BUT! That point
on the cam is going to be different when the incoming air
is, say, 100 degrees… that air needs to be cooled more to
make the blend 70 degrees, and it will be battling more heat
ingress from outside. So the ambient air sensor tweaks that
70 degree position more to the cool side of the servo cam.

So for any given set of prevailing conditions, there is a
stasis point on the cam for each requested temp… but that
point scoots around depending on prevailing conditions.

If you read the original patent, they also planned to have a
photocell, to tweak the behavior even further if it was
night time! Man, am I glad they didn’t follow through on
that…

I think I second that, Duncan - it might even have been combined to also
increase cooling in strong sunshine - or the angle of the sun…:slight_smile:

But in short; the stasis point shifts slightly according to
conditions…? As an extra; I have always assumed that when if you
switch to the fan high or low the servo just stops moving - and the
system goes on with the settings of the stationary servo. But at the set
fan speed of course. Correct…??

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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In reply to a message from Frank Andersen sent Thu 7 Jul 2005:

I think I second that, Duncan - it might even have been
combined to also increase cooling in strong sunshine

  • or the angle of the sun…:slight_smile:

I’m picturing the first time the Delanair engineers drove
into a tunnel and the cabin temp rose 10 degrees, they
thought ‘‘ohhhhhhhhhhhhhhhh’’ and bailed on the idea. At
least they didn’t think ‘‘OK, now we’ll add a tunnel sensor,
and…’’

As an extra; I have always assumed that when if you switch
to the fan high or low the servo just stops moving -
and the system goes on with the settings of the stationary
servo. But at the set fan speed of course. Correct…??

Absolutely not - the servo keeps spinning merrily along,
trying to do what it always does; the knob just overrides
the servo’s fan switches and sets the fan speed denoted
instead. In some cases you might be forcing the system to
alter the temperature faster, in other cases you may be
fighting its instincts, but it still keeps trying.–
Duncan Brown 1985 SIII XJ6
Grayslake, IL, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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frobozz wrote:

As an extra; I have always assumed that when if you switch
to the fan high or low the servo just stops moving -
and the system goes on with the settings of the stationary
servo. But at the set fan speed of course. Correct…??

Absolutely not - the servo keeps spinning merrily along,
trying to do what it always does; the knob just overrides
the servo’s fan switches and sets the fan speed denoted
instead. In some cases you might be forcing the system to
alter the temperature faster, in other cases you may be
fighting its instincts, but it still keeps trying.

That’s odd, Duncan…?

My servo doesn’t move in high or low yet comes on immidiately I set
to auto

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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In reply to a message from Frank Andersen sent Thu 7 Jul 2005:

That’s odd, Duncan…? My servo doesn’t move in high or
low yet comes on immidiately I set to auto

Didn’t we previously determine you have a whisper-quiet
servo that moves without you hearing it, heh?

Try this test: with the car fully warm, on a mild day, you
should be able to put the system in auto and turn the dial
to 85 and feel heat down by your knees, then turn the dial
to 65 and feel cool air from the center vent. (Keeping in
mind that it takes the servo 30 seconds to travel from one
end to the other…) OK, now set the fan to Low and try the
same temperature test. I believe you will still feel heat
at your knees on 85 and cool air out the center vent on
65… proving the servo is moving in response to the temp
knob, despite your having overridden the fan to Low speed.–
Duncan Brown 1985 SIII XJ6
Grayslake, IL, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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I see this an old thread, but was there ever a schematic/circuit diagram of the original Air con amplifier? After the de-potting of the unit.

1 Like

I cannot answer your question–electronics NOT my forte–but there are many 'lectrikkle gurus on here (@Paul_M_Novak comes to mind) who may be able to help ya.

Welcome to the best Jag source of info on the Net, and I love seeing these zombie threads brought back from the undead!

Thanks for your prompt reply, I will keep looking.

Bill Sent this from his iPhone

Bill,
I see that you are new to Jag-Lovers. Welcome.

I remember Duncan Brown’s posts on the A/C amplifiers and other XJ6 electronics very well. I was very impressed with his work. I do not remember him ever posting on Jag-Lovers a schematic of the A/C amplifier that he reverse engineered. If he did then you will find it if you searched the archives for “Duncan Brown” and follow those threads.

There were a few companies that sold aftermarket A/C amplifiers and about a year ago I posted pictures on Jag-Lovers of the ones that I have purchased over the past 15 years or so for my Series III XJ6s as I encountered A/C amplifier failures. They include the Australian “shotgun shell” design, the John’s Cars “grey brick” and the Jag-Aire design currently being sold by Gary Crosby (a Jag-Lovers member).

The OEM A/C amplifiers have long term reliability problems and I like to keep the heat and air conditioning sytems fully functional in my Delanair MK II equipped XJ6s. So as my A/C amplifiers failed I obtained a variety of replacement units to keep my climate control systems working as they should.

Paul

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Hey Bill, Good to see you here and happy belated B’Day!

I’ve followed some of Duncan’s work out of curiosity. I know he publicly posted his then-used email address on the forums in one of the AC Amp threads and maybe you can contact him with that. The site he stored his photos is still up, but there’s no final follow up for his work.

Some of his photos:
Chemically and mechanically de-potted Amp:


(I wonder if he tried boiling water to de-pot… may have worked better and been more gentle).

Breadboarding:

Near final product:

And that’s it… the end of it…

I found THESE pics… I can’t remember… somewhere else… but this could be reverse-engineered just from the pictures w/ some work I think…

Also,Besides Jag-Air, there appears to maybe be one place in the Neatherlands still making a replacement? Maybe? https://stengerjaguar.nl/nl/home/kachelregelaar-jaguar.html

~Paul K.

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Thanks Paul for the good wishes and all good information, it was mainly curiosity why I was asking as I don’t think there is a problem yet! Thought I would be prepared in case I need it later, I am building a file on the AC system and will add your input.

Bill Sent this from his iPhone

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That was a cool project. The one thing I would have done differently would have been to use only two LED’s, a red one that lit when the servo was moving in the warmer direction and a green one that lit when it was moving in the cooler direction.

This is the kind of amateur sleuthing/re-engineering that tells me, those who dogmatically assert these “gol-durned compooter cars ain’t NEVER gonna be able to be fixed, in 50 years!” are…wrong.

This is dead easy. It talks to nothing, isn’t checked, has no language, nothing. Just saying… take a modern high end car and it‘ll take you a decade to rebuild everything if you have as much as we here. It gets harder, but still, if they last 20 years (and they will) i don’t care.