I purchased the B&M Unimatic universal floor transmission shifter and proceeded to install it. After cutting the transmission hump, I realized that there was no backup light switch. I checked with B&M to see if there was a kit to add a backup light switch and there isn't! B&M sales literature is misleading as it doesn't state that there is no reverse light switch. Very easy to overlook. Who makes a transmission shifter without a backup light switch!
I checked on the Internet to see if anyone had a DIY solution for adding a back-up light switch to it and it seems that no-one had found one. Normally I would of returned this shifter but I had purchased it on sale a couple of years back and I had already cut a hole in the transmission tunnel. So on to retrofitting a backup light switch.
After lots of time pondering what to do, I realized that you can use the existing neutral safety switch mounting plates (passenger side) as guides for mounting a backup light switch on the opposite side (drivers side). Here's what needs to be done:
- Modify drivers side main plate for microswitch mounting screw plate clearance
- Drill drivers side main plate for swivel hole for the backup cam
- Backup Cam - this is the plate that swivels and it's cam lobe triggers the backup light microswitch
- Modify drivers side mounting angle for microswitch holes
- Make microswitch mounting screw plate
- Replace roll pin with longer pin
- Make swivel mounts
In addition, I replaced the shifter arm spring with a stronger spring as the original is pathetically weak. And I added torsion springs to both the neutral safety and backup cams. B&M had designed it so that only the weight of the cam held it down. I felt that road bumps could jar the cams enough that it might trigger the microswitches. Plus I don't like things rattling around.
Before you start, take lots of pictures! I did and am I ever glad because once everything was apart, it was difficult to put back together without them.
The main plate needs an opening to clear the backup light microswitch mounting screw plate. It's large because it allows the microswitch to be adjusted. Also a swivel hole needs to be drilled. Use the existing passenger side plate as a template. I made some mounting swivels on my lathe and found that 0.010" clearance left a perfect amount of play. A washer is placed between the main plate and the backup cam. I would post some dimensions here but I found that the stock washers that I used varied from 0.055" to 0.075" and the scrap sheet metal that I used for the backup cam was 0.085" thick while the original neutral safety switch cam was 0.135" thick. Getting the clearance took the most amount of time.
I used the original neutral safety switch cam as a template for the backup cam and only put a lobe where the reverse knotch is. The backup cam is much smaller than the neutral safety cam because it has to clear the tranny cable linkage as I found out!
The good news is that the drivers side mounting plate has the corner already knotched out. I used the passenger side as a template and drilled two slots for the microswitch. Clamped the two pieces together and used a 1/8" drill, drilled at either end of the slot and a small riffler file to file and connect the two holes. By the way, the microswitch is a standard size NO (normally open) microswitch which you can buy from any electronic parts source. I run mine through a relay so you just need one that can take enough current to drive a relay as finding one that will drive backup lights may be impossible.
A small plate holds the microswitch in place. Again, I used the existing plate as a template. I believe the holes are 4-40 screw holes which are figging small to thread.
Here's a closeup of the backup light switch assembly. It shows the backup cam (swivel plate) and also you can see the tranny linkage mounting point beneath the roll pin. That's why the backup cam is smaller then the factory neutral safety cam.
Normally the weight of the backup cam, stops the switch from being turned on. During normal driving the cam will bounce up and down depending on the road conditions - I don't like that as it may trigger one of the switches or make a rattle! So I added a torsion spring that wraps around the swivel mount and keeps the cam down. I did the same for the neutral safety cam. I couldn't find any torsion springs around so I rebent a regular spring into a torsion spring.
This is the the drivers side view which is where the backup light switch mechanism is located. The rear of the car is to the right. It is in the Park position right now. You can see the new roll pin which extends out on this side.
This is the passenger side with the factory installed neutral safety switch. I added a torsion a spring on this side too. And replaced the shift level spring for one with a little more oomph so that it feels like there's a spring there.