Outer Limits 4x4 Board

Anyone out there know anything about the circuit boards used to control the aircon on FJ/HJ60s ?

Problem: When I converted my G-pak petrol FJ to diesel, I lost my tacho operation. Turns out that the AC needs an RPM signal to activate the airconditioning. The G-pak petrol has a pot to control the on/off temperature, and another pot to change the RPM at which the AC cuts out. You are supposed to set the RPM knob so the AC is off at idle, but on with the 'Idle-Up' switch activated.

Now I checked the circuit board on my old man's barn door HJ60, and it only has a pot for temp, so needs no tach signal.

I figured I could just slip a card into the slot off a base model cruiser, but it seems it's not that simple.

I've jumpered the plug so the AC works, but it's on all the time and quickly ices up in the tropics.

Can anyone tell me which wires to jump on the G-pak FJ aircon controller so the AC will operate and cycle based on temp alone ?

Failing that, can anyone tell me which wires do what on the HJ60 base model system ? Maybe I can duplicate my FJ wiring to match the HJ circuit card.

Can't get an alternator with a tacho output? I thought these things were pretty common for diesels.

Scott

Yeah, I've considered this. However I'd need to muck about with alternator pulley sizes to get a correct (or near-enough) reading. I'd probably also require a diesel tacho (no biggie). I don't know of any alts with this as standard, though I've been told an auto elec could easily mod my alternator.

Someone did send me a diagram of how to rig up a hall-effect pickup to make the 6cyl petrol tach work with a diesel V8, but there's something missing, and even my electronics experts can't get it to work.

For interest, the V8 has 12 holes in the harmonic balancer. The theory is that the petrol six fires 3 times per revolution, so placing 3 bolts into equally spaced holes, and using a mag pickup, then suitably amplifying the signal, should see the standard tach give an accurate signal.

Something else I considered was 'faking' the tach signal to the AC controller, but no-one can explain what form such a signal takes ??? Is it simply 12V, is it current dependant ? I was thinking I could simply power the tach wire with a low voltage to simulate a constant RPM, but no-one seems to understand just how a tacho works.

Aftermarket petrol tachometers typically take their signal from the -ve side of the ignition coil. This point is grounded when the points are closed, and not grounded when the points are open. The tacho will see this as something resembling a square wave, which has a frequency directly proportional to engine speed.

The signal from the coil is probably "cleaned up" with some extra circuitry before the AC controller sees it. The bolts and hall effect sensor sounds like a simple way to generate a suitable frequency. If it's not working I'd guess that you have one or two problems.

1. The output of the circuit isn't "swinging" over the correct voltage range. The signal from the ignition coil will hit 0V, and hit 12V - with a lot of switching noise too. You should confirm that your circuit is reaching these voltages, and they're not being affected (pulled down) by the AC controller board.

2. The ignition signal will have a high slew rate - it changes from one voltage to another very quickly. Some circuits look for this "edge" as a trigger point. If your circuit changes voltage too slowly (slew rate is too low) the AC controller may not recognise the edges. The signal can be "squared up" pretty simply - I expect your "electronics experts" know how to do this.

Or I could be completely wrong - again.

Good luck,

Scott