280z tach capacitor identification

[heyitsrama]

Im trying to chase down why this 280z tach does not work unless the RPMs are +2000. I found that there is this capacitor that has peeled away it’s shielding. Does anyone know how to identify them? It does not appear to have any significant marking on it.

It’s the little teal one.

[Zed Head]

This one?  I wouldn't call that teal.

[heyitsrama]

@Zed Head my girlfriend and I get in arguments about green/yellow/teal the time, maybe she’s right 😂😂😂. Yeah that’s the one, I poked it too hard and took the plastic off, it looks “ok” on the other side. 
 

 

[Zed Head]

Not sure but I think that I've read about tachometer schematics somewhere in the past.  I found a couple of old threads but not sure they go in to that level of detail.

If you have the right meter and the capacitor still works you could measure it.

Or, rearranged words, if you have the right meter and you can measure it, it still works.

Edited November 28, 2020 by Zed Head

[heyitsrama]

I pulled out the capacitor, when comparing the resistance of the capacitor against a known good (different ratings) the resistance slowly increases, while the small green/teal one is stuck at 420ohms. I think this might be the issue. 

[heyitsrama]

I ended up replacing this with a random capacitor I had labeled 104M, the issue is still present there is no idle visible on the tach, it only moves after 1500 rpm. 😞

[Dave WM]

IF a capacitor was bad I would be more suspect of the electrolytic I think I see (cylinder shaped plastic cover wrap) should have polarity markings voltage and uF (capacitance) number on it. they are chemical in nature and can dry out/leak. I have never oped a tach so not sure if that is what I see in your pics. The others are unlikely to be bad. It would be best to test out of circuit, a crude test can be done with a VOM, some modern DMM have a cap test feature to check for loss of capacity.

Best way is with a proper cap tester for electrical leakage, ESR, capacity or just replace and try, but you want to use the correct uF amount and equal or greater voltage rating.

Edited November 29, 2020 by Dave WM

[AK260]

Ok, this is a topic close to my heart as my original rev counter would stick when hot and the second one has a tendency to bounce a little. So as far as I can see, you and I have two options:

1. Replace the electronics with this (which incidentally, would also give you a signal to plug into a data logging unit if that’s your thing or you ever plan to add an AFR meter and lose your life to tuning antics)

https://www.spiyda.com/smiths-rvi-rvc-conversion-board.html

2. There’s a great thread here ...

https://www.classiczcars.com/forums/topic/40718-tach-bounces-while-shifting/

And here’s the circuit diagram from it - given the symptoms you describe, I would wager that you are looking at the 500nf (0.5uf) cap on the pickup coil side:



Finally, while you are about it, replace the electrolytic one also. They tend to go with age. Here is a great write up ...

https://www.zdriver.com/forums/240z-280zxt-s30-s130-tech-tips-275/280z-tachometer-repair-40153/

[AK260]

Ps. Just realised, I have a tech open already so I will have a look for you tomorrow if no one else has responded by then.

[crayZlair]

I think that the "teal" component is a varistor. The resistor labeled "temp comp" in the schematic. It is physically mounted right next to a resistor. As the current thru the resistor changes, the heat dissipated from it (the resistor) changes (it will be affected by ambient air). The resistance of the varistor will change with the temperature of the resistor and allow for an accurate tach reading.

You can try holding a heat source (solder iron, heat gun) near the component while measuring with an ohm meter. See if the resistance changes.

Edited November 29, 2020 by crayZlair
added more for clarity

[Captain Obvious]

I can't tell what the component is from that distance, so I'm not a lot of help there.

However, I CAN tell you that the schematic posted above is for the earlier 240 tach with the inductive pickup, and it is not applicable to the 280 version. So not a lot of help there.

Can you take a closer up shot of the cracked component? Will your camera focus that close in? And can you take a pic of the circuit board bottom that it solders into? I might be able to figure out what it is by looking at the circuitry. Maybe. Might be able. Maybe.

[heyitsrama]

2 hours ago, crayZlair said:

I think that the "teal" component is a varistor. The resistor labeled "temp comp" in the schematic. It is physically mounted right next to a resistor. As the current thru the resistor changes, the heat dissipated from it (the resistor) changes (it will be affected by ambient air). The resistance of the varistor will change with the temperature of the resistor and allow for an accurate tach reading.

You can try holding a heat source (solder iron, heat gun) near the component while measuring with an ohm meter. See if the resistance changes.

I think you are right, I did not put too much heat to the part but i saw that the ohms were going down as the heat was applied, currently the thermometer says 70*f inside the room, the meter was ~358ohms. A good section of the shielding has been removed from the component, hopefully it does not effect the reading too much.

 

1 hour ago, Captain Obvious said:

I can't tell what the component is from that distance, so I'm not a lot of help there.

However, I CAN tell you that the schematic posted above is for the earlier 240 tach with the inductive pickup, and it is not applicable to the 280 version. So not a lot of help there.

Can you take a closer up shot of the cracked component? Will your camera focus that close in? And can you take a pic of the circuit board bottom that it solders into? I might be able to figure out what it is by looking at the circuitry. Maybe. Might be able. Maybe.

 

 

Let me know if you more another shot, It looks like the white wire that controls the arm on the tach is directly connected to it.

@Captain Obvious I uploaded here https://imgur.com/a/09qUOaL

I think the resolution might be higher.

I put the old component back on the board.