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XJ ( X308 ) XJ8 / XJR 1997 - 2003

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  #1  
Old 06-12-2012, 10:07 PM
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Default Help Explain the Function of the A/C Trinary Switch

There appear to be 3 functions in this switch (hence the name, I guess)...

1) A switch at 12 bar.
2) A switch at 20 bar.
3) A switch at 2-30 bar.

Okay, help me CONFIRM my thoughts on each piece of the puzzle here.

1) I'm thinking that when the pressure hits 12-bar that the switch closes, sends a ground to the ECM and the ECM turns on the LOW speed fan.

2) I'm thinking that when the pressure hits 20-bar that the switch closes, sends a ground to the ECM and the ECM turns on the HIGH speed fan.

3) I'm thinking that the switch, when in your hand uninstalled, is open. When installed at normal static pressure it is closed (let's say anywhere between 2-bar and 30-bar). When freon leaks out and pressure drops below 2-bar and the switch opens / when head pressure rises and gets higher than 30-bar the switch opens. When either scenario happens and the switch opens, the ACCM tells the compressor to turn off via that relay.


I'm pretty positive that #3 is correct above. It's just the other two that I really am looking for confirmation on.

So, to me, the ECM is not only using temperatures to control the fans, it's using the A/C system pressure to control the fans...assuming my theories are confirmed.

Anyone with ACTUAL KNOWLEDGE able to confirm these?

I've come up with some decent theories and I've come up with some pretty lousy ones...so I always like to ask the experts.

Thanks in advance!!!
Adam

Last edited by HectorM52; 06-12-2012 at 10:10 PM.
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  #2  
Old 06-13-2012, 07:22 AM
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Well there is no doubt that most cars since the 90s use one or more refrigerant pressure sensors to control A/C clutch engagement and fans. Some cars only have it on the high side of the system, some only have it on the low side. Some have both. If you want to see the pressures at which things happen, you can install a set of gauges and induce conditions that create the threshold pressures you are talking about. You can test the continuity of the the sensor using a multimeter and may be able to see more good info in a wiring diagram.
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  #3  
Old 06-13-2012, 07:59 AM
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It will be hard to induce those conditions on a car without an ECM...which is why I'm looking for THOSE specific answers.

I do understand that there are cars that have HP sensors, LP sensors, HP switches, LP switches, etc...

But what I'm looking to do is use this Jaguar trinary switch to not only control the ACCM via the third circuit described above, but I want to use the first two circuits to control the electric fans via direct links to the relay in lieu of going through the ECM.


I have a swapped GM LS-style engine in a '99 XJ8 that I have running and I'm trying to mimic the original Jag A/C operation... If I knew that those two theories above were correct, all I would need to do is just jump in those two wires at the "fan on" switches - in which case I could also delete my direct tie-in to the "fan on" switch which is currently tied into the "A/C on" circuit.


If my theories are correct, then the fans will not run FULL TIME when the A/C is on, just PART TIME when those two pressures are reached.
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Old 06-13-2012, 10:26 AM
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As for the Jag temp thresholds, I have no clue, but on my BMW 740il, it worked just like that.

When it goes over the normal operational temp by 20 to 30%, the low speed setting kicks in, until temp drops.

When it over the 20 to 30% of normal opertional temp the High speed setting kicks in (like stuck in traffic).

As for the compressor, it will not turn on if the freon level is low, because the freon also is a lubricant for the internal gears/pump. Regardless this will still turn on the high speed setting for the fans when the A/C button is pressed. Also on the BMW the A/C compressor will not turn on if the outside temp was below 55 degrees.

As for the switch in hand, I don't know know if there is a emergancy mode to turn the fans on if the switch fails. That is why most of the 740 owner switched over to electrical fans, the downside is they was always on, very loud, and draw to much voltage from other components.

Wow, just realized, this is the first car I even own, that didn't have a seperate engine driven fan to help with it cooling functions.

So, if the fan switch fails on the X308, the car will instancely overheat, WOW!! not good.

The ECM turns on the fans by the information it receives from the varies sensors, the sensors do not turn on the fans.

Hopefully I answered you questions.

SGB

Last edited by sbreeden; 06-13-2012 at 10:34 AM.
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Old 06-13-2012, 09:19 PM
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Quote:
Originally Posted by sbreeden View Post
As for the Jag temp thresholds, I have no clue, but on my BMW 740il, it worked just like that.

When it goes over the normal operational temp by 20 to 30%, the low speed setting kicks in, until temp drops.

When it over the 20 to 30% of normal opertional temp the High speed setting kicks in (like stuck in traffic).

As for the compressor, it will not turn on if the freon level is low, because the freon also is a lubricant for the internal gears/pump. Regardless this will still turn on the high speed setting for the fans when the A/C button is pressed. Also on the BMW the A/C compressor will not turn on if the outside temp was below 55 degrees.

As for the switch in hand, I don't know know if there is a emergancy mode to turn the fans on if the switch fails. That is why most of the 740 owner switched over to electrical fans, the downside is they was always on, very loud, and draw to much voltage from other components.

Wow, just realized, this is the first car I even own, that didn't have a seperate engine driven fan to help with it cooling functions.

So, if the fan switch fails on the X308, the car will instancely overheat, WOW!! not good.

The ECM turns on the fans by the information it receives from the varies sensors, the sensors do not turn on the fans.

Hopefully I answered you questions.

SGB
Okay, forgive my candor here, but I need to convey what exactly I know versus what I don't know.

So, did you answer my questions? Well, no, not really...clearly you're guessing here...

You first mentioned temp thresholds. Those have nothing to do with the trinary switch nor the fans turning on at certain line pressures.

Then you mention the high-pressure & low pressure cutoff, which I'm familiar with.

Then you mention the "fan switch" failing - what fan switch? Conversely, if the ECM failed and did not turn on the fans, the engine would eventually overheat, though I don't think it would instantly overheat.

I also understand that the ECM controls the fans, not direct links from the sensors/switches.



Okay, again, here is what I'm trying to figure out:
Do the two circuits inside the switch tell the ECM to turn on the fans at certain pressures? (If not, what exactly do those two circuits do???)

Also, should the fan AUTOMATICALLY come on when the A/C is switch on, or does it only come on when those pressures are reached???


Sorry for being blunt. But I know there's someone out there who knows these answers without question.
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Old 06-13-2012, 10:34 PM
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Quote:
Originally Posted by HectorM52 View Post
But I know there's someone out there who knows these answers without question.
Maybe there is and maybe there isn't. At least on this board. In the meantime those who are trying to help should not be subject to criticism that their attempt may not be exactly to your particular taste.

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Old 06-15-2012, 10:07 PM
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I know some sensors only relay data rather than just switching. The ECM will use the pressure data to make a/c clutch / fan decisions. If we could see a diagram of how the switch is wired up, we can infer l how it can be used.

I am lazy so if I were you, I would probably just wire a relay into the a/c clutch relay output that way the fans will run anytime the clutch is engaged, although its not really needed at highway speed.

Just out of curiousity, what ECM ru running and what is the swap on? I'm interested!
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Old 06-15-2012, 10:26 PM
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I should have put this up sooner. Look at the 4-way pressure switch. The two bottom-left circuits (WU - white & blue) and (RW - red & white) used to run to the Jag ECM. They'll now hop in on my "fan-on" circuits that run from the GM PCM to the Jag fan relay inputs.


Click the image to open in full size.

I've since found out that the newer Impala's run their A/C this way (the way I'm trying to confirm - using lower pressure to turn on the low-speed fans and using higher pressure to turn on the high-speed fans).

Much like everyone else is doing, I'm just going to assume this is the way the Jag worked originally (or at least something close to this) and run with it.

Last edited by HectorM52; 06-15-2012 at 10:30 PM.
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Old 06-15-2012, 10:35 PM
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Quote:
Originally Posted by ixj8it View Post
I know some sensors only relay data rather than just switching. The ECM will use the pressure data to make a/c clutch / fan decisions. If we could see a diagram of how the switch is wired up, we can infer l how it can be used.
Done, see above.


Quote:
Originally Posted by ixj8it View Post
I am lazy so if I were you, I would probably just wire a relay into the a/c clutch relay output that way the fans will run anytime the clutch is engaged, although its not really needed at highway speed.
That's currently the way it is wired. But I wanted to get the fans running CLOSER to what is stock. To me, it just makes more sense the way I want to do it. Like you said, the fan isn't needed at highway speed!!


Quote:
Originally Posted by ixj8it View Post
Just out of curiousity, what ECM ru running and what is the swap on? I'm interested!
I'm running the '99 Silverado PCM (powertrain control module - it controls both the engine and tranny). Swap is in a '99 XJ8 sedan.

I'm currently about 99% of the way through the swap. Once I get the A/C running I'll be able to button up the wiring harness. Which will only leave one or two small items left before giving the car back to my client.
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Old 06-15-2012, 10:42 PM
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Cool! I wanted to do the chevy engine swap on mine I didnt though. Yeah I think you are 100% right on using those two wires now that we have seen the diagrams. One to turn on low speed, the other to turn on high using a couple of relays. Looks like it grounds one or both depending on pressure.
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Old 06-18-2012, 08:27 AM
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Actually I wasn't guessing.

If you was looking to bypass all the electronics, that control the fans automatically; then that is something you should have stated.

We still cool.

SGB
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Old 06-18-2012, 08:27 AM
 
 
 
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2000, 99, ac, clutch, foru, function, jaguar, location, pcm, pressure, relay, switch, test, trinary, wire, xjr, xjs



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