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Old 02-12-2012, 09:31 PM   #1 (permalink)
Cyborg ECU
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Location: Coastal Southern California
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Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

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How to Install an alternator cut-off ("kill") switch on a 1998 Honda Civic

Several Ecomodder members and many others have installed alternator "kill" or "cut-off" switches on their cars. “MetroMPG” tested and reported the potential fuel economy improvements "going alternator optional" on his Metro. “Daox” has a very good installation post for his Paseo. Some others have posted install DIYs for older Civics. On my 1998 Civic it seems a little trickier than it is on some other cars. This "how to" will outline how I did it, thanks in part to advice from “drmiller” and “brucepick”. Comments, corrections, and tips for improvement are all welcome. I will edit this post as appropriate, based on feedback.


The Civics have a "smart alternator". Through four wires running from the on-board computer (ECU) and the Electronic Load Detector (ELD) to the alternator, the sixth gen Civic applies a logic that modulates how much power the alternator produces, maximizing potential fuel savings by generating electricity when it will have the least negative effect on fuel economy and power. By deleting my alternator entirely and recharging the battery exclusively by plugging the car in at night, I have periodically observed a 3-5% increase in fuel economy over the last four or five months. This mod allows me to have that advantage, while still allowing me to revert to stock operations with the flick of a switch whenever I need more range.


1 - You must disable two of the wires to the 4P connector, not just one: the sixth gen Civic's "smart alternator" has a four wire connector (a "4P" connector) and a separate large gauge white power wire for the alternator's power output to the battery. On the connector, one wire is for the charging light signal (blu/wht), another is the power line (blk/yel) for the field windings, and the two others are signal and control wires to and from the ECU. If you cut only the power line (blk/yel) the alt will still power using the blu/wht charging lamp wire. I know this because I tested for it using short wires with male and female spade connectors. I interrupted one line at a time and then two at a time until I was able to shut the alt off. You must interrupt both the blu/wht and blk/yel lines. A similar discovery was made by "busypaws" for his 1997 Protege DX alternator switch (he resolved the problem differently).

2 - You will have to cut into the blk/yel wire at the 4P connector: It will not seem ideal to cut into the wire at the 4P connector into the alternator. So you might be tempted to try to cut into the wire closer to the fire wall. The Honda Service Manual wiring diagram for the alternator might encourage you to think that you can cut in near the fire wall or in the cabin. I tried that. I cut a wire that also disabled an O2 sensor. The only way to avoid affecting other components on the harness in to cut in at the 4P connector.

(1) Butt splice connectors
(2) Two female spade connectors
(3) a few feet of adequate gauge wire
(4) a wire hanger
(5) an on/off switch capable of handling just a couple volts
(6) an exact-o-knife and a box cutter
(7) duct tape
(8) philips head screwdriver.

1) Locate alternator and its light-green 4P connector, and unplug it.

2) Identify and cut yel/blk wire an inch or two from the 4P connector.

3) Connect your switch wire to each end using the butt splices.

4) Unplug blu/wht charging light wire at connector (on the left in the pic below).

5) Cut a slit into driver's side firewall grommet using the exact-o knife.

6) Run switch wire through the grommet by taping it to the wire hanger.

7) Cut hole into dash panel and run wires through it.

8) Connect wire to switch using female spade connectors.

9) put panel and installed switch back into dashboard.


Brucepick came up with a clever alternative to cutting into the harness. He went to a junkyard and got a junker’s 4P connector and attached it to the existing connector on his car using spade connectors. With one connector essentially plugged into the next, he need not alter the wiring harness on his car at all. Pretty clever.

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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

Last edited by California98Civic; 02-13-2012 at 08:36 PM..
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