Tach Signal wire ????

[bobby v]

So my tach (after market) does funny stuff when it gets to around 2500rpms, bounces all over. The gauge maker says I need a filter for it. Ok sure, Ill try that, but in looking at videos that say the white wire coming off the grey connector on the coil which is an empty wire, is the wire to tap into for the tach. The maker says to check it for voltage and if it has power to not use it, I did and it does have 12 volts. The PO has it tied into the light colored wire coming out of the other black connector on the coil, so since it does work I will install the filter when it gets here and hope for the best.
But in the meantime does any one know what the wire in the photos is for that has the blue masking tape on it with question marks, it is just there with nothing connected to it
Bobby V

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[Jglew82]

That wire with the question marks is your tach signal... Try tapping into that - it's the lead for the factory tach.

 

[Ken K]

GRY is tach wire on GM trucks and passenger vehicles, same circuit number and same wire. Either at a distributor or PCM generated.

Starting in 1984 passenger cars, GM made an announcement at the beginning of the electrical system portion of the service manual. They pointed out circuit numbers, wire colors and the style of laying out the circuit using "Marshaling" with B+ at the top and ground at the bottom.

 

[Schurkey]

That "Scotch Lock" blue-plastic splice connector has GOT to GO. They attract corrosion. Some of them have silicone sealer inside, most don't. All of them are "bad news". You'll be lucky to save the original wire.

What does the Scotch Lock connect? Appears that it splices a green wire to a yellow wire. What is the green wire for? The tach that doesn't work right? (HOW is the tach connected now? How many wires does it have, and where do they go?)

Y'know, a faulty tach can cause all sorts of engine driveability problems...

 

[Jglew82]

That "Scotch Lock" blue-plastic splice connector has GOT to GO. They attract corrosion. Some of them have silicone sealer inside, most don't. All of them are "bad news". You'll be lucky to save the original wire.

Seriously. Ask anyone who has ever touched trailer wiring or shoddy service body lighting. These things are meant for the trashcan.

 

[df2x4]

X3 on the ScotchLock bashing, they suck. I have exactly one of them in use between all of my vehicles, and it's on a relayed remote turn-on wire for amplifiers in my red truck to power a Bluetooth receiver. The only reason I used it was because I know I'm ripping all that wiring out at some point and I wanted to test the receiver for proper operation in a hidden location.

 

[bobby v]

That "Scotch Lock" blue-plastic splice connector has GOT to GO. They attract corrosion. Some of them have silicone sealer inside, most don't. All of them are "bad news". You'll be lucky to save the original wire.

What does the Scotch Lock connect? Appears that it splices a green wire to a yellow wire. What is the green wire for? The tach that doesn't work right? (HOW is the tach connected now? How many wires does it have, and where do they go?)

Y'know, a faulty tach can cause all sorts of engine driveability problems...

I didn't do the original install but Ill tell what I know.
The tach has red, yellow,black, orange, and the green signal wire you see going into the engine compartment, it is the one with the blue connector to another wire coming off the coil. The red & Yellow go to power fuses in the cab fuse panel as they should, the orange is not connected as it is for dimming the gauge, black obviously for grnd. The tack works as is but bounces around at 2500rpms. Maker of gauge says the green wire should NOT be connected to any wire that has power, that the one labeled with ?? marks is the one everyone says is the tach signal wire and to hook that to the green wire coming off the gauge. But the wire with the masking tape has power to it, 12v so wont that blow up the tach with 2 power sources going in it, its supposed to be a negative wire hook up, God this is so ****** up. Im not so great with wiring so Im trying to learn but for Gods sake Im getting so many suggestions I don't know what to do. One guy on youTube guarantees that the white wire with masking tape is the right one and to hook it up, if I do and blow up the tach then what, but another gauge??
I love working and learning about cars but this kind of stuff get depressing, I hope you can understand what Im trying to convey
Im going to bed
Bobby V
PS its especially hard when you are trying to figure what some other person did, did he do it right ?? or is it all screwed up, like that connector they used, bad choice so now I have to try and correct it, ****!!!!!!!!!!!!!!!!!!!!!!

 

[Jglew82]

Bobby, that wire with the question marks is the tach signal wire, no question. You are measuring 12v on it because it isn't grounded, but it is indeed the negative side. This is understandably confusing because the instructions are telling you not to connect it to the POSITIVE side. Think of it this way, say you have a simple lamp circuit - 12v in from the battery on the positive, negative wire not connected to anything. If you connect a meter to the negative battery terminal and measure the negative side of the lamp, guess what you'll see.. 12v. That power has not been provided a path to ground, so the "positive juice" is trapped at the end (in laymans terms). Your tach will let those juices flow like they should. Connect it up and report back. Here's a diagram to ease your worries. Circuit 121 is what you are looking for, which I have highlighted. Notice it goes to a tach lead, and also through the firewall to the factory cluster, which yours no longer has, but is the correct wire you need.

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[PlayingWithTBI]

On my 88 the white wire (your ??? one) hooks up to the tach in my Dakota Digital dash and works fine.

 

[bobby v]

D

Bobby, that wire with the question marks is the tach signal wire, no question. You are measuring 12v on it because it isn't grounded, but it is indeed the negative side. This is understandably confusing because the instructions are telling you not to connect it to the POSITIVE side. Think of it this way, say you have a simple lamp circuit - 12v in from the battery on the positive, negative wire not connected to anything. If you connect a meter to the negative battery terminal and measure the negative side of the lamp, guess what you'll see.. 12v. That power has not been provided a path to ground, so the "positive juice" is trapped at the end (in laymans terms). Your tach will let those juices flow like they should. Connect it up and report back. Here's a diagram to ease your worries. Circuit 121 is what you are looking for, which I have highlighted. Notice it goes to a tach lead, and also through the firewall to the factory cluster, which yours no longer has, but is the correct wire you need.

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Okie Dokie, seems everyone has come to the same conclusion, even the maker of the gauge so Im going to go for it. If it blows up the tach then its only money right.
Ill report back