Powering Laptop - Broken Cig Lighter

SnailTail · 07-20-2008, 10:09 PM

Hello,

I have a T40 Thinkpad which I would like to power in my car, as the battery doesn't stay charged for very long. I bought an adaptor that can plug into the cigarette lighter, but unfortunately my cigarette lighter doesn't work. I checked the fuse, and it is fine. I have a voltmeter and would like to check the wires, but I'm not too sure how to use it - what setting should it be on to check the wires that go to the cigarette lighter and how many volts should it be at if it is working good?

If all else fails and I can't fix the cigarette lighter, how else can I power my laptop? I have a 1994 BMW 318is.

Any help is greatly appreciated. Thanks.
-Adam Weglarz.

lbridges · 07-20-2008, 10:21 PM

Voltmeter should be on DC voltage, symbol-wise AC usually has a ~ symbol while DC is a straight line. Pick something that has a range that will include up to 15V.

With the key set to the ON position, put the black lead on the outer metal part of the socket (where the lighter would plug in) and the red lead touching the center back of the socket. Voltage should be between 12 and 15 (depending on if the engine is running or not when you check).

If you read zero volts, the socket is bad. There is usually a small fusible link on the back of lighter sockets. The link is put there by most OEMs since lighters are designed to both get hot and involve electricity - a bad combination that could lead to a fire if something was holding the lighter "in" when it was trying to pop out. The fix (if that's the problem) is to replace the lighter socket assembly.

SnailTail · 07-20-2008, 10:34 PM

On the DC side of the voltmeter, there are 20, 200 and 2000 and also 200m and 2000m. On all of them except 200m, nothing happens at all. Should I get a volt if I put both prongs together? (cause I don't). If I put it on 200m, it reads between 0 and .8 of a volt, but I think this happens if I touch it on any metal. I have also tried putting the prongs on all of the setting directly on the wires that connect to the bottom of the cig lighter and nothing happens.

Sorry I'm such a newb at this. Any other suggestions are welcome. I really want to get this working.

-Adam.

lbridges · 07-20-2008, 10:41 PM

20 would be fine. It should be zero if you touch the two probes together.

You touched the two probes to the two wires leading to the back of the lighter socket - was the key on - are you sure you got bare metal (many times they have a plastic covering - might be clear and hard to tell)?

If the key was "ON" and you touched both probes to the two wires, then you have a wiring or fuse problem.

Your alternative to fixing that is to find another set of "good" wires that switch off/on with the key and wire in another lighter socket (like from Wally-World or similar).

SnailTail · 07-20-2008, 10:55 PM

Ok, just checked again to make sure - there definetly isn't any power from the 2 wires. I put the voltmeter in 20 and made sure that I got metal wire that connects to the socket. I put the black probe on the brown ground wire and I put the red probe on the red wire (positive); there's surely no volt at all.

Now, I'm hoping I'm just a moron when it comes to checking fuses. It says on the inside cover of the fuse box that the cig lighter is fuse 32. How do I know which fuse this is, as their not numbered? The only numbers on them is how many volts they do, such as 7.5A and 30A. I checked all of the wires that are in the fuse that should be broken if the fuse is bad and they all seem fine.

How would I go about checking the wiring for the cig lighter - I can't see where it leads...

Can somebody please tell me how I can wire another socket somewhere else, or atleast point me to some tutorials? Please help if you can. I'm desperate.
-Adam.

lbridges · 07-21-2008, 05:12 PM

Set the voltmeter to check resistance (Ohms).

Pull one fuse at a time out and see if there's any reading. Each one should read zero (or close to that if you have a cheap meter). It should read infinity (or something that means the same thing - look in the meter's book) if the fuse is bad.

Sometimes some things actually require multiple fuses to be good in order to work. If this doesn't do it for you, then I suggest signing up on a BMW board where someone can help with wire colors, further testing etc.