Advanced Scamp Electrical Question - Fiberglass RV


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Old 03-07-2016, 06:48 PM   #1
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Advanced Scamp Electrical Question

I call this an advanced question because this one really has me stumped and will likely require your very best thinking cap also if you choose to try your skill at solving the puzzle. Only read further if you like a challenge.

The outside front light (over the propane tank and battery) on my Sept 2015 vintage Scamp was working fine last fall. On the off season, I installed a TriMetric meter and companion solar charger with a connection to a receptacle. All of this was put in the compartment under the front sofa (door side). While load testing the solar charger by turning on every 12 volt item at once (and while off shore power), I discovered that the front outside light would not come on.

It is certainly possible that I pulled a connector loose or something while installing the meter and solar wiring, so I started with standard troubleshooting steps.

Removed bulb and noticed that since Scamp uses such cheap light fixtures the bulb socket was a little corroded after only six months of being outdoors. Cleaned it all up well, still no light.

Checked the bulb visually and for continuity. It checked out as good.

Substituted a LED bulb and there was light. Operation was normal with the LED bulb.

Tried another incandescent bulb with same results as the first bulb, no light.

Hooked up both incandescent bulbs to a 12 volt supply with alligator clips and they both lit up fine. The bulbs are good.

I can actually see the center pin make contact when I put the bulbs in. Everything sure seems to be making good contact.

Measured the voltage at the blub socket (with the switch on) and read 12.7 VDC at the center positive contact and the part of the socket where the bulb base and tab make contact.

Scratched head since that seemed to be the next appropriate step.

At this point in my story I might offer kudos to those of you who 1: noticed that the LED bulb with low current draw worked fine but the higher amperage incandescent did not, and then, 2: concluded that maybe there is a bad connection that is allowing enough power to get the 12.7 VDC reading but is not good enough to pass the amount of current which the incandescent needs. If that were the case I would expect the voltage to drop under the heavier load, so next step…

I removed the wire nut off the positive wires, made sure they were well twisted together, and hooked up the multimeter positive lead to the positive wires. With the incandescent bulb in and the switch on, I read the voltage with the negative meter lead on the outside of the socket, and then on the base of the bulb. Voltage was constant when the switch was on - there was no voltage drop. At this point there was only one more test I could think of and it was just a little risky.

With extreme care as to not cause a short, I removed both wire nuts in the light fixture to expose the positive and negative feed wires and touched them to the incandescent bulb, which I almost dropped when there was a small spark and the bulb lit up! So we know the wiring to the fixture is OK.

That means the fixture seems to be the issue. It worked fine with an incandescent bulb a few months ago but no longer does. It does work with LED bulbs however which are equivalent with the same base. Explain that!
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Old 03-07-2016, 07:30 PM   #2
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The LED will sometimes work when you have a coroded connection like on the ground side. You need to remove the bulb and do a resistance check on both the hot side and ground side.
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Old 03-07-2016, 08:21 PM   #3
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I agree with your assessment that the fixture is at fault, especially if the wires that end at the fixture will light the incandescent bulb, but it won't work in the fixture itself. Perhaps some dielectric grease on the switch contacts might help and keep corrosion at bay?
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Old 03-07-2016, 08:31 PM   #4
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The contacts in the light switch may be corroded. Jumper the switch and see how it works.
Eddie
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Old 03-07-2016, 08:42 PM   #5
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All good suggestions that point to a bad connection, probably due to corrosion, in the fixture. I'll look at it again in the daylight but if this happens after only six months I think I might just have to upgrade the fixture.
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Old 03-08-2016, 10:18 AM   #6
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Might not hurt to give the light socket and the switch a spritz of WD-40. If corrosion is the problem that will help. Also if you check an auto parts store or online there are dielectric greases that are made for coating the metal bases of automotive bulbs to prevent corrosion.
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Old 03-08-2016, 10:45 AM   #7
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The LED bulb draws way less amperage, so yes, if you had a bad connection the power hungry incandescent may not light up.
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Old 03-08-2016, 01:34 PM   #8
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We have multiple winners for this puzzle. I don’t know who said this was so difficult!
(Oh wait, I think I said that )

@Eddie Longest: You sir were dead on. Jumper-ed the switch and the light works fine.

@ Captleemo: Removed switch and sprayed it with contact cleaner that I have here (the kind that is used on old style TV tuners and potentiometers. That helped, but the switch is still flaky. it works most of the time, and always lights if I bypass the switch.

@ Radar1 and Captleemo: Good idea - and I do have dielectric grease here too. Its something all trailer owners should keep around. It will help but the corrosion on the base of the bulb was not the problem (or maybe only part of the problem).

@ Darwin Maring: Resistance of the switch only was all of .2 ohms with no load.

So.. bad switch allowed enough power for the LED to work but not the incandescent bulb.

By the way, I found so much sawdust and stuff in the light fixture that I am sure it trapped water on the bottom where the switch is. It looks like Scamp did a lot of sanding, cutting and fiberglass work while the lens was off and never cleaned it out when the light install was finished. If I can’t get a replacement switch for free under warranty than I will spring for a better light fixture.


Thanks to all.
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Old 03-08-2016, 01:56 PM   #9
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The following is from Wiki. Dielectric grease does not conduct electricity so it's not intended for use on the contacts.

Dielectric grease is electrically insulating and does not break down when high voltage is applied. It is often applied to electrical connectors, particularly those containing rubber gaskets, as a means of lubricating and sealing rubber portions of the connector without arcing.
A common use of dielectric grease is in high-voltage connections associated with gasoline engine spark plugs. The grease is applied to the rubber boot of the plug wire. This helps the rubber boot slide onto the ceramic insulator of the plug. The grease also acts to seal the rubber boot, while at the same time preventing the rubber from becoming stuck to the ceramic.
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Old 03-08-2016, 02:32 PM   #10
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Quote:
Originally Posted by Glenn Baglo View Post
Dielectric grease does not conduct electricity so it's not intended for use on the contacts.
I might be confused about the terminology but this is what I was referring to. Used it for years on all sorts of electrical contacts, such as the trailer to tug umbilical cord.
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Old 03-08-2016, 02:36 PM   #11
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That says it enhances conductivity, so should do the trick.
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Old 03-08-2016, 02:43 PM   #12
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In the electrical world a dielectriic is an insulator. Dielectric grease has the resistivity of glass. That said there are two EE's on this board, Byron and myself. Byron loves the stuff. I think its a mistake. Good luck Raz
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Old 03-08-2016, 02:50 PM   #13
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Dielectric means it won't conduct electricity but won't hinder conductivity so it actually is not an insulator because if it was it would hider conductivity.
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Old 03-08-2016, 03:04 PM   #14
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Quote:
Originally Posted by Captleemo View Post
Dielectric means it won't conduct electricity but won't hinder conductivity so it actually is not an insulator because if it was it would hider conductivity.
Wiki entry says it is an insulator.
And defines insulator: An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.
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