I found a new use for a recycled 1990 740 defogger switch that I thought others might be interested in.

My V8 Volvo 740 has a T-56 with a reverse lock-out solenoid. To actuate the solenoid I had been using a recycled fog lamp switch which I placed in my dash in the slot formerly occupied by my sun roof switch. (I don't use the sun roof much, so it was not a big deal to lose the switch.) To shift into reverse I would simply toggle the switch on and off as necessary. It helped to remind me that I left the switch on as it would light up when engaged. Unfortunately I would still forget, and while I never accidentally shifted into reverse, it bothered me that I might do so.

What I really wanted was a timer that would automatically shut of the solenoid. This is where the defogger switch came in as it has a built in timer and power relay, and a light to indicate when it is on. I found one at a salvage yard for just a few dollars.

The problem is that the timer lasts for 12 minutes, which is too long. After some experimentation I found that I could reduce the timer to about 1 minute. The trick was to replace the resistor labeled as R2 on the circuit board. R2 has a value of 560K ohm, and I replaced it with a 1/4 watt 47K ohm resistor which shortened the timer by about a factor of 10. It was very easy to do once I figured out which resistor to replace.

You might also want to note that the three spade connectors on the switch function as follows (each is labeled on the switch):

Label "151" is power
Label "31" is ground
Label "87" is the switched output

A common alternative solution is to engage the reverse lock out solenoid through the brake light switch. This means the solenoid is engaged every time one steps on the brake, which seems like it could lead to excessive wear of the solenoid.

Perhaps the ultimate solution is to use the signal from the VSS to engage the solenoid only when the car’s speed is, say 5 MPH or less. I have not figured out how to do this yet!