I just finished installing a tach for my motor and thought I'd share this for those needing a tach but not sure how to drive it. I didn't really find out much about going this route but there were lots of articles about how to drive a factory tach.
I've been wanting to do this for a while. Upon first building my truck, I checked the ratio of each gear to have an idea of the rpm in a certain gear at a certain speed. My calculations came out pretty close to the tach readout so I feel good that it's pretty accurate.
I purchased an Auto Meter 3707 tach
. It has a back lit display with a green or red rubber sleeve to slide over the lamp which is removable from the back for your night illumination color. I spoke to the factory people to see what they could tell me about the type of input to drive the thing since it normally gets a signal from the ignition system and I was thinking it was high voltage. They told me no, it's looking for a 12V square wave input with less than 50% duty cycle.
Because of my industrial background I thought a proximity switch should work and it did. I picked up a used DC three wire switch from a machinery company near the office. These things are typically rated 10-30VDC. I found one which was a PNP Sourcing switch.
I don't know if an NPN sinking switch would work or not. Oh when you look for one, be sure to get one that is sealed to keep water out. One with a replaceable cable would be nice since it could get damaged should something get loose. If that happens, replacing it is very quick and you don't have to get into the wiring again.
I have an Advanced FB1-4001 which has a 3/4 keyed shaft on the front and two 3/8 holes tapped into the front face of the motor face. I made an L shaped bracket and drilled a hole in it large enough for the 18mm switch to fit in and another hole for the 3/4 bolt to fit in. I attached it to the face of the motor so that the switch end was facing the motor shaft.
I picked up a shaft collar at Fastenal, an industrial parts supplier. It's basically a donut shaped collar about 1/2" wide with a 3/4 hole in it and a 5/16" allen set screw to fasten it on the shaft. No keyway but that's not needed. I removed the allen screw, drilled and tapped another hole directly opposite it. Then I took two 5/16 bolts and cut them off so they just touched the shaft when screwed all the way in. Now I have a device to give me two pulses for every rotation of the motor, the same thing you would have on a four cylinder engine.
Once the switch is installed, you will need to adjust it so that when the motor rotates, the LED will illuminate to show you it picked up the bolt heads. Put the transmission in neutral and turn the motor by hand to make sure it doesn't physically strike the sensor. Adjust the sensor as far away from the bolts as possible yet close enough it will still sense them. Then adjust it about one turn of the nut closer. This way it is less likely to get hit during rotation but assured sensing the bolts as well.
I removed a screw on the back of the tach with an allen wrench to access the dip switches and set the pulse count. With the screw removed the cover comes off. The diagram on the instructions shows how to set the switches for 4,6 or 8 cylinder operation. I selected the four cylinder option which tells the tach for each two pulses the engine rotated once.
As for the wiring, I picked up a hot from the ignition circuit after the inertia switch. There's a white wire on the tach to connect for illumination. I tied it to my parking lamp circuit off the switch in the dash.
Here's an installed pic. I put it on the A-pillar which is easily removable on my truck with two screws and allows you to run the wire down along the door molding toward the fuse box.
I'll take a photo of the mounted sensor and prox switch tomorrow if I don't forget.