Tach issue. Pic Request

[HawkX66]

Originally Posted by Lynn

Nice pics Dave.

I bought the same kit. Posted this on TC earlier today.

I recently purchased a new circuit board from Greg on ebay: $25 and free shipping. What a deal: https://www.ebay.com/itm/NEW-69-77-G...53.m2749.l2649

I swapped it out last night. The only thing I want to point out is that I had to gently squeeze all of the female bullet connectors to insure a tight fit on the new board. Other than that, install was very straight forward.

Started the car to make sure the tach worked (it did) and then finished putting things back together. For you slim dextrous guys, you might be able to pull the tach without pulling the radio, but not me. To pull the radio, I had to pull the add on gauges below the radio, meaining I had to put all that back together. Not a big deal, but just 9 weeks after a reverse total shoulder joint replacement, not a pleasant task.

Came home from work early today and took the car for a drive. Had a blast. Burnt some rubber (at least one perk of the insane covid-19 restrictions is that there is less traffic). But, my tach reads even HIGHER than it did before. I sent Greg a message asking what to do. I suspect he will advise that I send him the tach for repair.

I did notice that on the white wire going to the circuit board, there is an in line resistor. I assume that is the resistor that gets replaced by the "adjustable" resistor referred to in the bulliten.

Anyone do this? Of course, I can send it in, but come on. I do almost EVERYTHING myself on my cars. There has to be a way to get this done.

Originally Posted by Lynn

Is that small brass screw in the blue part an adjustment?

Anyone know?

Haven't heard back from Greg (the seller) yet.

It sounds like he didn't calibrate it correctly. Yes, the brass screw on the blue plastic part (variable potentiometer) is how it gets calibrated. You have to tell them what your red line is. Thankfully mine seems to be spot on. I've put several hundred miles on since I installed the new board.
I don't know if I'd send your tach in. If he doesn't get back to you in reasonable time, I'd buy another board from another vendor. It seems like you'd be paying the purchase price in shipping back and forth.
edit: I was just thinking... Are you sure it wasn't off before? Is it starting at zero? My guess is Greg will get back to you and that he just has something going on right now. He was pretty attentive with me. I still wouldn't send my tach. That's just me though. It sounds like you're the same...

[Lynn]

Yes, my tach was off before changing the board. It read high. Now it reads even higher.

It is an 8K tach.

I may just pull it, make a harness extension so a helper can hold the tack while someone else is driving, and adjust it on the fly.

[SuperNovaSS]

You should be able to adjust it while the car is parked. Hook up a known good tach along with this one and adjust until they match at different RPMs. I don’t understand why you would have to tell them what your redline is. A 7k versus a 8k tach certainly makes a difference but the redline does not make a difference.

Jason

[HawkX66]

Originally Posted by Lynn

Yes, my tach was off before changing the board. It read high. Now it reads even higher.

It is an 8K tach.

I may just pull it, make a harness extension so a helper can hold the tack while someone else is driving, and adjust it on the fly.

Originally Posted by SuperNovaSS

You should be able to adjust it while the car is parked. Hook up a known good tach along with this one and adjust until they match at different RPMs. I don’t understand why you would have to tell them what your redline is. A 7k versus a 8k tach certainly makes a difference but the redline does not make a difference.

Jason

You're right Jason. I misspoke. It was not the red line, it was the max. Mine is 7k.
Lynn, did your new circuit board have the max written on it? He wrote 7k on mine. Like Jason said, you shouldn't need to do anything other than in neutral/park. Can't hurt to try I suppose.

[Lynn]

Jason; I get that I could do it with the car still. But, my experience is that factory tachs that are accurate at one rpm (say at idle) are rarely accurate at cruise. I figured the best compromise was to pick a cruising speed (my speedo is accurate within 1 mph all the way from 45 to 70), calculate the correct rpm at that cruising speed, and dial the tach in there.

Sounds like a lot of trouble, but it will just be one drive. Besides, the only other "known good tach" I have is an old Sun tach that is hooked up on my Studebaker farm truck, so I would have to remove and then re-install that one. I have a cool old Moroso (Jones) cable drive tach, but the distributor on my Z doesn't have a cable drive.

Dave; I don't remember anything being written on it. Will look when I get it back off.

[SuperNovaSS]

Dave,

I figured that is what you meant. I just wanted to clarify so others were not confused. You can certainly handle this Lynn. It just sucks you have to go through the whole procedure again.

The lack of another tach definitely makes it tough. I agree, it can look accurate at idle but be way off higher up since the difference between 700 and 800 rpm is nominal but the difference between 7,000 and 8,000 rpm is significant.


Jason

[Lynn]

Thanks for your help. It would suck a lot more if I didn't enjoy piddling on cars.

[Lynn]

Got the tach back out. A lot easier the second time around.

It is going to be impossible to adjust on the fly unless I drill a small hole in the side of the back housing.

Sent Greg a question, asking about the inline resistor on my black wire. He said he hasn't seen one with a resistor. Hmmmm. Looking at the pics of Dave's disassembled tach, I notice his does not have a resistor.

I am wondering if this was a fix at the factory to calibrate, and someone over did it with the resistor.

Check out the pic on mine. Definitely a resistor in line on the black wire, and it does not looked hacked. Looks factory. There was a time I could tell you the value on that just by looking at the bands, but that has been almost 50 years ago! I can either look it up or just measure it with an ohm meter.

Also asked Greg if it makes a difference that my tach is an 8k tach, and the potentiometer has "7k" written on it.

Will report back.

[Lynn]

Got this response from Greg:

"Hi Lynn. This is a new one on me. I’ve never seen a resistor in line before. I’m guessing a dealership repair done way back in the day?"

[Lynn]

Here’s the remote set up. Ran three wires for the tach. Bent the potentiometer over a bit and drilled a hole in the back housing (with the tach apart). I will seal that hole up before putting the tach back into the dash, just so no dust can get in there.

Hooked it up and started the car. Turning the screw clockwise slows it down. I was just getting ready to go upstairs and get Sherri to take me for a drive so I could dial it in when it started raining. Crap. This will have to wait.

I know my speedo is dead nuts accurate. Already dialed that in a few years ago with gear changes and the correct adapter. There is a radar warning sign on one of the local highways that has a read out of your speed, and I double checked the other day. Still right on. So with my current 27.04 inch rear tires, and the 4.10 gear, I should be turning 3567 rpm at 70 and 2293 at 45. I figure if I have both of those pretty darn close, that is as good as I can expect out of a factory tach.

One question: since the resistor in the black wire is apparently not factory, would I be better off eliminating it and using the potentiometer to dial it in? Any electrical gurus in the house?

One other thing; I had forgotten that one of the tabs on the lens housing is broken off. Probably no big deal, as I still have three of the four screws holding it in. I may post in the classifieds to see if someone has a good to excellent original they would part with. Not interested in a repop.