Please Help A Newb Tach Question

#1

I can’t explain how bad we are at electrical. We have a 87 (1986 build) 325i convertible. I need to install an aftermarket tach and shift light. Can someone give me directions in language that can readily be understood by a 3 year old?

Thanks

#2

Both should have 3 wires, 12V, Gnd and Signal. For the 12V wire, find a wire under the dash that only has 12V when the car is on, abbreviated K-On for key-on. Also called “Switched 12V”. Find this with a multimeter. If you’ve not used a multimeter before read some how-to’s on the web because it’s a necessary male skill.

Alternately you could go to your fuse box with the multimeter and with the car at K-On, find which fuses are hot with 12V. Then go K-Off and see which fuses went cold. This will tell you which fuse is ONLY hot at K-On and gives you a place to wire your tach to in order to get K-On 12V (switched 12V).

12V that goes right to the battery is “unswitched” 12V, in that you can’t switch it off.

Maybe this is obvious or maybe not, but if you wire the tach to something that is hot with 12V at K-Off, then that something is going to be always on and drain your battery down.

The Gnd wire is pretty self explanatory. You can either use the multimeter to find a Gnd wire under the dash, or you can just fasten it to the car’s metal somewhere. That’s the car’s “metal”, not the car’s “paint”.

The signal wire probably goes to the -12V connection on the coil. Sometimes the tach gets wired to the +12V lead, but that’s unusual. The coil’s 2 threaded connectors are probably labeled - and +. If not the - connector is the smaller of the two.

The shift light has to be programmable in some way. You’ll have to see the instructions for how to set it for different rpms, but it wires up the same way.

#3

Any solid green in the car is switched power. Any solid red is always hot. Solid brown is ground. Purple and purple/w/green are hot in start and run or start only, but you shouldn’t need those, and will want to check those as I’m not sure. I am confident about the green, red and brown though. You can find power and ground distribution splices under the instrument cluster. you may need to uncover the harness to find the red and green ones. The brown ones are easy to see bolted to the sheetmetal. I believe tack signal is a small solid black wire. I’ll check the old iq3 thread to make sure.

#4

quoted from jlevie.

“C1 pin 7 (Black) is the tach signal from the DME or you can use the wire off pin 6 of the DME connector.”

Not sure if you want you’re original tach to work or not. I’m guessing you can tee into it.

#5

PO, turbo and I aren’t disagreeing with each other. He’s just providing an alternative solution.

I didn’t know that. You should create a thread for posterity titled “Wire color codes” or something like that. Good info, thx.

I would add a caveat that maybe red is fused unswitched. Otherwise we’ll confuse newbies looking at the 2 black wires that run from batt to the engine compartment power cluster.

#6

I wouldn’t call it an alternative solution. I didn’t reply to the original post because I have a logger dash which is a whole lot different than an external tach. I suppose I could make a spreadsheet of definitive wire colors but you’d have to give me a month.

Bigds01 you need a Bentley manual to work on e30’s. It helps to have access to alldata pro for wiring diagrams but I don’t think most have that.

#7

Reading schematics is another necessary male skill. But can often be offset by the complementary male skill of buying beer.

#8

The first step is to check the doc’s for the tach to see what it will accept as a tach signal. Years ago all tachs so it was simple. Now some can do that and others will only take a 0-5v signal.

If the tach can be connect to the coil, run a wire from the coil positive to the tach’s power lead and a wire from the coil negative to the data lead, Pick up a chassis ground anywhere.

If the tach can only accept a 0-5v signal, locate C104 near the DME connector. It is a three pin connector with blue/black wire (tach), a green wire (START input), and a white wire (econometer) on the body side. The START wire is hot in start & run and can be used to power the tach. Splice the tach’s power lead into the green wire, the data lead into the blue/black wire, and connect the ground lead to a chassis ground.

Note while this car has a C104 connector, later cars (from 9/87) route the tach, econometer, and START through C101. On those cars, splice the data lead into the wire that runs to DME pin 9 and the power lead to the wire that runs to DME pin 7.

#9

Is this car being built for SE30? Convertibles are no longer legal if it is.