The A/C Amp Thread (1 Viewer)

[TrickyT]

This thread is all about FJ60/FJ62 A/C amplifiers. You know, that little black box that mounts on the evaporator case behind the glove box. There are plenty of other threads concerning A/C amps and I thought about adding to those, but in the end decided that the information here breaks enough new ground that it deserves a dedicated thread.

There are details here about the A/C amps for the FJ60 (p/n 88650-60020) and the FJ62 (p/n 88650-90A03) . Both are fairly similar and designed to perform three functions:
1) prevent the compressor from engaging during engine start-up or at low idle
2) increase idle speed when the compressor is engaged
3) disable the compressor if the evaporator coils get too cold (i.e., near/at freezing)

The major difference between the boards is how #1 is implemented. The FJ60 board has rpm sense circuitry that is driven by a signal from the igniter that inhibits the A/C clutch relay from closing at low rpm, while the FJ62 uses a signal from the ignition switch that disables the relay when the starter is engaged and for a short time thereafter.

 

[TrickyT]

Here’s what the circuit boards look like. FJ62 circuit board (p/n 077300-0841) is the top photo, FJ60 (p/n 077100-1690) on the bottom.

 

[TrickyT]

For the EE’s out there, here are the schematics that I reverse engineered for the two boards.

Both boards use a proprietary IC for the controller. On the FJ62 it’s a Toshiba T2333 and on the FJ60 it’s a Nippon Denso SE012. I could not find data sheets for either.

 

[TrickyT]

I first got interested in these boards a few years back when a friend was having a problem with the A/C on his FJ62. In the end the A/C amp was not at fault, but I did reverse engineer the circuit.

More recently, @FJ60Cam was doing a V8 swap in his 60 (see https://forum.ih8mud.com/threads/cams-fj60-is-gittin-a-heart-transplant.809304/) and obviously wanted the A/C to still work. Trouble was, the ignition signal from the ’07 5.3L Vortec was not compatible with the FJ60 A/C amp. Now others have dealt with this problem by either buying a signal converter module to drive the amp or by re-purposing an amp from a FJ62. (See for example https://forum.ih8mud.com/threads/ac-amplifier-not-working.849576/). But Cameron and I got to talking and I told him to send me his amp and I would figure out how to disable the rpm sensing circuitry so that he could just use his original amp. (At 310 hp compared to the 2F’s 135 hp there wasn’t much concern about the Vortec being able to spin the compressor at startup.)

It’s really easy to disable the rpm sense circuit. You just need to remove one resistor from the circuit board. If you want to be able to reverse your change you can unsolder the resistor, otherwise just use a small pair of wire cutters and snip the leads on the resistor. Here’s the resistor’s location on the board:

 

[TrickyT]

While I was at it, I wondered about the red trim potentiometer on this same board. It’s used to adjust the temperature at which the compressor cuts out to keep the evaporator coils from freezing over. There is a thermistor mounted inside the evaporator whose resistance changes as a function of temperature according to this graph taken from the FSM:

Note that the graph shows a range of values for any given temperature, indicating that the thermistor has a tolerance of +/- ~8%. The red trim pot is meant to be a factory-only adjustment to account for board-to-board differences in resistor and thermistor values. But with careful adjustment a user could adjust this potentiometer so that the compressor would continue to run until just before the evaporator begins to freeze up. So while I was figuring out how the rpm sense circuitry worked, I used my same test setup to calibrate the red trim pot. Here’s what that looks like:

If you mess with your trim pot setting just remember that the resistor values in the circuit are not exact and therefore these marks are only approximate. Make sure you make a reference mark on the board before you make any changes so that you can always reset the pot to where you started at. Also, exactly when the evaporator coils freeze is going to depend on humidity and other factors so be a bit conservative in your setting.

 

[TrickyT]

Lastly, if any of you are interested in what it took to figure all this stuff out, here’s a picture of my test setup:

The white breadboard with the small PC board and blue LED is an Arduino microprocessor that I programmed to simulate the 2F ignition system, with the knob controlling the “rpm”. In the photo it’s set at 32.6 Hz which corresponds to 650 rpm.

 

[Spike Strip]

Very nice work. Get thee to the FAQ!

 

[MANUCHAO]

This is great info...

Thank you for posting it and doing all the work....!!!

 

[daytonadogie]

Awesome! Thanks for sharing

 

[MickTLC]

Nice work! Good for future reference. Thanks.

 

[JohninFla]

You are the bomb! Awesome summary! Million dollar answer to AC amp questions!

 

[MANUCHAO]

TrickyT,

I wanted to say thanks again for the info you have provided.

I finally took my rig to have the AC recharged.

After doing the mods posted above to the amplifier........... DAMN...!!

I had to lower the temp on the AC....
BBBRRR It's COLD....!!!
And that's at idle...(528 rpm's)

Thank you ....!!

 

[TrickyT]

TrickyT,

I wanted to say thanks again for the info you have provided.

I finally took my rig to have the AC recharged.

After doing the mods posted above to the amplifier........... DAMN...!!

I had to lower the temp on the AC....
BBBRRR It's COLD....!!!
And that's at idle...(528 rpm's)

Thank you ....!!

Glad this worked well for you. I'll get an IPA out of the fridge and meet your toast right now.

 

[elripster]

TrickyT, would you know if the 62 uses the same thermistor as the 60 as far as the resistance value is concerned?

 

[TrickyT]

TrickyT, would you know if the 62 uses the same thermistor as the 60 as far as the resistance value is concerned?

Pretty sure that the FJ62 and "later year" FJ60's use the same thermistor. But I don't recall what years of FJ60's are considered "later." Beno would know for sure.

 

[elripster]

I believe if is November of 85 and later. I have a later November of 85 FJ60 and all of the parts align with later 85 specs.

It looks like that 62 amp must self calibrate to that thermistor. I noticed no more red potentiometer and all those voltage dividers and inputs on the IC.

Frank

 

[elripster]

TrickyT, one more question, do you see an issue with the 60 thermistor being grounded Vs. the 62 not? Or is that the later 60's have a non-ground thermistor? The reason I ask is that I have a 3FE in my 60 and am getting a used amp from a 62. I would think though that I could just remove the ground if I needed to.

Frank

 

[TrickyT]

I don't know about the "self calibrating" aspect, or if it's not more that the Toyota engineers figured out that the potentiometers were always getting set to the same place during the factory calibration process and so decided they could just eliminate the pot in favor of fixed resistors. The only way to know for sure is to get the full specs on the Toshiba T2333 integrated circuit and better understand how the complete circuit works.

 

[TrickyT]

TrickyT, one more question, do you see an issue with the 60 thermistor being grounded Vs. the 62 not? Or is that the later 60's have a non-ground thermistor? The reason I ask is that I have a 3FE in my 60 and am getting a used amp from a 62. I would think though that I could just remove the ground if I needed to.

Frank

The thermistor is grounded in both the FJ60 and FJ62. I just didn't show the thermistor in the FJ62 circuit schematic.

 

[elripster]

Excellent, thanks for the prompt replies, this is most helpful.

Frank