On 8/14/2022 at 1:00 AM, Patcon said:

Does the 240 current sensing tach only have one loop out the back of the tach?

Yes.

One thing I didn't experiment with was polarity. I can run more tests tomorrow.

Also, were you splitting current between two tachs or just the 510?

  On 8/14/2022 at 1:04 AM, Patcon said:

Also, were you splitting current between two tachs or just the 510?

They were in series. That would be the same current since neither should be a load. I might be able verify that with an oscilloscope.

I ran a couple of more tests.

1. Reversed polarity - Tachometer did not register at all.
2. Original polarity - Tachometer in car registers just as it should. 
3. Tachometer on multimeter put in 4 cylinder mode - 510 tachometer seems to be reading about 1/10 of the multimeter. The needle did appear to indicate proportionally.

Also a friend who used to own a 260Z had his tachometer repaired at Bob's Speedometer. https://www.bobsspeedometer.com/

I think you have choices.

Is there something in the tach that would reduce the current by 90%? Too much resistance on the loop or a bad component on the board?

  On 8/14/2022 at 2:26 AM, Patcon said:

Is there something in the tach that would reduce the current by 90%? Too much resistance on the loop or a bad component on the board?

I doubt it's resistance in the ignition circuit. It would have to be something internal to the tachometer. I don't know enough about inductive loop tachometers to say what, though.

This article on Dime Quarterly is interesting: http://dimequarterly.blogspot.com/2011/08/tech-how-to-510-factory-tachometer.html

It says the sensing should be done on the negative side of the coil, and it does mention polarity for the sensing.

I'm not sure if I will have time to run another test today. We'll see.

 

My thoughts? If the two tachs were in series and the one in the dash worked fine, then I think there is an internal problem with the 510 tach. I mean, there's a slight possibility it has something to do with the ballast resistor, but I suspect the gauge itself.

I also think there is nothing to be gleaned from the seeming "proportionality" of the tach readings. I think that's a red herring.

Steve, If you want to send me that tach, I'll open it up and do some reverse engineering on the guts? If nothing else, it'll be a great story about a tach that traveled all over the country. "510 Tach Tour!"    

  On 8/14/2022 at 3:16 PM, Captain Obvious said:

My thoughts? If the two tachs were in series and the one in the dash worked fine, then I think there is an internal problem with the 510 tach. I mean, there's a slight possibility it has something to do with the ballast resistor, but I suspect the gauge itself.

I also think there is nothing to be gleaned from the seeming "proportionality" of the tach readings. I think that's a red herring.

Steve, If you want to send me that tach, I'll open it up and do some reverse engineering on the guts? If nothing else, it'll be a great story about a tach that traveled all over the country. "510 Tach Tour!"    

I'll try wiring it like it suggests on Dime Quarterly, and if it doesn't behave better, I can send it to you.

  On 8/14/2022 at 6:17 PM, SteveJ said:

I'll try wiring it like it suggests on Dime Quarterly, and if it doesn't behave better, I can send it to you.

Not surprisingly, there was no change. But I did take the time to mark the polarity on the tachometer wires to make it easier to identify which wire goes where in the future. I couldn't find my red electrical tape (that had been sitting in the same place in the garage for about 10 years until I reorganized last year), but I did find some red heat shrink that fit over the spade connector.

At least I had fun testing. I'm interested in Bruce's analysis. I could do a visual on the discrete components, but I think Bruce has better skills in that realm.

Edited August 14, 20222 yr by SteveJ

Well this may all be for naught... I got a head start based on the pics from that Dime Quarterly thread I started looking for info on the IC chip - AN603.

Based on the logo, it looks like a Matsushita (now part of Panasonic) part, but I couldn't dig up any info about it at all. So if I can't find anything about what's going on inside that chip, I might not be able to do much with it. I mean, I can certainly do a macro level reverse engineering and infer what's going on inside that chip, but that may be about it.

And if it looks like there's something wrong inside the chip, I certainly won't be able to get a replacement except from another old donor part.

I wonder if they used that same chip in the current sense Z tachs... Those are at least easier to find at this point.

Anyway, regardless, I'll do what I can.