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Old 09-19-2016, 07:16 PM   #1
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12 Volt Mystery - Going slightly insane

So I'm in the process of installing a small vented heater that has a tiny 12volt fan inside it - to help distribute the hot air. Draws next to nothing. Simple wiring coming out of the heater - one red wire (hot) & one black wire (ground).

When I run the heater wires direct to a 12v battery the fan runs just fine. The switch & speed control dial works just fine. When I tap into some 12v wiring in the airstream (confirmed/tested 12V with the OM meter). The fan does not run.

At first, I thought it might be a bad ground, but there's some DC lights that are tapped from the same source and they work just fine. I test the wires direct, I get 12v. I test the unit at the fan, I get 12 V.

Very weird. So I have the correct voltage from the airstream wires, but it's not running the fan. Nothing is wrong with the switch or fan, because it works when hooked direct to the battery.

I'm pretty much as frustrated as you can get with this -- It can't get any easier!!! but I still can't seem to figure out what the heck is going on.

Can anyone shed some light on the situation for me?

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Old 09-19-2016, 07:20 PM   #2
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Here's a semi-educated guess:

The fan draws a lot more current than the lights. That current draw pulls the voltage down in that circuit to the point where the fan motor won't run.

How to check: Connect the volt meter across the fan leads as you connect the fan leads to your two 12 volt wires. Iin other words, you want to measure the voltage getting to the fan as it's trying to run.

Let us know what happens.

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Old 09-19-2016, 07:34 PM   #3
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Hi gomotomoto, I thought I might share a go crazy problem I had when I installed a new SeeLevel water tank level sender in my Airstream. This is a 12v device that uses very little voltage or current. Maybe like your "computer fan" in your little heater. My read out was very erratic.

So the SeeLevel tech told me the instrument will not operate without a ground resistance less than 20 ohms. You might check to see if you have very low resistance to ground from the heater wire to the Airstream frame, or shell. Older Airstreams can get corrosion between the ground bolt and the frame member. Mine did, I cleaned it up, and measured 2 ohms resistance between the meter and the ground.

But it didn't solve my erratic instrument. Then SeeLevel asked about my "voltage ripple" in the DC circuits produced by an old converter. Voltage ripple can make 12 volt radios sound terrible (static) and the like. My SeeLevel instrument would work fine when I was on battery power alone. But it was erratic when I was plugged into shore power and thus using the converter output. So I purchased a new converter and my problems went away. Luckily before I went insane.

So try your heater on 12v battery power alone. Try your heater with a "pure" 12v ground wire straight to the negative of the battery, and then try your heater while plugged in using the converter output.

Chances are good none of these experiments will solve your problem. I'm just helping you along the path to insanity.

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Old 09-19-2016, 07:39 PM   #4
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Sounds like you figured out where the 12VDC is, so my guess is that you have not picked up a ground for the black wire but another 12VDC wire.
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Old 09-19-2016, 08:55 PM   #5
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It sounds like you know what you're doing and what to look for and from your descriptions, well I got nothing. From what you're saying seems like it should be working. It'll be interesting to see what the fix is.
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Old 09-20-2016, 05:04 AM   #6
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Yes, so the question becomes: What would give a 12v reading at the heater (with OM meter) but keep the fan from running when the switch and speed control switch is confirmed to be ok?

My guess is some sort of bad ground - but if the ground was bad, wouldn't the OM meter read something other than 12v?

Running right to battery fixes the issue, I may just have to route those wires directly to battery. Kind of sucks not knowing what's going on though.
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Old 09-20-2016, 06:08 AM   #7
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A poor ground can do some really strange things to dc devices. I would persue that first. Even if logic tells you otherwise. Just to test, you could attach the + side at your heater location and then run a separate ground wire direct to the battery, see what happens.
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Old 09-20-2016, 06:21 AM   #8
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I'll join the group that suspects you have an inadequate ground connection.

First, if you haven't already done so, check for corrosion at the negative terminal of your battery and also (especially) at the point where the negative battery cable connects to the shell or frame of the trailer. Clean and tight, or the whole system will be affected.

Run a temporary test connection between the ground wire on your heater fan and the metal shell or frame of the trailer. See if the fan runs in that configuration. If it runs, your problem is in the ground wiring.

Turn everything off, set your meter to read ohms, and check between the ground connection you're using and the shell/frame. Meter should read very close to zero ohms (1 or 2 max). Keep probing until you find a good ground. (Power should be off when checking resistance (ohms) or you will blow a fuse in the meter.)

Good luck.
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Old 09-20-2016, 11:06 AM   #9
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The thread title made me think of the Queen song I'm Going Slightly Mad.
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Old 09-20-2016, 11:18 AM   #10
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The thread title made me think of the Queen song I'm Going Slightly Mad.

If an electrical problem doesn't drive you at least slightly crazy, you don't understand electricity at all.
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Old 09-20-2016, 01:32 PM   #11
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Amen to that! I am trying to noodle through a replace my batteries with LFP's project and there are a number of wickets I am addressing.
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Old 09-20-2016, 02:24 PM   #12
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Wild thought but could there be a diode in the fan to preclude it running backwards. To test this reverse your wires where you are trying to get it to work.

Can your meter show + and - readings CD readings?
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Old 09-20-2016, 02:40 PM   #13
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What kind of heater are you installing ??? I'm planning to do a similar project soon with a catalytic heater. It also has a small fan and is vented to the exterior made by a company in Washington. Just curious what I have to look forward to when I get into it.
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Old 09-20-2016, 03:23 PM   #14
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12 Volt Mystery - Going slightly insane

In your discussion of 12 volts for your heater, there is one thing you are missing, and that is amperage. Your small heater is taking much more power than you probably realize, and it is the heating coils rather than the fan that uses the power. (amperage.) There should be a tag on your heater that specifies AMPS.
A Multimeter will read volts in wiring that is large or very fine, and will give the same reading. If, however, the wire and/or connections won't carry the load, the heater won't run AND the point of restriction will rapidly heat up until a fuse blows, a circuit breaker trips, or a wire melts. None of these results are good. Hooking directly to the battery with the wires supplied with the heater covers the bases, but more research into wire size, connections, etc. is required to wire it in directly.
You should have 12 volt outlets. If you can attach the proper plug to the heater wires, this should give you an acceptable wiring set up.

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12 volt

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