I recently purchased my 86 Volvo, this is my first Volvo I've owned and I am slowly learning all the nuances with the vehicle and how to fix them thanks to this forum. My 86 has the 4-speed manual with overdrive but like many vehicles out there the overdrive is not engaging. Since this is by far the most unique overdrive I've seen I'm not sure how to go about diagnosing the issue. I noticed that the button on top of the shifter does not "click" but does depress and goes right back to its original position. I thought that may be the issue, but I do not know if the button is supposed to "click"? I'm not sure how frequently these buttons fail or if I should just replace it outright?

 

before replacing anything, including the button, its best to do a series of electrical tests to verify that indeed what you're replacing is the root problem.

the buttons on my automatic overdrives don't click, they are momentary contact buttons and just press in and pop right back out, I assume the manual OD is similar.

The 'toggle' on/off function is provided by a relay. I'd test the pushbutton with a digital multimeter (DMM)... the relay is behind the center console, high up under the dashboard behind the upper radio cubby, my parts catalog says its red, rectangular and mounted sideways.. the pushbutton in the shifter is connected to relay pins 15 (blue wire) and 86 (black wire). pin 15 also goes to fuse 11 and powers the whole OD system

relay pin 31 (black) goes to ground. relay pin 85 (red) goes to the gearbox switch on the top/front/side corner of the gearbox, I believe this switch goes to ground when the gearbox is in 4th gear.

relay pin 87 (yellow) goes to both the dashboard OD indicator light, and to the OD solenoid on the bottom back of the OD unit behind the transmission. if this yellow wire gets power, the OD /should/ engage.

if the light isn't toggling, its almost certainly an electrical problem

relay pin 15 should always have voltage when the ignition is on (measure with voltmeter relative to chassis ground). relay pin 86 should be +12V when the button is being pushed. if no, then the pushbutton or its wiring is the problem. the pushbutton wiring goes through a pair of connectors on the right side of the transmission hump, one has three pins (red, yellow, brown on one side, and red, yellow, black on the other) and the other has one pin (blue on both sides). the button connects the brown wire to the blue one.

I would then test the gearbox switch, ignition switched off, connector unplugged, use the ohms mode to test that the red wire goes to ground when the transmission is in 4th gear, and is open in neutral or any other gear. you can also test this at the relay socket, pin 85 would go to ground if gear in 4th. (unplug the relay and test this at the relay socket).

at that 3 wire connector next to the transmission hump, disconnect it, and test the resistance of the yellow wire on the side going to the gearbox, measure its ohms relative to ground, it should be a low single digit ohms, this is the solenoid. if thats open, either the solenoid is burned out, or there's a broken wire between that connector and the solenoid. unplug the relay, turn the key on without starting the car, and jumper the relay socket between pins 15 and 87, and both the OD light should light up, and the OD solenoid should click. if that IS the case, and the other tests are OK, its highly likely to be the relay itself.

if its the relay, open it up, and odds are either the contacts are all crusty (gently clean with a fine emery fingernail board) or there's a cold solder joint (reflow all solder joints). worse case, thats a $40 relay at TascaParts.com, PN 3523805 according to VADIS (older Volvo parts catalog).