Mystery Short Circuit

I have a 2008 Access Cab Tacoma, 4WD, 4 cyl, 5 spd, 219K miles.


Thursday morning the truck would not start. The solenoid only clicked. I had to rush to the doctor, so took my wife's Subaru.


After I got home, I disconnected both battery cables and hooked up the charger. Six hours later the battery was only 56% charged. It could be time for a new battery. I looked in my maintenance file and discovered the battery was purchased in July 2014. Yep, time for a new one.


So I bought a new battery on Friday and brought it home. I'm good at dropping stuff. I lost the little square headed bolt that clamps the positive cable terminal.


On Saturday I replaced the missing bolt with an ordinary M6 x 1 x 30mm bolt. I installed two flat washers with the edges ground to fit where the square headed bolt was. I put it all together with an M6 nyloc nut. There is a small wire with a ring terminal on the end and a fuse in the middle that connects to the positive terminal. I attached the ring terminal under the nyloc nut. Then it started raining hard.


Today the truck started normally, just fine. I pulled into the garage to do more work out of the rain. I replaced the standard headlight bulbs with brighter H4 bulbs. Turned on the lights and both high and low beams worked just fine.


When I went to start the engine and pull out of the garage, I heard a pop and then everything electrical went dead. No starter, no lights, no nothing.


I pulled the negative cable off the battery and checked with my multimeter. I have 12.6 volts between the negative battery terminal and ground. So, there is definitely a short somewhere. I don't have a clue where it is, or how I might have caused it.


Your thoughts, guys?

 

What do your battery cables look like (pics?)?

Sounds like a classic bad battery cable connection or possibly the main fuse popped but that would be very unusual.

 

The horn doesn't go through the Main 120/140A fuse.
Try the horn......

It's highly unlikely you blew the horn fuse.

Easy step 1.

If the horn doesn't work...Check the 10A horn fuse for power.
If no power, start looking at the cables. If power, check fuse.....(not likely)

If horn works, Check brake lights. If not working....Check main 120/140A fuse.

 

Thanks, guys. I will do some more diagnosis and take photos when it is daylight.

I just reconnected the battery to test the horn, and it worked. I discovered that the map light was also on. The headlights worked, which they did not after the popping sound yesterday. That's all the testing I have done so far today.

I remembered that in the beginning the old 11 year old battery had zero charge when I put the charger on it. That could have been caused by discharging through a short, or through a map light that was left on for several days. The old battery would not take a charge, and at 11 years it was at the end of its life span anyhow.

I will check the items you guys listed above, but now think the map light was the problem.

Thanks again.

 

Dm93, you were correct about the bad connection. When I installed the new battery I did not get the positive terminal tight. That is fixed. I cleaned the terminals when I installed the new battery. They really had very little corrosion on them, surprising after 11 years.

Now the truck starts and runs normally. Everything electrical works as it should: horn, headlights hi and low beam, tail lights, turn signals, brake lights, interior lights, A/C, and radio.

But -- with everything turned off and the negative cable disconnected, the multimeter still reads 12.6 volts between the battery negative post and the end of the cable. Shouldn't that be zero? Or is it normal due to some feature I don't understand?

You guys are really welcoming and helpful. It's great.

 

You mean from the Negative post to the body or engine Ground?

 

body computer draws a little, keyless entry receiver would draw a little as well
parasitic draw it looked for via current, not voltage.

 

From the negative battery post to the terminal on the end of the black cable, 12.6 volts.

So CraigF, are you saying it is normal for the computer to use some electricity even when everything is turned off?

No keyless entry on this truck, unless somebody breaks in. Remember, I have hand crank windows. When Toyota said this was the base model, they meant really basic.

I love my Tacoma, but I miss the simplicity of my '51 Ford F-1 pickup.

 

Both cables are black...
If you had 12.6v from the negative post to Ground nothing would work.

Got pics of where your testing?

 

12.6 volts between negative post and ground. Everything works.

 

Yea that's normal, if you wanted to check parasitic draw you would have to set your meter to current and move your positive lead to the amps port on your meter (don't forget to move it back when you are done).
Typical current draw after everything goes to sleep (which can take up to an hour on some vehicles) should be less than 50 mA (0.05A).

If you aren't experiencing a dead battery when the truck sits several days you almost certainly don't have a parasitic draw issue.

 

Thanks, Dm93. It's a relief to know I am seeing something normal. Measuring current the meter shows only 1 mA, truck was turned off overnight.

I suspected a short because the battery was completely dead after sitting for several days, and would not take a charge, consistent with discharging through a short circuit. Turned out that somebody left an interior light on. The old battery was 11 years old, so about due for replacement anyway. Better for this to happen now than in mid winter.

I still miss my super simple '51 Ford.

 

Yea between the light being left on and the battery being that old a new battery should fix it.

11 years would be unheard of for a battery where I'm at, average lifespan for a battery here is 3 years. Oldest working battery I've seen was 10 years old.

 

3 years is about it where I am

 

3 years? Yikes! Does the heat in Texas wear them out?

11 years is exceptional here in Virginia, but I have usually gotten 6 or 7.

 

There's many factors but heat is a major one.
Quality of batteries (like everything else) is nowhere near what it used to be, I've seen batteries less than a year old that were crap already.

Also modern vehicles demand alot more from a battery than old vehicles that really didn't need it for much more than starting.