Tachometer needle jumps sometimes - cause?

[Bitsyncmaster]

Hi, I have same problem with the needle but very rarely in idle, mostly at 3000-3200 RPM.
Btw, my alternator is also delivering 12,5V with AC and Fans on and 14.0 with everything switched off.
Can you confirm that with the new alternator it is now improved? Thanks.

Just another observation: I think it got worse after the engine overhaul and I can see that checking the alternator cables should me on my list (after I fix the engine).
Interestingly, almost all of my instruments are now reading lower than before engine overhaul:
- Voltmeter is showing 1,5V less than actual (same as my aftermarket radio which has screen mode showing voltage)
- fans kick in on 190 instead of 210 before and switch off at 150 instead of 160 before.
- oil pressure showing less than of measured with the manual gauge
- fuel is showing 1/4 less than in reality

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

[Jonathan]

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

Thanks Dave. I'll look at these connections on my own car as well.

[Jonathan]

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

I got the connections cleaned up behind the rear board. They didn't look terrible nor were they loose to the touch. I took them apart and cleaned/sanded off any surface corrosion on the cable lugs and then reinstalled with new nuts and washers.

Car started right up but it still seems to have voltage issues. Like the voltage doesn't go up to it's usual spot right from the beginning. I noticed it hovered a little under the 13 volt marking for a few seconds and then eventually came up above the 13 volt marking. The belt doesn't seem to be loose or slipping, but maybe it is?

I turned on some accessories, including the headlights and stereo and AC fans and the voltage dropped a small amount with each new thing added. With those running it was right at or just below 13 volts. No cooling fans running up front mind you as the engine was cold still.

Then after turning all those accessories off again and just letting it idle in the garage, I noticed the voltage "hunt" a little. Or vary might be a better word. Not hunting like our idle can do sometimes, but the voltage needle moved some up or down and then back like it was having a hard time adjusting itself and finding it's spot.

I would describe it as having a problem with regulating the voltage, but then I don't know what that might mean exactly. Help me understand, is the "voltage regulator" function something going on inside the alternator and the only connection it has to our car's system through that single yellow/brown output wire? I'm going to go double check that wire and any connections it has are all good still.

Is there anything else to check regarding this voltage not being stable? I can tighten up the belt some more, but I just don't think that's all it is.

[Bitsyncmaster]

Yes the alternator regulates the voltage internally in the alternator. There is an external connection to remotely sense where the voltage reference it uses but most just leave it open and it uses the battery terminal on the alternator. However my DMCH alternator I used that remote input jumpered to the battery terminal on the alternator because I could see a dip every 18 seconds. That jumper also fixed the error code flash it did before starting the engine.

[Jonathan]

Not sure I follow what you mean about the ground, Dave.

I was under the impression the alternator got it's ground from the mounting bracket and bolts that secure it directly to the passenger side engine manifold. Are you saying there is another way to provide a ground to the alternator? And it uses this as it's ground reference?

[Bitsyncmaster]

Not sure I follow what you mean about the ground, Dave.

I was under the impression the alternator got it's ground from the mounting bracket and bolts that secure it directly to the passenger side engine manifold. Are you saying there is another way to provide a ground to the alternator? And it uses this as it's ground reference?

Not talking about any ground in past reference. I assume your alternator has a good ground via the mounting bolts. I was taking about the +12 reference that the regulator uses to provide the correct voltage.

FYI:
The alternator should hold a higher voltage when it is cold. This was always designed into the regulation because a lead acid battery requires a higher voltage when the battery is cold. Real cold you should almost get 15 volts but hot can drop to as low as 13.5 volts. Normally my car puts out 14.6 volts but I don't drive it on cold winter days.

[Timeless]

My car has this intermittent issue as well. It doesn't happen at idle only when cruising at 2000-3500 RPM. (auto trans) Voltage is ok and it doesn't matter if A/C, lights, stereo are on.

To my knowledge the binnacle/cluster have never been removed judging how original everything looks under the dash. Id did replace the battery today so I'll continue to monitor it.

[Timeless]

Ignition coil replaced with DMC NOS Bosch and tach still erratic/jumping while cruising. With a recent trip in the VOD and everything in there solid I'm starting to think it's tachometer age issue.

[Ron]

Have you tried a hand held tachometer?

[Timeless]

Have you tried a hand held tachometer?

I have not. If I had one I would test with it.