Electrical Is A/C compressor harness diode necessary?

wiseguyk · Apr 1, 2018

Hey guys, I'm nearing completion of wire tucking my 91 LX and had a question about eliminating the stock A/C compressor harness that has the diode in it.

I did a r134a conversion and have an aftermarket Sanden compressor which only uses the single black/yellow wire to engage the compressor (the housing has a built in ground to the bracket) so I'm wondering if I need to retain the diode present in the stock harness or if it's unnecessary.

Here's what the harness looks like:

And here's mine with all the tape removed:

I'm wondering if I need this harness or if I can eliminate it. There's already so much wiring I'm cramming up in the driver fender well so if I can save some room by getting rid of this that would be great.

Thanks!

Mustang5L5 · Apr 1, 2018

It’s a flyback diode. Its necessary. Here's how it's wired up on a 91

wiseguyk · Apr 1, 2018

Thanks for your reply Mike! I see the A/C clutch diode there and A/C clutch field coil but I'm not experienced with reading wiring diagrams so it's a little confusing

I guess the easy thing to do is to just leave it connected but I guess I was hoping to understand what the purpose was and why it's needed in the first place. What book did you find that diagram in?

BMW Rider · Apr 1, 2018

The diode is there to protect the rest of the cars electronics from an induced voltage surge when the clutch is de-energized. Think of the clutch magnet as being basically a coil. When the power is cut, the collapsing magnetic field creates a high voltage discharge. A diode is basically a one way valve for electricity, the normal power can pass through to energize the coil, but the induced backflow voltage is blocked. Older cars without electronics (or with simple early electronics) did not need protection from such a surge, but more modern computers are very sensitive to it.

wiseguyk · Apr 2, 2018

Ah ok thank you for the explanation. I didn't realize that occurred when the clutch disengaged and it makes sense now why that diode would need to be there. I certainly don't want to risk frying/damaging my ECU so I'll make sure to wire it back up and tuck it up there along with all the other wiring

wiseguyk · Apr 2, 2018

Mustang5L5 said:
I don't think so. Here's how it's wired up on a 91

Hey Mike, is there any chance that you can tell me what the two wires going into the A/C harness (with the diode) are?

I see the black/white one shown in the diagram you attached but there are 2 wires that go into that harness from the car side.

Mustang5L5 · Apr 2, 2018

wiseguyk said:
Hey Mike, is there any chance that you can tell me what the two wires going into the A/C harness (with the diode) are?

I see the black/white one shown in the diagram you attached but there are 2 wires that go into that harness from the car side.

The other wire is just the black wire to ground. If you look at my wiring diagram I posted, you'll see both the wires go to connector C100 even though they are split on the diagram.. The stripe wire comes off the WOT relay on the pass side of the engine bay and the other one travels to ground

wiseguyk · Apr 2, 2018

Mustang5L5 said:
The other wire is just the black wire to ground. If you look at my wiring diagram I posted, you'll see both the wires go to connector C100 even though they are split on the diagram.. The stripe wire comes off the WOT relay on the pass side of the engine bay and the other one travels to ground

Ah ha! Did I mention I suck at reading wiring diagrams? I see now, thank you so much for taking the time to explain it to me.

When I thought all I needed was the positive wire to engage the clutch (my aftermarket compressor only uses the black/yellow wire from the harness w/diode) I cut the other wire coming from the car harness. I thought I remembered it being a ground but wasn't sure since I started this project a couple months ago.

Thanks again! By the way, what is the name of the book you took the wiring diagram picture from? It looks very detailed and like something I should have in my arsenal.

Mustang5L5 · Apr 2, 2018

wiseguyk said:
Thanks again! By the way, what is the name of the book you took the wiring diagram picture from? It looks very detailed and like something I should have in my arsenal.

1991 Mustang Electrical and Vacuum troubleshooting manual.

There are originals on Ebay but rockauto.com sells reprints cheaper under "Manuals"

wiseguyk · Apr 2, 2018

Mustang5L5 said:
1991 Mustang Electrical and Vacuum troubleshooting manual.

There are originals on Ebay but rockauto.com sells reprints cheaper under "Manuals"

Awesome thanks again Mike!

Blown88GT · Apr 4, 2018

BMW Rider said:
The diode is there to protect the rest of the cars electronics from an induced voltage surge when the clutch is de-energized. Think of the clutch magnet as being basically a coil. When the power is cut, the collapsing magnetic field creates a high voltage discharge. A diode is basically a one way valve for electricity, the normal power can pass through to energize the coil, but the induced backflow voltage is blocked. Older cars without electronics (or with simple early electronics) did not need protection from such a surge, but more modern computers are very sensitive to it.

Close, but not quite. The diode does not conduct when the A/C clutch is energized. It only conducts when the A/C clutch is de-energized because the voltage spike is reversed polarity. The function of the diode is to protect the relay contacts from a spark that would arc across them when the relay opens. The diode suppresses the spark. It operates much like a condenser did in a points distributor. Suppress the spark so the points don't erode so quickly.

BTW, Ford didn't add anything that wasn't necessary & didn't add things that were necessary. Saving $.10 here & there adds up to $1.00/vehicle which is a lot when you make millions of vehicles.

wiseguyk · Apr 5, 2018

Blown88GT said:
Close, but not quite. The diode does not conduct when the A/C clutch is energized. It only conducts when the A/C clutch is de-energized because the voltage spike is reversed polarity. The function of the diode is to protect the relay contacts from a spark that would arc across them when the relay opens. The diode suppresses the spark. It operates much like a condenser did in a points distributor. Suppress the spark so the points don't erode so quickly.

BTW, Ford didn't add anything that wasn't necessary & didn't add things that were necessary. Saving $.10 here & there adds up to $1.00/vehicle which is a lot when you make millions of vehicles.

Thanks for clarifying, and yeah it makes sense that Ford would have omitted anything that wasn't actually necessary.

Electrical Is A/C compressor harness diode necessary?

wiseguyk

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