New A/C wiring harness build advice

[Littlebeefy]

Hi all,
As many of you already know, there is very little of my original truck left, so everything I do somehow has to Frankenstein together some old parts and some new parts. This is no different.

It's an '84 Bronco, 460 transplant using an Edelbrock Pro Flo 4. The original EEC has been pulled out, but most of the original wiring for everything that wasn't connected to the EEC remains. The Bronco came with the factory A/C and has the original hardware installed for the most part, except for the compressor. All of the other parts have been replaced with new OEM style where available. The compressor is a Sanden 5 series, I believe.

I am trying to wire the A/C controls to the clutch cycle switch and the compressor. As I understand it, the light green wire with pink (or purple?) dashes coming from the firewall harness connects to the clutch cycle switch. That is as far as I get before confusion sets in. It appears that the other connector on the cycle switch should go to the only wire on the Sanden (it doesn't have a ground because it grounds via the engine mount). That makes sense, except it doesn't account for the diode that I have on the original harness. In looking at the diagram on the EVTM, it seems like the wire that goes to the Sanden should also splice off, go through the diode and then go to ground, which does not make sense to me at all because then it would never actually result in power being sent to the clutch (it would just follow the path of least resistance and go to ground instead).

I never trust myself in these situations; I had to take circuits twice in college and still got lucky to get through it with a C so it is not a strength for me. Any advice here?

[85lebaront2]

The diode is wired backwards across the clutch terminals, it is called a "snubber" diode. Simply use the wire from the switch to power the Sanden clutch, leave the diode and ground side intact so it will still function.

The snubber is due to the reverse pulse a mangnetic coil produces when the circuit is broken (like an ignition coil does), this keeps it from damaging the switch or relay.

[Littlebeefy]

So is this right?:

[85lebaront2]

Chad, I am thinking that's backwards, I will have to see if I have a loose AC compressor harness I can check.

[ratdude747]

Diode wise, that's correct. You don't want the diode to conduct when the coil command is on (otherwise, it'll behave as a short to ground and pop the fuse). But you do want it to conduct when the coil command turns off and said coil induces a reverse voltage to keep current flowing.

[ArdWrknTrk]

Let's look at the actual EVTM diagram....



The diode is blocking the alternate path to ground, but preventing ringing (or pulse) from getting back to the EEC through the pressure switch by creating a loop to the field coil.
There's also .6-7 voltage drop across most diodes, but I don't think that's a factor here.

[Littlebeefy]

Thanks. Based on the EVTM, it seems like my drawing is correct. I can ignore the EEC connection and the vacuum solepot, so all that's left is what I have drawn. I still don't understand it (like I said, extremely weak with circuits; simply does not compute so I think I'm missing that part of my brain) but understanding it is not a requirement for making it.

[ArdWrknTrk]

If there's no computer and the only thing is the clutch cycle switch you shouldn't need the diode.

[Littlebeefy]

That's even better, Jim. So one side of the cycle switch connects to the LG/P wire and one side connects to the Sanden and Bob's your uncle?!?

[viven44]

I moved the bullnose FS10 compressor into my 78 Bronco (460/Carb). The 78 system doesn't have a low-side pressure switch but a de-icing switch.... past that switch, a 12V power wire runs to the compressor to turn on the AC clutch... The AC compressor needs 2 wires... 1 for clutch and the other one is ground.... the clutch control wire came from the de-icing switch... I ran a wire to the body and grounded it there... I had no idea I needed a diode as well....