Replacement tachometer not working right

[alcodiesel]

I can't get the replacement tach to read steadily. It'll read the rpm then sporadically jump then settle back to the correct rpm just to do it again.

I tried a 2k ohm pot in series with it as the instruction suggested if it wasn't reading correctly- no joy. The install instructions said nothing of where in series to put it so I put in the wire going to the coil.

I connected it directly to the neg. coil term. to determine if there was a wire issue- no joy.

This A4 is a '76 with Indigo electronic ignition. The PO died so I know nothing else.
Here is a link to the tach I bought:

http://wemausa.3dcartstores.com/Tachometer_p_47.html

I got the 0-4000rpm, gas version.

Any ideas before I send it back?

Thank you,

[hanleyclifford]

If the tach has it's own 12v power feed that is where to put a 1 ohm resistor.

[joe_db]

You can also add a filter capacitor between the power and ground. Might be an RFI issue. Also try a ferrite on the signal wire from the coil.

Those tachs IMHO look like relabeled VDO tachs - just FYI.

[Administrator]

I can't get the replacement tach to read steadily. It'll read the rpm then sporadically jump then settle back to the correct rpm just to do it again.

This doesn't sound like a resistance mismatch.

A one ohm resistor, Hanley?

Bill

[ndutton]

Could be any of a number of issues (gauge defect, EI defect, who knows?) but my thought went directly to intermittent voltage spike. This may be an indicator of an alternator/regulator problem.

Try measuring the voltage at the tachometer input source (not the coil '-' lead) and see if there's a correlation between voltage and erratic tach readings. If not, that's good info, you can move on to other possibilities.

I'll spare the lecture for now about fixing rather than masking a problem. Let's find out what's going on first.

edit:
After a little research, if this tach uses a stepper motor for needle sweep, there was a known issue with some (a whole lot) stepper motors in the early to mid 2000's. GM in particular had a huge recall spanning several model years. The behavior you describe was typical. My Silverado was a victim.

It affected all full size trucks and truck based products including Suburbans, Tahoes and Escalades

[hanleyclifford]

This doesn't sound like a resistance mismatch.

A one ohm resistor, Hanley?

Bill

Yes, Bill - often a "12 volt" accessory suffering from too much voltage in a "12 volt" system will display the indicated symptoms. Typically occurs with sounders and the like, but it's worth a shot. Maybe first check voltage actually being delivered to the device.

[hanleyclifford]

The issue seems to affect automotive as well. Note the focus on voltage. http://www.studebaker-info.org/text3/tachy.txt

[alcodiesel]

Ah ha! The tach instructions say a 2k ohm pot. I will put it in the feed. I will check the voltage running and report back here. Thanks fellas.

[ndutton]

In basic electrical terms not specific to this instrument I'm curious how the addition of a fixed resistance resolves an intermittent problem. I suppose we should accept the manufacturer knows the idiosyncrasies of his product better than anyone else.

[alcodiesel]

I know when a 12v auto system is running it's above 12v- maybe 13 to 14v. I am guessing this tach needs 12 or less. The instructions weren't clear- something was lost in the translation. It's a mystery.
That's why I put the resistance in the send line- I just couldn't imagine a component was made for auto/marine not able to handle up to 15v or so.

[ndutton]

Since typical charging voltages are a little over 14 volts these days, higher with AGM technology batteries, and the ABYC specifies narrow system voltage drop limits it's unlikely we are expected to pepper resistors all over the place to keep things working. We recently discussed this with the Johnson pump many of us use for electric FWC. It's voltage specification is 12V but that specification also has a tolerance of +/- 20% meaning it's specified for all voltages between 9.6 and 14.4.

I'd be surprised if the tach didn't have a similar voltage tolerance. Even 40 years ago the preferred charging voltage was 13.8V so this isn't a new concept.

[joe_db]

You put a resistor in the sense line from the coil or the 12 volt supply line?
I cannot imagine anyone making a tach for 12 volt vehicles that can't deal with 14+ volts. 12 volt systems started appearing as long ago as the 1930s and system voltage was around 13.8-14 volts even back then.
Our ignition coils are a somewhat special case, but every single electrical device on the boat is not actually limited to 12.0 volts.
A resistor in the sense line makes much more sense The signal might be too strong and generate random errors without it.
The other special case is the Johnson pumps. They get used in all kinds of cars and run fine at 14+ volts, but what I got from Tom at Indigo is he talked to their engineers who told him they would pump a bit less water but last quite a bit longer at 12 volts instead of 14. I am adding a resistor to mine and will see how it does with the engine temperature. This is a bit counter-intuitive, most motors HATE low voltage. If you ever wire up electric winches, a windlass, a starter, or an air conditioner, undersized wires and voltage drop is hard on them.

I know when a 12v auto system is running it's above 12v- maybe 13 to 14v. I am guessing this tach needs 12 or less. The instructions weren't clear- something was lost in the translation. It's a mystery.
That's why I put the resistance in the send line- I just couldn't imagine a component was made for auto/marine not able to handle up to 15v or so.

[alcodiesel]

I put it in the sense line.

I just called the place in Ft. Lauderdale from which I bought the thing from. They said the sense line is correct and something is wrong w/ the unit. They'll replace or refund.

[hanleyclifford]

In basic electrical terms not specific to this instrument I'm curious how the addition of a fixed resistance resolves an intermittent problem. I suppose we should accept the manufacturer knows the idiosyncrasies of his product better than anyone else.

Companies like Pertronix?

[hanleyclifford]

You put a resistor in the sense line from the coil or the 12 volt supply line?
I cannot imagine anyone making a tach for 12 volt vehicles that can't deal with 14+ volts. 12 volt systems started appearing as long ago as the 1930s and system voltage was around 13.8-14 volts even back then.
Our ignition coils are a somewhat special case, but every single electrical device on the boat is not actually limited to 12.0 volts.
A resistor in the sense line makes much more sense The signal might be too strong and generate random errors without it.
The other special case is the Johnson pumps. They get used in all kinds of cars and run fine at 14+ volts, but what I got from Tom at Indigo is he talked to their engineers who told him they would pump a bit less water but last quite a bit longer at 12 volts instead of 14. I am adding a resistor to mine and will see how it does with the engine temperature. This is a bit counter-intuitive, most motors HATE low voltage. If you ever wire up electric winches, a windlass, a starter, or an air conditioner, undersized wires and voltage drop is hard on them.

Which is why you should know the exact voltage being delivered and keep it near the specification. In many cases, 12 volts means 12 volts, and no spiking allowed.