Tachometer Swap : 280z into a 240z series 1 housing

[Zed Head]

On 5/13/2020 at 8:26 PM, Zed Head said:

If too much current flows through the tachometer the coil might not discharge.  Make then break.  No break, no spark.  Maybe why it's 2200 ohms.

I wasn't joking here.  2200 ohms is a lot of resistance.  If the tachometer circuit already has resistance there's no reason to add it in-line.  Unless it's for flyback(?) blocking, from the coil discharge.  I'm going to guess that without the resistance in the circuit either the tach won't work or the ignition system. 

Could you measure the resistance through the connection at the tach to ground?  That might offer a clue.  It's probably in the schematic but in the meantime.

I'm not an electrical engineer, I'm just trying to figure things out.

[zclocks]

I currently don't have a test bed to check the tach. as the injection system is off my car. First I would need to see a schematic to see what is going on and second you have a different set up than my stock 280.

I've converted several of the 240-280 swaps and each was different on how the owner got the tach to work.I do recall that some had to add a 1 to 5 k resistor in line with the coil to get the tach to work.   Each after market system  has slightly different requirements from 240 to 280. I can appreciate you wanting to minimize the pain of installation, but there needs to be more homework done before blindly trying  to get this to work. I have a tach somewhere and when I find it I can make a schematic.   The more I see that video of the "5 ohm" resister removal/jump the more questions I have.  

[Zed Head]

If you have a  spare distributor and an ignition module (unless it's a points distributor), and a coil and a terminal wire and a battery, you could make your own tester.  Spin the distributor, make sparks, watch the tach.  Wish I had parts, it would be fun.

[Captain Obvious]

I looked here for a spare tach (aka not installed in my car) and I couldn't find one. I know I had one some time ago, but I also remember that I sold one to a deserving desperate forum member at some point in the past. Maybe that was my only spare?

If I had one, I'm sure I could come up with a way to test it on the bench.  (Easy for me to say, right?    )

Anyone want to send me a 280 tach for investigative purposes?   Haha!!

[zclocks]

I can't find the ones I have stashed away , but did call in a favor. I should have one sometime next week. 

 I do have  a jumper harness  extension for testing the tachometers in my car. Now if I can just find the harness!!

[Captain Obvious]

Haha! One thing at a time, right?

So do you have a signal generator? I bet you could just use a 12V square wave to test the tach on the bench. It might require the flyback spike from the coil primary as a trigger, or not. It might work fine with a clean 12V square.

[zKars]

For the sake of reference, here are pictures of the resistor's where-abouts and appearance in the harness. And a picture of my Fluke VOM showing the resistance value.

 

 

located just inside the firewall grommet.

ok, 2226 ohms.  Nominal 2200 ohm. If you were to go buy one, the color code is RED RED RED (2, 2, X100) (then one more band for tolerance).

Blue taped to the harness

Labelled as a resistor

Edited May 16, 2020 by zKars

[zclocks]

Thanks. I have seen that in the past and wondered what it was. This will narrow down the search.

[zclocks]

Hey zKars, 

I found my tach  resistor on my 75 280z . It was very close to where yours is, but my resistor holder is completely different. It would appear to be the old style FUSE holder. I guess in Jan of 75 that's all they had!

My  resistance (2.237K ohm) is close to yours and it looks like a 2 or maybe 5 watt resistor. Hard to tell anymore. The connectors are good , but I like all connections crimped and soldered. Just another something to do.

Edited May 18, 2020 by zclocks

[zclocks]

Captain,

Yes, I do have a signal gen  and I found my Tach extension harness.  Now if I could only find the 2 tachs I have.

All this searching I found parts I didn't know I even had.

Edited May 18, 2020 by zclocks

[Blitzed]

Hi All,

Well the dash re-instillation did not go well. Dash will be in and out of the car for a third time. Sorry for the lengthy response but I have compounding issues now. 

The 280Z tach conversion into the 240Z housing did not work as listed above. Tach needle is not moving, thought a missed a step so removed the dash/tach and covered all steps again, second re-install, tach needle is not moving.

Worst case scenario, my concern from the start no way to test the function of a used tach before turning the key and praying it will work. Purchased the used tach from a reputable source in AZ but they do not test used parts before shipping. So I'm searching for another 280Z tach. 

Bigger problems now. All the cars electronics functioned well before the dash removal, now electrical issues. 

Turn signals - Do not operate. No green arrow lights and a single weak click (right not left) from the TS relay (can under the drivers side) then stops. Can I assume the lights not working could be related to the tach not working?  Will the tach not working effect the TS switch?

Hazard lights function  - green arrows on tach light and relay clicks fine from the passenger side. So I know the lights on the tach function. 

Brake lights - Do not work now.

Headlights- Worked before the dash removal and worked upon the first re-install, turned them off, they will not come back on.  

Running lights -work fine. 

All dash gauges / gauge lights (including the tach), clock, emergency brake light and high beam light , dimmer, and interior lights work fine. Car starts and runs fine. 

Will a short in the system begin to take down relays? 

Have a FSM with a 240Z wiring diagram (no electrical experience), it's great I can locate colors of wires. What I need to understand is where the connectors/connections located?

Any advise would be appreciated. 

[Blitzed]

Hi All,

Hope all is well. After sending the "tach not working" post last night, searched the archives on the site, may have touch on the electrical issue. Another lesson in too many changes at once! 

The original 73 240z hazard light switch broke, the center wire on the knob post broke off (plugs into the small can in dash). Purchased a used hazard light switch from the same reputable source in AZ as the tach (and untested).

So the turn signal and brake light issues can all be related to the replacement hazard switch, yes?

While the dash and wiring are in the car intact, would like to test if the issues are related to the replacement hazard switch. 

The small can in the dash has three inputs holes for the two wire plugs, center seems to be static. does it matter which input hole for the second wire?

Anyone experienced the issue with the post wire breaking and can it be repaired? broke at the back end of the post, can't solder a slice, not enough of the original wire exposed at the end of the post. Wire appears to travel into the steel post and is crimped at the knob end. Would like to repair the original and test. 

Tach is still not functioning, all conversion steps have been confirmed and reconfirmed . Without a clear method to test the tach internal function, assume the 280z tach internals are the issue. Will re-install the 240Z internal and live with the tach as is. 

Thanks for all the info.