USDM AE86 TACH

[jakes_hachiroku]

so ik this is probably on the site somewhere but the search is kinda weird lol but this will prob be a quick one for you guys. so when i bought my usdm 86 3 years ago it just never had a working rpm gauge. finally got sick of looking at my aftermarket rpm gauge and decided to find what i think is the rpm wire (black with red rings?) and connect it to my ignition coil. still doesnt do a thing motor is a 16v 4age with weber dcoe carbs and msd ignition. would i be able to take said tacho wire and splice it into my msd ignition or am i missing something here? thanks!

[UNLIMITED 86]

With the MSD ignition, you will probably need a tach adapter to get either tach to work.

[El Gato Felix]

Yes you will need Msd's tach adapter Part Number is 8920 if you want to use the STOCK TACH. Otherwise your msd box should have a tach wire, BUT only works on AFTERMARKET tachs. This is what I bought when I installed MSD programmable ignition on my carbed 4A-GE. It's a little pricey, but if you want that stock tach to work you will need this plug in part.

[jondee86]

Or perhaps one of these ?



https://www.ebay.com/itm/TACH-TACHOMETE ... 2171581596

Cheers... jondee86

[jakes_hachiroku]

Thanks guys! Always good help. I just hate my aftermarket rpm gauge and want to get my oem one working. So I'm assuming this msd adapter will work like the oem igniter would in terms of the rpm signal? Should be super simple to set up to my msd box

[jondee86]

MSD puts out a 12V square wave tacho signal and you just need to convert that
to a "voltage spike" to drive an old school tacho. Most DIY converters use an old
relay coil and a transistor with some other bits and pieces set up like this...



That is basically what will be inside the Ebay item. I don't know what is inside
the MSD part or how it works, but I guess they go a different route to get the
same result.

Cheers... jondee86

[UNLIMITED 86]

It's likely similar in operation but my guess is that the MSD parts are potted into place to prevent failure from vibration (hence the box shape with flying leads).

[jakes_hachiroku]

Ok guys! so ive finally had the time to install the msd tach adapter annnnnnnnnnnd………… NOTHING lol so now im lost lol is there another wire for the usdm tach that looks similar to the tacho whire?

[jondee86]

On the USDM GTS the tacho gets its signal from the black wire on the igniter.
The SR5 will get a signal direct from the coil negative. The black wire finds its
way to the big square(ish) plug on the firewall behind the engine, and from
there to the OEM tacho.

According to this diagram...



... all you need to do is connect the black wire (that leads to the tacho) to the
violet wire out of the tach adapter. Note that the tach adapter needs its own
key-switched 12V supply and chassis ground.

Cheers... jondee86

[jakes_hachiroku]

Yes I wired everything just like the picture with the black wire to the violet on the adapter. but did I grab the wrong wire? I used the wire that is black with what looks like brown dots on it? is this not right?

On the USDM GTS the tacho gets its signal from the black wire on the igniter.
The SR5 will get a signal direct from the coil negative. The black wire finds its
way to the big square(ish) plug on the firewall behind the engine, and from
there to the OEM tacho.

According to this diagram...



... all you need to do is connect the black wire (that leads to the tacho) to the
violet wire out of the tach adapter. Note that the tach adapter needs its own
key-switched 12V supply and chassis ground.

Cheers... jondee86

[aukword1]

Sounds like the right wire, you might have to trace it back and see if there are any shorts on it. Even possibly that it could be the wrong wire there are a few that look the same.

[jakes_hachiroku]

IT does look like the idle up and rpm gauge wire look similar so I will look again it just sucks cuz the previous owner cut a lot of those wires haha

Sounds like the right wire, you might have to trace it back and see if there are any shorts on it. Even possibly that it could be the wrong wire there are a few that look the same.

[UNLIMITED 86]

Hopefully you have a multimeter.

Set it to check for continuity and test between the "black" wire you think is for the Tach, and the black wire going to the igniter.

[jakes_hachiroku]

ok so i have double checked that it is definitely the correct wire that i used for my tach adapter so now im not sure why it wouldnt work unless the tach is actually bad lol the one thing i havent tried doing is simply putting the tach directly to the msd box without the tach adapter so until i can figure this out i guess my autometer tach is going back in thanks for the help and pics guys you always help me out