I have read the other posts on this topic and they helped some. My car has a new engine harness, under dash and tail light harness. When i disconnect the neg battery terminal and put the volt meter between the cable and the battery post i get a 6.2v reading and the battery only shown 7.7v charge. Now when i disconnect the main dissconect plug at the far right of engine compartment that reading drops to 3.04v. and to narrow it down further the black wire with yellow stripe is the wire causing the 3v jump. could it be the ignition switch? and is a 3v reading ok or should there be no reading at all.
Thanks for all your help.

 

Car?
Year?
Radio?
Clock?
Pull fuses one by one (a JamesW trick).
Jim

 

66
mustang coupe
Radio new install constsant power run seprate from harness.
No clock
Pulled fuses dome light affected slightly .6v or so left fuse out still get 6v reading.

 

Remember that a voltmeter reads potential, not current. The question is - do you have a problem, and what is it, or are you just taking readings? And what type of harnesses?
Jim

 

The problem is the battery drains from full charge in a very short time when running. if it is let stand in the garage sometimes it will only loose a very small charge some times it will loose 6-7 volts. all of the harnesses were bought from John Mustang in Houston tx. The battery was draining before the new harnesses were installed. i thought they would be my cure all because they were in sad shape but it seems i have issues elsewhere. are there any other suggestions anyone has?

 

Take a voltage readingbetween the positive terminal of the battery to ground with the battery disconnected. The reading should be between 11.5VDC (a low battery) and 12.5VDC.
Take a voltage reading from the same two points with everything hooked up and the engine not running. There should not be a drop from the above. Disconnnect your new radio to do this.
Take a voltage reading from the same two points with the engine running. You should read 14 to 18 VDC, the charging voltage. If not, have your alternator checked and if bad, replace it and get a new regulator.
Don't bother disconnecting the battery and reading the voltagebetween the battery terminal and the cable. It don't work that way. You need an ammeter to take that type ofreading.
Jim

 

I had a battery drain issue last month. Here is what I did. Hope it helps.

I though I had a drain so I checked my current draw by disconnecting the negative cable and checking the current flow with the car off. It was basically 0, so I knew it was not an accessory draining the battery

I said shoot, I guess it is a bad battery, so I charged it. The dang thing was dead the next day after driving once. So I replaced the battery. That helped some, but mostly because the new battery has 1000amp while the old one had 650amps.

I finally decided that I had a bad harness connection, so I ordered and installed a new alternator harness. That helped my dash light to work correctly, but did not solve the overall drain problem. What I noticed when the alt light started working was it would glow at low rpms.

I tested the charging voltage with the car running. It was 14V one time I checked it, 17V another time, 12.5V another time. This told me it was a voltage regulator issue. Replaced it, but still did not fix the issue. Finally I replaced the alternator and voltage regulator. This corrected the issue. What was happening was the alternator was working, but not being consistant. This would allow the battery to receive a charge some of the time, but would also cause it to drain some while driving. When the car would sit for a day or two, the battery would just not have enough to start the engine.

It was a hard problem to diagnose, but I wish I would have just had the alternator checked to begin with. Instead I got a whole new charging system![8D] The good new is I now know everything is new and working.

 

Us retired auto electricians love dealing with the folks that suscribe to the shotgun approach to electrical repair. Take your car to a repair shop, SEARS, auto parts dealer and let them test your electrical system in the car (usually for free) . You will always be money ahead!

 

To start from the beginning--Do you have a battery going dead on you after sitting a period of time? I would guess not. A reading of less than batteru voltage with the volt meter in seies with a disconnected cable is not significant--That indicates a draw in low mili-amps. If a full batt reading then put an ampmeter in series and read the actual draw. Also note if there is ay noticable spark when the cale touched to the terminal.

Note--Clocks--Older ones rewind and cause a high voltage reading until the cable touched to the batt to rewind. Newer ones are always drawing. If a problem park the car in a dark place, go out after dark to look for light leaks from hood light, glove box light, trunk light etc.

I have seen a draw indicated from touching the top of a dirty battery.

 

Man, you just said a mouth full. I absolutely hate automotive electrics, so I usually never screw around with trying to fix it myself. I did replace my rear wiring harness though because the PO had it all screwed up inside the trunk because he had a trailer hitch attached to Ol' Yeller.[:'(] It surprised the hell outa' me that everything worked when I got through with it.