Tachometer maxes out when Ignition switched on. Loss of power too! (Help!)

[Jughead]

 

HELP!! I seemed to have created an issue in removing my radio, and I need help diagnosing/fixing. 

Car:  is a ’72 240Z with standard/stock points ignition.  The car ran great prior to radio removal. 

Current Symptoms:  1) Tachometer pins to max when ignition switched on. (Goes to zero with Ignition off)   2) Poor performance (low power) while driving. Everything else seems to work fine.  The radio and heater control panel are currently out of the car.

Events leading to issue:  The car was running great.  The radio did not power on, although the antenna switch on the the radio worked to raise/lower the antenna.

I carefully removed my radio to diagnose it, which involved removing the center console and heater control panel, and loosening the fuse box to move out of way.  With the heater control panel removed,  the wiring revealed appears to be stock and unmolested (see picture).   In trying to diagnose the symptoms above, I have replaced all the fuses with new ones of the correct amperage.  (My cigarette lighter now works!) 

Questions

1.       What hypotheses would explain the symptoms?

2.       How would I test the above hypotheses?

3.       Is it possible that simply reinstalling the radio will fix the symptoms?!

4.       Can anyone identify the plug with black and red wires identified with a question mark ?  (I don’t think I unplugged it, but it drives me crazy I cannot identify it !!)

 

Thanks in advance for your help – I will report back for posterity!

[SteveJ]

Please make it easier to provide help. Wiring diagrams differ by year. 

As for why your tachometer is maxing out, I would posit that there is too much current flowing through the tachometer. Maybe the black/white wire from the tachometer has been damaged and is slightly coming into contact with ground. This would cause more current to flow through the tachometer sensor and less to flow out to the ignition. Maybe you reconnected something wrong, though there shouldn't be anything connected to the black/white wire between the tachometer and coil positive.

While the car is running, measure voltage to ground at the green/white wire on the ballast resistor and also measure voltage to ground at the black/white wire on coil positive. There shouldn't be much of a difference in voltage between the two. Also measure voltage to ground at the black/white wire on the ballast resistor. Pleas report your results.

After you supply the year of your car, I can look at the FSM to try to identify the mystery connector. I'm guessing you have a 72 since I see short pigtails on the fusebox, but I don't see sockets for bulbs around where the control panel goes.

 

[SteveJ]

After looking through the wiring diagram, I am guessing that mystery connector is for the fog light switch. You don't have a fog light switch? Well, North American cars are wired for fog lights, but Nissan didn't install a switch. It's just as well. The engineers designed the circuit to come off the parking lights. That circuit already has enough issues without fog lights added.

[Jughead]

@SteveJ  The car is a 1972 240Z.  (I've updated my signature - thanks for that tip!)

Test results at idle:

• Green/White @ ballast resistor:  varied but roughly 10.77
• Black White @ Coil:                      varied but roughly 10.22
• Black/White @ ballast resistor:   varied but roughly 13.97

 

Re: the connector: Are you thinking it was never used for this car? (I don't recall unplugging it.  and I don't have fog lights.)  I can tell you that the green is hot (juts under 12V ) with the ignition switched on and the headlight switched on, so I like the fog light idea.

 

Edited July 18, 2022 by Jughead
typo

[SteveJ]

The voltages look right for a 240Z with points. 

Unfortunately that probably means your tachometer has seen better days. Read through this thread: 

 

Of course, @beermanpete figured out how to fix his. I wish he was still hanging out around here.

As for the fog lights, that green/white wire should only be used to control a relay along with a switch. Don't use that wire to power the fog lights. You would need a fused 12 volt source connected to the 87 terminal of a relay with the 30 terminal connected to the red wire. (Relay explanation here: https://fiddlingwithzcars.wordpress.com/2012/12/22/relays-simplified/)

[Captain Obvious]

9 hours ago, SteveJ said:

I would posit that there is too much current flowing through the tachometer.

Actually, on the points cars, the ignition current does not actually flow "through" the tachometer. It kinda flows "in close proximity to it". Inductive pick-up, right?

And if that's the case, anything steady state won't get picked up at all. So a DC current, even one that is "too high" would not be noticed by the tach at all?

[SteveJ]

9 minutes ago, Captain Obvious said:

Actually, on the points cars, the ignition current does not actually flow "through" the tachometer. It kinda flows "in close proximity to it". Inductive pick-up, right?

And if that's the case, anything steady state won't get picked up at all. So a DC current, even one that is "too high" would not be noticed by the tach at all?

You're right. I was speculating based upon the dropped performance. That's why I revised to dying tachometer once the voltage readings came back.

[Captain Obvious]

And I should have added before, I'm having a hard time coming up with an answer to "why is the tach maxxing out when the ignition is turned on". I haven't been inside the points style tach at all, so other than having a rough conceptual idea how they work, I'm at a loss. It may still have something to do with the ignition pickup even though the actual ignition current doesn't flow into the tach body.

One thing I could suggest... Disconnect the B/W wire from the coil and see if the tach still pegs? Then go from there?

[Jughead]

@Captain Obvious:  I disconnected the black & white lead from the coil (+ side ) , and when I switch on the ignition, it DOES still peg to max.  Thoughts?

[Captain Obvious]

Gotcha. So no... I don't have any theories at this time other than your tach has failed internally. However, the coincidental nature of that failure occurring at the exact same time you were messing around with other stuff in the dash makes me think it's something else.

Maybe SteveJ has some additional theories? 

[SteveJ]

Well, everything I could find about the tach pegging says that the tach is going bad, and for many it's the failure of a discrete component, like in the thread I linked. The good news is this can be repaired.

This place fixed a friend's 260Z tachometer about 11 years ago. https://www.bobsspeedometer.com/

[Jughead]

I agree that it's super coincidental.   

Questions: 

1. How would a bad tach affect the engine performance as in my case (hypothetically)?  
2. Is there a way to bench-test the tach while it's still in the car?   
3. I have another Tach that looks to be in good shape.   How would I bench test that one?
4. Removing a Tach seems like it would be a major P.I.T.A.   Is it? 

BTW:  This is very helpful - thanks.