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post #1 of 6 Old 07-03-2015, 07:34 PM Thread Starter
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Eratic Voltmeter

I installed a $20, rectangular, red LED, digital VM and connected it to 14 AWG wire powering a 12 v. cig lighter outlet using Posi-Taps, connected to my fuse block. After about 20 engine run hours I have the following: 10% of the ERT there is no display in the VM; 40% of the ERT the VM slowly fluctuates between 13.9 and 14.2 and it usually does this at idle; the rest of the time it wildly fluctuates between 10 and 20 v, going up and down the scale and it especially does this on acceleration from a stop light. It has power, and I "feel" if I got another one nothing would change. Any ideas? Thanks.
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post #2 of 6 Old 07-03-2015, 07:54 PM
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I installed a $20, rectangular, red LED, digital VM and connected it to 14 AWG wire powering a 12 v. cig lighter outlet using Posi-Taps, connected to my fuse block. After about 20 engine run hours I have the following: 10% of the ERT there is no display in the VM; 40% of the ERT the VM slowly fluctuates between 13.9 and 14.2 and it usually does this at idle; the rest of the time it wildly fluctuates between 10 and 20 v, going up and down the scale and it especially does this on acceleration from a stop light. It has power, and I "feel" if I got another one nothing would change. Any ideas? Thanks.
You've got a bad connection. My initial hypothesis would be that your "Posi-Taps" should be called "Maybe-Taps". Those things are chancy even when put onto the wire type for which they were designed. While your statement of what hooked to what is a bit ambiguous, I think it likely the new connection ended up loose. If your voltmeter has exposed terminals, take a look at them using a known good (and battery powered) voltmeter. Then check that outlet. If those act differently, you've got "Maybe-Taps".
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post #3 of 6 Old 07-03-2015, 08:27 PM
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I think the OP has a bad VM or isn't hooked into "clean" power, may be ignition interference.
Does it vary with key on engine off?

A bad connection isn't going to make the VM show 20V on a 12V system.

As for Posi-Taps- I think they are fantastic and have been trouble free for me, used them for years.

Trepidator- Are you sure you aren't thinking of something like ScotchLok squeeze connecors?

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post #4 of 6 Old 07-03-2015, 10:00 PM
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Originally Posted by AlH View Post
I think the OP has a bad VM or isn't hooked into "clean" power, may be ignition interference.
Does it vary with key on engine off?

A bad connection isn't going to make the VM show 20V on a 12V system.

As for Posi-Taps- I think they are fantastic and have been trouble free for me, used them for years.

Trepidator- Are you sure you aren't thinking of something like ScotchLok squeeze connecors?
(Responding to points in order

Yes, it could be a bad VM, either defective or which responds to noise rather than filtering it out well (as virtually all analog-to-digital converter do which are used for DVMs.)

I got the impression that the wild variation was fleeting, and possibly induced by extra movement. That together with the VM going blank indicates a bad connection somewhere, maybe even within the VM. But the way modern electronic products are made and tested, bad connections within them are rare once they are deemed salable. Seeing a 20V reading transiently during rapid connection and disconnection of power to the VM need not implicate the VM build quality. (It does indicate a scrimping design approach, but so does the $20 retail price.)

I have no specific experience (that I remember) with Posi-Taps. I just know that I've never seen a non-crimp, non-solder wire tapping device that looked reliable in the face of wire size variation, strand count variation, and installation with common hand tools. Maybe my view of such things has been soured by those ScotchLok things. In my years of work, I have come to appreciate reliable connections and despise questionable ones. (That's why I have crimpers and a willingness to solder when needed.)

Until the OP makes some measurements, getting at the cause is a guessing game. Improving the guesses is why I recommend getting and using another volt meter.
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post #5 of 6 Old 07-03-2015, 10:25 PM
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Disconnect the VM, then make a temporary test connection direct to the battery. If this is flakey, the meter is bad. If this is solid, then your wiring is bad.

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post #6 of 6 Old 07-13-2015, 11:36 PM Thread Starter
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Connection Was Bad

Removed meter and it was rock steady on the battery. I used 18 g wire to make a new, short connection to the 12 v outlet wires, and used a Posi-Tap as before. Now, rock steady. I use a "Battery Tender Automatic" all the time. It charges initially at about 14 v then maintains the battery at about 12.5 v. That is what I now read on the meter on start up. Over time, the new SH775 Series R/R slowly brings up the battery voltage to 13 + I have not done any freeway runs but I expect the battery voltage will come up to 14. OTH this battery is now 4 yrs, 6 months old. The VM is connected to a fuse block with diode and cuts off the battery at about 11.8 v. So, if aux lights fail and VM shows no display, I know to pull fuses for the lights and get home pronto. Thanks, for the help.
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