Replacement tachometer not working right

[hanleyclifford]

Having the EI does influence the discussion, of course. Isn't a potentiometer supposed to divide or reduce voltage?

[JOHN COOKSON]

Isn't a potentiometer supposed to divide or reduce voltage?

Yes. It's a variable resistor.
It's use would be to calibrate the instrument to various input voltages would be my guess.

TRUE GRIT

[edwardc]

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. ...

My tach was doing this a few years ago. I traced the problem to the little rotary selector switch on the back of the tach that is used to select the number of cylinders on the engine. Apparently the contacts got a little corroded and it was intermittant. By simply turning it back and forth through its full range a few times, it scrubbed the contacts clean and it settled down to a steady, correct reading. That was 4 years ago, and it hasn't acted up since.

[hanleyclifford]

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.

Imagine it. I have personally corrected intermittent spiking problems with instruments as diverse as DC 10 sounders and ST 2000 auto helms by means of resistors.

[hanleyclifford]

Yes. It's a variable resistor.
It's use would be to calibrate the instrument to various input voltages would be my guess.

TRUE GRIT

It appears that according to the manufacturer the potentiometer is also used to influence output voltages. Sure would like to see a link to those manufacturer instructions.

[JOHN COOKSON]

It appears that according to the manufacturer the potentiometer is also used to influence output voltages. Sure would like to see a link to those manufacturer instructions.

Could be.
My other question is what are 2K ohms doing in a 12 volt system? Wouldn't resistance of just a few ohms drop the voltage to almost zero? What am I missing here?

TRUE GRIT

[hanleyclifford]

Could be.
My other question is what are 2K ohms doing in a 12 volt system? Wouldn't resistance of just a few ohms drop the voltage to almost zero? What am I missing here?

TRUE GRIT

Same thing I'm missing. This is not adding up.

[ndutton]

Alco Bill, even though the chatter continues it's good to hear you reached a resolution quickly. Funny how not a single one of us advised a call to the dealer. We do get caught up with ourselves don't we?

[hanleyclifford]

Could be.
My other question is what are 2K ohms doing in a 12 volt system? Wouldn't resistance of just a few ohms drop the voltage to almost zero? What am I missing here?

TRUE GRIT

It is quite possible the potentiometer in the sensing to meter line is to adjust for variations in the signal from coil - . We have seen how the introduction of EI affects tach readings. The resistance in the supply is a different matter, so we could be looking at 2 issues here. Once a sensitive instrument gets spiked, however, there is not usually any fixing it with a belated installation of the resistor - do the primary resistor first; you can always take it out, but it doesn't work the other way.

[alcodiesel]

AlcoTach

Hahaha, you guys are too much. I returned it. They'll send another. I'll report here.
AlcoBill-
Alcodiesel comes from one of my old hobbies- model railroading. ALCO is American Locomotive COmpany of Schenectady. Great steam engines then diesels before they went broke.

[romantic comedy]

Bill, I used to drive right by the old ALCO plant in Schenectady, on Erie Ave. Used to see a sign that said "American Locomotive". Sadly, they demolished that plant a few years ago.

[hanleyclifford]

Scary Stuff

Found some instructions from the company that makes the tach: http://www.wemausa.com/instructions/...ometer_led.pdf

[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.

Are you referring to these instructions? http://www.wemausa.com/instructions/...ometer_led.pdf

[alcodiesel]

Horrible customer service. WEMA USA

Finally, finally got refund for the tach I sent back November 23rd, 2015! They said it was a diesel tach even though I clearly ordered a gas one and the instructions were for gas. They said they didn't have the one I wanted in stock.

It seemed that they would do nothing till I called- repeatedly. I had them on speed dial! They kept telling me they moved. Who cares? I didn't force them to take my money.

Wow, I will never order from them again. Here's their website:

http://www.wemausa.com/index.shtml#.VqN_kPkrLDc

Turns out there's an auto shop http://www.autoelectricservice.net/gauges.html
selling VDO tachs way less than Defender ($112 v $140) that will work just a few minutes from my house. Sheesh.

[ndutton]

Diesel tachs typically have one of two input signals, alternator tap or tach generator (sender). During troubleshooting did you ever determine which input it required?