Rear Window Defrogger – Accord 90-93

by spartanix on June 15, 2010

Honda Accord 1990-1993 rear window defogger system uses a heating grid wire bonded to inside of window. Heat is regulated by a dash-mounted control switch with indicator light, relay and Integrated Control Unit (ICU).

Power is supplied to control switch through a fuse in fuse/relay box. On Accord and Prelude, ICU will supply power to grid for 25 minutes, or until ignition is turned off. On Civic, timer is located in rear window defogger switch.

Grid Filament Repair
Clean broken wire tips thoroughly. Place masking tape along both sides of broken wire. Apply Repair Paste (Dupont 4817) to broken section of grid.

Remove masking tape after paste has dried. Wait 24 hours before using defogger.

Defogger Switch Test
Carefully pry switch from instrument panel. Using an ohmmeter, ensure continuity exists between wire terminals “D” and “F” when control switch is pushed.

If continuity does not exist, replace switch. Continuity should exist between wire terminals “A” and “C” (indicator light), and wire terminals “B” and “E” (light) at all times.

Defogger Relay Test
Remove defogger relay. On Honda Accord and Prelude, relay is located in underdash fuse/relay box at left side kick panel.

On Honda Civic, relay is located in underhood fuse/relay box on right side of engine compartment.

Apply battery voltage to terminal “C”, and ground terminal “D”. See Fig. 3 . Use an ohmmeter to ensure continuity exists between terminals “A” and “B”. Ensure continuity does not exist between terminals “A” and “B” when battery voltage is removed.

Grid Filament Test

  1. Turn ignition and rear window defogger on. Using a voltmeter, check for voltage between defogger positive wire terminal and body ground. If voltage is present, go to step 2). If voltage is not present, check for faulty defogger relay. Check for open in Black/Green defogger relay wire.
  2. If voltage is present, use an ohmmeter to check for continuity between defogger negative wire terminal and body ground. If continuity does not exist, check for open in defogger ground wire. Connect voltmeter negative probe to defogger negative wire terminal. Connect voltmeter positive probe to center of each defogger grid wire.
  3. If voltage is approximately 6 volts, defogger grid wire is okay. If voltage is greater than 6 volts, an open exists between center of defogger grid wire and negative side of center. If voltage is not present, an open exists between defogger grid wire and positive side of center. To locate break, move probe along grid wire until meter needle moves abruptly.


Defogger Inoperative
Check the following fuses: No. 1 (10-amp) and No. 8 (7.5-amp) in dash fuse/relay box (located at left kick panel). Check defogger timer circuit input (in ICU). Check for poor ground connections.

Check for short or open in Black/Green, Yellow/Black or Yellow/Green wires.

Indicator Light Inoperative (Defogger Operates)
On all Honda Accord 1990-1993 models, check for burned-out bulb. On Accord, check for short or open in Yellow/Black or Yellow/Green wires.

On Honda Civic and Prelude, check for short or open in Black/Yellow or Black wires.

Indicator Light Operates (Defogger Inoperative)
Check for blown fuse No. 17 (40-amp on Honda Accord coupe and sedan, 30-amp on wagon) in underhood fuse/relay box.

Check for poor ground connections. Perform defogger grid and defogger relay tests. Check for short or open in Yellow/Green or Yellow/Black wires.

Operation Time Too Long or Too Short

  1. On Honda Accord and Prelude, check defogger timer circuit input (in ICU). On Honda Civic, check defogger timer circuit, located in defogger switch.
  2. On Honda Accord, check for open or disconnected terminal in Yellow/Green or Yellow/Black wires. On Civic, check for open or disconnected terminal in Blue/Yellow, Black/Yellow or Black/Green wires. On Prelude, check for open or disconnected terminal in Yellow/White or Black/Green wires.

Got comments or questions? Feel free to leave them below.

{ 5 comments… read them below or add one }

Xavier January 6, 2011 at 5:44 pm

Rear defroster switch. The tests outlined above are useful. If the two small bulbs in the switch are replaced the corresponding continuity requirements can be met for 2 of the tests. The bulbs can be purchased from an electronic store for under $3.00. The depressed switch test requires the switch to be opened up to repair it. This also can be done excepting that the bakelite casing is fragile and could break.

spartanix January 10, 2011 at 11:19 pm

Thanks for the awesome info. Keep it comin.

Keith March 19, 2011 at 1:35 pm

Does this also apply to 2005 Accord LX?

Jake August 11, 2012 at 9:49 am

Thank you for the info….awesome job. I used the pin diagram to determine my wire terminals “B” and “E” do not have continuity.

Please add an ABS post for the 90-93 Accord as that seems to be a common issue on older Accords!

mike November 13, 2012 at 11:53 pm

wich realy is the rear window defrost. there is three or four relays in the fuse box, located left side of car near the clutch pedal. the car is a 91 honda accord ex two door

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