Electrical Is A/C compressor harness diode necessary?

wiseguyk

Active Member
Sep 23, 2016
302
83
48
Florida
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:

1522593971019.png


And here's mine with all the tape removed:
1522594159508.png


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! :)
 
  • Sponsors(?)


wiseguyk

Active Member
Sep 23, 2016
302
83
48
Florida
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 :confused:

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

Mustang Master
Oct 9, 2014
140
277
83
Calgary, AB, Canada
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

Active Member
Sep 23, 2016
302
83
48
Florida
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

Active Member
Sep 23, 2016
302
83
48
Florida
I don't think so. Here's how it's wired up on a 91

IMG_7348.JPG
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

i'm familiar with penetration
Mod Dude
Feb 18, 2001
33,071
9,212
224
Massachusetts
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

Active Member
Sep 23, 2016
302
83
48
Florida
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? :oops: 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.
 
Last edited:

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
33,071
9,212
224
Massachusetts
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"
 
  • Like
Reactions: wiseguyk

Blown88GT

Founding Member
Nov 13, 1999
2,038
468
134
Palm Beach Gardens, FL
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.
 
  • Like
Reactions: wiseguyk

wiseguyk

Active Member
Sep 23, 2016
302
83
48
Florida
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.
 
Thread starter Similar threads Forum Replies Date
6 Looking for the short harness that goes to the A/C compressor Fox 5.0 Mustang Tech 3
nickyb Ac compressor clutch$$$?. Fox 5.0 Mustang Tech 34
91GTstroked Engine AC compressor stays running? Fox 5.0 Mustang Tech 16
T AC Compressor Short Cycling 2005 - 2009 Specific Tech 7
H A/c PROBLEM think it maybe a compressor Fox 5.0 Mustang Tech 32
M For Sale 78 King Cobra Parts - Heads, Intake, Exhaust Manifolds, Radiator, AC Condenser, AC Compressor Mustang II Parts 0
NurseGuy AC Compressor cycle 1996 - 2004 SN95 Mustang -General/Talk- 5
G A/C Compressor not engaging SN95 V6 Mustang Tech 1
Cnorth94 01 GT Compressor keeps kicking outs Freon 1996 - 2004 SN95 Mustang -General/Talk- 0
O A/C compressor - York/Tecumseh style '67 Classic Mustang Specific Tech 0
R Engine 93 Ac Compressor Fox 5.0 Mustang Tech 1
NateKerr AC Compressor Issue 1996 - 2004 SN95 Mustang -General/Talk- 2
S Engine 99 GT - A/C compressor question SN95 4.6L Mustang Tech 6
I 1998 V6 - AC compressor clutch wont engage 1996 - 2004 SN95 Mustang -General/Talk- 5
MoDriver SOLD New Ford A/C Compressor w/Clutch Model HR980 83-88 Ford SVO Mustang Tbird Cougar Engine and Power Adder 0
TrophyHead Sanden A/C Compressor brackets? 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 1
M A/C compressor will not turn on 2005 - 2014 Specific V6 Tech 0
J A/C compressor clutch 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 1
A 06 ac problem The Welcome Wagon 3
billison Let’s Talk Air Compressors/set Ups. 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 29
J Ac Clutch Problem On 2006 Gt 2005 - 2009 Specific Tech 2
E Possible Ac Compressor Problem? 2005 - 2009 Specific Tech 0
Konrad Kocon Electrical Ac Compressor Clutch Is Acting Weird... SN95 4.6L Mustang Tech 11
D Back Of Ac Compressor Hits Valve Cover Fox 5.0 Mustang Tech 7
R Help Identifying Ac Compressor Fox 5.0 Mustang Tech 2
tannerc91gt Air Compressor Recommendations 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 20
mpeach1976 Ac Compressor Failure 2010 - 2014 Specific Tech 10
jlangholzj Expired Fomoco A/c Compressor Engine and Power Adder 0
C 98 Mustang Air Conditioning Compressor Isn't Grounded Help SN95 4.6L Mustang Tech 3
C A.c. Compressor Broke 1996 - 2004 SN95 Mustang -General/Talk- 3
A A Little A/c Issue.. The Welcome Wagon 0
chapters707 A/c Compressor Will Jump On But Can't Get It Charged 1994 - 1995 Specific Tech 4
W How To Power Air Compressor Other Auto Tech 5
B Please Help Ac Compressor Will Not Engage SN95 4.6L Mustang Tech 15
G Ac Compressor Clutch/coil Replaced Won't Engage SN95 4.6L Mustang Tech 5
squeak93 Best Valve Spring Compressor For The Money 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 7
madspeed Compressor Water Seperator? Other Auto Tech 3
mikestang63 Chrome Ac Compressor 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 1
stykthyn Rebuild Ac Compressor Or Buy Sanden? Fox 5.0 Mustang Tech 5
SoFlStang Compressor Connector Fox 5.0 Mustang Tech 8
jayrw 2000 Mustang Gt - Ac Compressor Clutch Not Kicking On Unless I Jump It. SN95 4.6L Mustang Tech 12
N A/C compressor preassure valve keeps engadging Fox 5.0 Mustang Tech 1
S A/C Air Conditioning re install compressor and fittings questions Classic Mustang Specific Tech 3
Americansilver Engine AC Compressor SN95 V6 Mustang Tech 2
fastgtracer Ac Compressor Clutch Diagram Needed Fox 5.0 Mustang Tech 3
tealtiger93 A/c Compressor, Factory Or Dealer Installed? 2.3L (N/A & Turbo) Tech 14
boostfrk Engine Testing A/c Compressor Fox 5.0 Mustang Tech 3
A What Spring Compressor For The Front?? Fox 5.0 Mustang Tech 6
90lxwhite a/c compressor and the paper clip tirck Fox 5.0 Mustang Tech 7
bbayne35 A/C Compressor Question SN95 4.6L Mustang Tech 1
Similar threads