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

[SteveJ]

As long as you don't have a dash cover glued onto the dash, it's not too bad to remove the tach. It just involves contorting yourself some. I'm in my mid-50s, and I can do it. Heck, I did it a couple of weeks ago when I was helping with diagnosing a car.

There are two wing nuts holding the tach in the dash. You pretty much have to lie down in the footwell and reach up to find them . Once you pull the tachometer out enough, you can disconnect it from the wiring harness. 

[Captain Obvious]

35 minutes ago, Jughead said:

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?

1. I shouldn't. There should be no way a bad tach should do anything at all to engine performance. Another reason that I question the coincidental nature of the events. Unless the insulation on one of the ignition wires has split under the dash and is shorting to ground or something, ignition should not be affected. But even that I don't think that should peg the tach at full scale.

2. Not really. You would have to unplug it from the harness running through the dash and pipe your own signals to it.

3. Bench testing would be a little complicated. Power and ground are easy... Generating the RPM signal for the other two wires could be a little trouble without some electronics equipment. I don't think you would be able to generate a signal fast enough just by hand. You could do it using your car with the tach sitting on the fender near the coil and some wires connecting to appropriate locations. Not the simplest thing.

 

[Jughead]

UPDATE

So I swapped out the Tachometers this weekend,  and the Tach now works great.  ( @SteveJ: You're right it was surprisingly easy (I'm 63 yo).)  

HOWEVER:  Engine performance is still low.   It seems to be better than it was before I swapped Tachometers, but it's still not good.  With a warm engine and the clutch out, it take a long time to increase RPMs.   

Questions:

1. Under the assumption that the replacement Tach is good, any hypotheses regarding what the issue might be? 
2. How would I check the wiring for shorts, and where should I focus?

Thanks!!

 

     

[Zed Head]

Check your points and the coil.  Might be that the various problems are related.  Check coil resistance of the primary and secondary circuits, verify supply voltage, reset the points gap, or, ideally, set the dwell.  Verify a good ground for the points.  Spark strength is all about current through the coil.  Check the advance mechanisms in the distributor.  Verify proper ignition timing.  Retarded timing can kill engine performance.

Once you know that spark and timing are good, you can focus on carburetion.

[SteveJ]

42 minutes ago, Jughead said:

UPDATE

So I swapped out the Tachometers this weekend,  and the Tach now works great.  ( @SteveJ: You're right it was surprisingly easy (I'm 63 yo).)  

HOWEVER:  Engine performance is still low.   It seems to be better than it was before I swapped Tachometers, but it's still not good.  With a warm engine and the clutch out, it take a long time to increase RPMs.   

Questions:

1. Under the assumption that the replacement Tach is good, any hypotheses regarding what the issue might be? 
2. How would I check the wiring for shorts, and where should I focus?

Thanks!!

 

     

In addition to what @Zed Head suggested, do you have an electrical or mechanical fuel pump? Do you have a heat shield over the exhaust manifold? Have you thought of insulating the fuel line where it could get heated up by the exhaust? When I switched over to headers, I eventually fabricated my own heat shield to protect more than stock, and when I installed the Patton Machine fuel injection, I routed the fuel line so it didn't run near the valve cover. Both efforts were to protect the fuel system from heat.

[Captain Obvious]

20 hours ago, Jughead said:

I swapped out the Tachometers this weekend,  and the Tach now works great.

Glad it was that simple despite the super coincidental nature of the failure. Occam's razor.

So you want to send me your dead tach so I can open it up on the operating table and do some reverse engineering? 

[SteveJ]

14 minutes ago, Captain Obvious said:

Glad it was that simple despite the super coincidental nature of the failure. Occam's razor.

So you want to send me your dead tach so I can open it up on the operating table and do some reverse engineering? 

I already asked in a PM, so I call dibs!

[Jughead]

Thanks gents.   Answers to @SteveJ's questions:

1) Do you have an electrical or mechanical fuel pump? :   Mechanical.   I see a steady suppluy of fuel in the fuel filter

2) Do you have a heat shield over the exhaust manifold? Yes. 

3) Have you thought of insulating the fuel line where it could get heated up by the exhaust?:  I have not considered doing this because the car's been running great.   The manifold is the original.

4)   I'm going to hold onto the Tach - at least until the problem is sorted out.

One more clue:   Last night while test driving I noticed the that amperage gauge was occasionally swinging wildly.  I've never seen that before.   Full disclosure:  I did install some LED dash lights the other day that I got from Datsun Garage...

 

[Zed Head]

1 hour ago, Jughead said:

amperage gauge was occasionally swinging wildly

Zero to plus or zero to minus.  Might be that you have a short or an open circuit somewhere.

[SteveJ]

Have you noticed a pattern when the ammeter is swinging? The alternator is possibly losing excitation. It could be an intermittent connection on the battery sense or a failing voltage regulator.

Can you disconnect the voltage regulator and take some clear photos of the pins in the connectors in the engine harness and on the voltage regulator?