Hello,

I have a 81 F100 and have suspected for some time that I may have a short. I finally get around to narrowing it down and it's in the circuit for the horn/cigarette lighter. I put a multimeter between the negative post on the battery and my negative battery cable. I got the following readings.

With Fuse In - 11.69 volts - 1191 Ohm (on 200K setting)

With Fuse Out - 0.95 volts - 248 Ohms (on 200k setting)

One other thing. When working on my steering wheel recently I need to pull a good portion of it out of the column. A yellow wire was preventing it from pulling out fully so I cut it then reattached it afterwards. For the heck of it I decided to try and separating the yellow wire, leaving the fuse in, and see what the volts and ohms were. Here is the result.

Fuse in with Yellow wire cut - 1.95 volts - 245 Ohms (on 200K setting)

On the surface this seems pretty similar to when the fuse is out and makes me wonder if I have accidentally discovered at least the wire that is the culprit.

I know a bit about electronics and understand the Ohms is a measurement of resistance. I am hoping someone that knows much more than I can translate the readings I am getting.

P.S. With the yellow wire separated the turn signals and emergency flashers work.

Thanks a bunch

 

You can't use the voltmeter the way you are using it and get a good reading on most vehicles. Your readings are not that bad, but you must not have a digital radio or some other device that uses a little bit of current at all times. If you are looking for zero volts, I don't believe you will ever get it.

You should be hooking the meter up in a current measuring mode, or you can use a testlight and hook it up like you are the meter, between the negative cable and the battery neg terminal.

If you hook your meter up to read amps, look for something less than about .07 amps or 70ma. If you are above that, then do your wire test again and see what you have.

 

Thank you Franklin,

When I set the multimeter to read amps should I connect it in the fashion I am now or differently?

 

Yes, connect like you have it, in series with the negative side of the battery. Be careful, most meters require you to unplug the red lead and move it to a new plug-in used for amps only. What I mean by "being careful" is to make sure you move this lead back to it's normal position before you put the meter away. If you forget and then try to take a voltage reading on something else later, the meter will be a dead short and you could let the smoke out of whatever you are testing(been there, done that).

 

So I evidently blew my 200ma fuse somewhere along the line. I have checked with Sears, Ace, and Walmart to try and find the 200ma fast acting fuse but to no luck. Any ideas on where this is sold?

One interesting thing is that when I used the 10amp setting I didn't get a reading...just "0". Concerned the multimeter wasn't functioning properly I turned the ignition key to "On" and the 10am reading read around 7.5amp or so. This tells me the it is at least working. I am a bit surprised I didn't even get a fraction reading on the 10amp setting. Hence the reason I am trying to replace my 200ma fuse to get a reading at a lower level.

 

Radio Shack sells a 200ma fuse part number 55048555, but the stores don't carry it you have to order it over the web.

 

At the 10amp setting shouldn't I have expected to see some sort of reading, for example 0.01 or 0.02?

 

On that meter, if you hook it up to higher amperage than you select, it may pop that fuse every time. I wouldn't attempt to try to read an unknown amperage with a multi-meter. I'd buy, rent or steal a real amp meter (ammeter) - prob with a clamp-on cable instead of probes.

 

When you are in the 10 amp mode, do you have 2 places to the right of the decimal point in the readout?

 

Sorry for the delay. I got a pack of 5 200ma fuses mailed to me and tested the truck today. I was a bit surprised and I guess happy at what I saw. With the "mystery" yellow wire split the mAmp reading was 0.0. With the yellow wire connected it read 6.5 at the 200ma setting which I assume means 6.5ma's.

At a reading of 6.5ma wouldn't it take quite a while for the battery to discharge to the point where it had trouble starting? The vehicle is not being used and seemed to have trouble starting after about 2 weeks or so. I think I may have found the culprit in the yellow wire considering the reading is 0.0 after it's disconnected but as before I wouldn't have thought that 6.5ma were enough to weaken a battery in 2 weeks.

 

6.5ma is not enough to hurt anything. Like I said in a previous post, I read somewhere that generally if you are under 70ma, you are ok. Modern vehicles are pushing the limits on this, and I also read a about a warning sent to dealers to start and run the cars on their lots at least every 2 months or the batteries will go dead.

If your charging system is in good shape(around 13.5-14.5v on the battery while the engine is running) I suspect you have a battery problem. Also make sure the top of the battery is clean, if it gets dirty and a little bit of acid leaks out, the top can be somewhat conductive and discharge the battery over time. This is a rare occurrence though.