EricSosa

So I've been reading through this forum and I've been trying to do some digging on a project I'm currently involved in but haven't found anything related to my issue.

The car is a 1988 RX-7 Convertible.

Tachometer
I've been trying to install a tachometer in this car because the stock tachometer doesn't work properly, it will either not work at all or the needle will move around erratically. I've currently run into an issue when trying to connect the input wire to the signal wire off of the coil pack. First off, the tachometer is eBay brand, I couldn't find any 4,6,8 cylinder options on it. Secondly, the tachometer has 5 wires that come off of it; Red, Yellow, White, Black, and Green. Red is 12v constant which I connected to the 12v constant wire from the radio, Yellow is ignition which I used from the radio as well, White is for illumination which is also tapped from the radio, Black is ground, and the Green wire is signal input. I've tried tapping the signal wire to the Yellow/Blue bullet connector wire off from the trailing coil pack but I get no signal back. The aftermarket tachometer turns on with no issues, illuminates properly when turning dash lights on etc. but I just can't get a signal to it.

I've done some diagnosing to the coil pack according to some other posts and this is what I've come across. The trailing coil pack is supposed to have 3 nuts on it that bolts it to it's mounting bracket in the bay, this coil pack only has 1 nut there. What I've done was taken the 1 nut holding the trailing coil pack down and put it on 1 of the 2 lower bolts to fasten it down and that appears to have fixed the stock tachometer for the time being. After starting the car, I've connected one end of my circuit tester to the negative terminal on the battery and when touching the black bullet connector on the leading coil pack, my tester lights up. When I touch the Yellow/Blue wire on the trailing coil pack, no light appears on my tester and the stock tachometer in the car stops working completely. I've put the aftermarket tachometer input wire to both bullet connectors on both coil packs and gets no signal back. If I ground the Yellow/Blue wire on the trailing coil pack, the stock tachometer stops working. Underneath the trailing coil pack, there is a white 4 wire plug that plugs into it. The previous owner of the car cut the Yellow/Blue wire, poorly reconnected it, and just taped it. I re-soldered a wire to it to give it a proper connection and taped it back up but same issue persists. Under the dash is what looks like a module for the cluster and a multi wire plug is connected to it with the Yellow/Blue wire from the trailing coil pack. When I circuit test that wire, the stock tachometer stops working and if I connect the input wire of the aftermarket tachometer to that same wire, nothing happens.

I've checked the trailing coil pack to see if it gives off a spark and it does when the stock tachometer is working, if the stock tachometer isn't working, then the coil pack isn't giving a spark. I haven't tried checking if there is a spark if I ground the Yellow/Blue wire but I'm assuming it'd give the same result when the stock tachometer stops working. I don't know rotaries very well but when the trailing coil pack didn't give off spark, the engine still ran fine with little to no notice of any issues. I've connected an aftermarket tachometer to that same wire before on another RX-7 FC and it worked just fine, but this car is just blatantly giving me issues. I'm assuming those 3 nuts on the coil pack is what properly grounds it and because it's not 100% fastened correctly, the stock tachometer either doesn't work or just acts sporadic.

What do you guys think the issue could be?

Oil Pressure Gauge
This is another eBay brand gauge that the owner decided to add to the car. 3 wires off of the gauge and 3 wires off of the sensor. Red on the gauge goes to the same ignition wire from the radio, Black is ground, and Green is input. Gauge turns on with no issues. A sandwich plate was installed where the oil filter goes and the sensor is supposed to be tapped into that. The sensor has the same 3 wires, I'ved tapped the Red wire to the alternator, Black to the negative terminal on the battery, and Green back to the gauge. The gauge signifies 3 --- when it isn't receiving a signal from the sensor to which it currently isn't. For some reason, when I ground the input wire, the gauge gets a signal and briefly will give a number then just read 0. If I disconnect the red wire for the sensor from the alternator, I still receive the 3 ---, but when I ground the input wire again, it reads a number then quickly goes down to 0.

I also setup a water temp gauge for this car which is actually working properly. It reads about 84-86 degrees Celsius at idle. The sensor is connected to a little sandwich plate with 2 holes in it and is between 2 hoses by the Radiator. 1 hole is for a ground wire and the other hole is for the sensor (and Green wire). I currently won't be around this car for the next couple days but I'm curious if the oil pressure sensor could work the same way. Yes, it's a 3 wire sensor but since I didn't need the power wire connected to it to get a reading, I'm wondering if I'd be able to just connect the ground wire to the oil sandwich plate and only run the input wire from the sensor, just like the water temp sensor.

Hot_Dog

>>> I've checked the trailing coil pack to see if it gives off a spark and it does when the stock tachometer is working, if the stock tachometer isn't working, then the coil pack isn't giving a spark.

Appears to me that your trailing coil pack is the problem. It's working intermittently which is causing your tach to either work or not work.

>>> Oil Pressure Gauge -- This is another eBay brand gauge that the owner decided to add to the car. 3 wires off of the gauge and 3 wires off of the sensor.

You need to get hookup instructions from the gauge manufacturer. Most common oil pressure sending units only have a 1-wire sending unit like the stock unit. Mostly all gauges have a built-in voltage regulator and operate on 5 volts. Some GReddy oil pressure sending units have multiple wire send units, but they are multi-function analog and/or digital gauges. One of their gauges has a data link box between the sending unit and gauge, and the other has a harness where the sending unit's 3-wires plug int the back of the gauge. Best if you can contact the gauge manufacturer for hookup info.