Tachometer kills engine

[pepedod]

Acquired 1974 260z, UK spec, part of reasonable price was the fact that the electrics were not perfect. Most items now working fine, however, tach that was not working when purchased but plugged in, now kills engine when plugged in, and will not start if plugged in. Everything else works fine when removed, any thoughts, ideas would be appreciated?

[FastWoman]

The tach is probably shorted and shorts the (-) post of the ignition to ground, ergo no spark.

[pepedod]

I am suspecting knackered tachometer, but struggling to understand why engine would run with tachometer plugged in prior to fixing other electrical items, i.e. Fuel gauge, hazard lights and wiper functions. Although the tachometer was not working before I unplugged it, the engine would run with it plugged in. Any suggestions on checks I could perform before investing in tachometer replacement.

Thanks to date.

[FastWoman]

Oh, I didn't get that part -- engine previously running with tach plugged in. Hmmmmm....

Are you sure your tach is wired correctly? Is there any part of the tach's wiring that you worked on? I'm sorry I can't be of much help here, as the 260's tach circuit is wired differently than my 280's, which is electronic.

[EScanlon]

Both are electronic.

Check the wiring at the coil, if you inverted the wiring from the ignition to the tach (start position) with the Run position (Ig Switch - Tach - coil) then that's a strong possibility. If your tach is a 4-wire tach, then this is very likely the situation. If it is only the 3 wire tach, then this might not be the problem at all. The 4 wire tach ran the power to the coil from the ignition switch THROUGH the tach to the coil.

Your car being a 74 260, is the first year (worldwide) that the 260 was offered, and the wiring may be very similar if not identical to the late 73 240. There were also model transition vehicles in both models (i.e. 240's with 3 wire tachs, as well as 260's with 4 wire tachs).

[Jetaway]

Acquired 1974 260z, UK spec, part of reasonable price was the fact that the electrics were not perfect. Most items now working fine, however, tach that was not working when purchased but plugged in, now kills engine when plugged in, and will not start if plugged in. Everything else works fine when removed, any thoughts, ideas would be appreciated?

Shot in the dark here. In 240Zs the tachometer forms an electrical loop in the ignition system and if not plugged in, regardless of whether the tach functions or not, the engine will not run. If the 260Zs have the same set-up and assuming that the PO was somewhat responsible for bollixing up the electrics then ...

The PO, by chance or design, created an ignition loop that bypassed the tachometer altogether. Then you came along and by cleaning up the wiring inadvertently changed this ad hoc loop such that the previously electrically dead tach wires now produce a short to ground.

I'll admit the three devices you mentioned as working in a follow-up post don't seem like likely candidates for the ad hoc loop, though POs can be awfully creative at times. If I had to choose the most likely candidate, it would be the ammeter. Did you work on the ammeter but not include it in your "i.e.?"

Chris

[pepedod]

Thanks folks, these posts are excellent...

PO had made some fairly radical fixes, and I have found them all yet. Interior lamps, door switches and defogger etc still to unravel.....

It's a 4 wire tach, and one issue that I have not mentioned so far but has been triggered by the posts is the ignition was hot wired for starting, as I have cleaning and reinstating wires, I have also re-installed wires to the ignition and the ignition is now starting with key, so don't know if this bypass was introduced to get round the fact it didn't start previously with the tachometer plugged in, possibility that I will need to work through.

The ammeter not touched but not sure if it is working as it just sits there doing nothing really....... Not familiar with the workings of the ammeter being really honest.

Will do some working out based on suggestions so far and keep you posted.

Many thanks again

[pepedod]

Fantastic news, the tach and engine are alive thanks to all your guidance and thought provoking comments.

It turned out to be a mixture of some creative wiring at the ignition coil and the addition of a hot wire at the ignition by the PO's, and I do mean plural as I have records of it being at auction in 2007 with a reported non functioning tacho. Some patient disassembly, cleaning, identification and patient assembly has the ignition and tachometer all working perfect.

Thanks again, you kept me on the straight and narrow, even with a few thousand miles between us.

Have uploaded the evidence. http://www.youtube.com/watchv=VwwicqbioHM&feature=youtube_gdata_player

Hope it works

Regards

Philip

[pepedod]

http://m.youtube.com/#/watch?desktop_uri=http%3A%2F%2Fwww.youtube.com%2Fw atch%3Fv%3DVwwicqbioHM%26feature%3Dyoutube_gdata_p layer&feature=youtube_gdata_player&v=VwwicqbioHM&gl=GB

See if this post works, if not search "260z Tach is alive" in you tube......