Battery Electrical Advice

Karter18 · 08-16-2014, 10:19 AM

I have a relatively new to me 87 Carrera. It had sat for a few years so I changed the battery and have been keeping it on a tender. The tender has been reading red for a long time each week but if I unplug and replug it usually goes to green right away. Last week I decided to leave the tender off and went to start it this week and nothing totally flat. I jumped it off the wife's car and drive it about 15 minutes. Tried to start it again and got about half a crank and then had to bump start it. When I got home I put a voltmeter on the battery while the car was running at idle in the garage with the reading shown below. I then put the voltmeter on the battery after I shut the car off as below.

First, I am not an electrical genius but it looks like the car is reading 20 volts on and off?

Next there is no difference between on and off so this leads me to believe that I have an alternator issue? If so can these be rebuilt or is it best to get a new one? Is there another way to test this? Go to an auto parts store or garage and see if the battery is ok and if the alternator is working?

Also, is there a way I can tell if there is something draining the battery while the car sits. I usually leave it in the garage with the windows open and the doors unlocked. There is not an alarm option on this car. I put new hood struts on the hood but I assume the light is going off when the hood is closed but I cannot check. I did find a couple of connectors that were unplugged and I plugged them back in. One was by the passenger headlight in the trunk. The other was above the fuse box and I found it while putting hood strut in.

Any help is appreciated. Thank You.

steely · 11:01 AM

I would recommend that you test the meter on your wife's car battery - just to see if it is sane.
I would also have the battery checked before moving on to checking the alternator & internal voltage regulator - which can be rebuilt.

If you are interested in checking drain, I would do a search here. With a good battery, you'd disconnect the battery NEG terminal, attach the (+) side of the meter to it, and the (-) side to the battery cable. I think at that point you pull fuses while keeping track of them to see which results in a current reading drop.

I see that your meter goes to 500mA or 1/2 an Amp. If you happen to draw more, you will blow the internal meter fuse.

mrdieseldude · 08-16-2014, 12:07 PM

If you are getting 20 volts with the car running, you have a bad regulator. It should not allow voltage to rise that high. A 12 V battery is going to have a tough time holding that kind of charge (with the engine off). Are you sure the meter is working properly? As steely suggests... put the meter on the wife's car. With it running, you should see around 14 volts. First things first.... and yes, your alternator can be repaired, and if required, I'm sure that is the route you will want to go (but that may not even be your problem).

Michael

Justin@Athens · Justin@Athens's Garage

I am going to surmise a bad voltage regulator as well, most likely you have killed this battery as well. Not a huge job, but not exactly putting air in your tires either. The VR is located on the back of the alternator, and I would take it the alternator to the auto parts store for testing as well. They most likely won't be able to tell you anything about the regulator, but can insure the alternator is functioning.

Karter18 · 02:05 PM

All right I had some user error on the voltmeter as the lead was not in the DC lead. Plus I think the thing is toast so off to get another one of those.

Took the battery to Autozone and they checked it under load and it seems ok but the charge was very low. They are charging it now so I will pick it up in a couple of hours.

I started to do the fuse checking as suggested above. I disconnected the negative terminal and then put the voltmeter on a setting resistance between the negative terminal and the cable. When I turned up the resistance the car acted like the terminal was connected with the trunk light coming on, etc.

Next I started to plug and unplug fuses and everything stayed fine until the fourth from the top which is the:

clock - Has not been consistent since I got the car and only seems to move when the car is running. I just checked the wiring and it is as pictured which seems strange to have the black wire plugged into the +12V? See Photo. Also I put all new LED's into the dash lights as well. Edit: Upon some searching it appears that the 12V wire is just in a series for that fuse so it goes to the clock and then to the next item such as radio, etc. so this may be correct as in the photo.
Glove compartment light - Seems to work ok
Interior Light - Passenger side always worked and drivers side started working or I never noticed it. They both only work with the passenger side door.
Luggage Light - Replaced the bulb and it works. One bulb already went out and replaced that one. Not sure if it is staying on with the trunk closed. It used to go off when the hood was unlatched before I put the new struts in.
Radio - Seems to work fine except skipping on the bots dots.

After this fuse was unplugged the voltmeter stopped working and I pulled the battery to go get it checked out.

Thanks for the responses.

mrdieseldude · 08-16-2014, 03:01 PM

The luggage compartment lamp should have a push button switch that is very obvious to see. You can tell if this shuts the lamp down by depressing it. If the lamp stay illuminated... that is a problem.

Michael

PS - if you've discharged your battery.... even once, into the 11 volt range (there abouts) its shot (or soon will be). A Battery Tender is cheap... replacing batteries gets expensive. I buy'em by the 6 pack.

mrdieseldude · 08-16-2014, 03:04 PM

Quote:

Originally Posted by Karter18

... seems strange to have the black wire plugged into the +12V? See Photo. .

gotta love those Germans

steely · 08-16-2014, 06:21 PM

Quote:

Originally Posted by Karter18

I disconnected the negative terminal and then put the voltmeter on a setting resistance between the negative terminal and the cable.

Use the current mode instead when you do this, not resistance. Set the knob to the highest DCmA range. Do not let the leads touch anything and do not let any part of yourself get in between them. The mA reading is thousandths of an Amp. I think 200mA may be normal, but please do a search. Good luck

Karter18 · 08-16-2014, 07:46 PM

Steely I used volts with the leads on the negative terminal and the negative post. Everything read 12.70 Volts until I pulled that same fuse and then it dropped to 12.00 Volts.

I did find that the dome light for the truck was staying on as the back of the fixture was grounding to the hood a staying on even with the hood down. I filmed it with my phone while closing the hood for confirmation. I am not sure why this would be happening as there is a white nylon casket around the fixture. I put some stick on motorcycle rubber adhesive tank pad on the inside where it was grounding and added two washers to the screws and that seems to do the trick as the light now goes off. I am sure this is what drained the battery and caused the battery tender to not go green a lot of the time. Heck maybe even the clock will work now

I put it all back together except the clock and have it on the tender after getting the battery charged and hope to see a green light on the tender in the morning. Maybe this will solve the problems? I guess the only way to find out is to disconnect the negative terminal again and do the test with the fuse.

I will try it with the method Steely mentioned above. Thanks for everyone's assistance.

mrdieseldude · 08:45 AM

The lit trunk lamp could easily be your problem. I seldom believe the story that the problem was caused by numerous issues (could be..., but not the norm). That said, one problem can cause another problem. If you drained the battery sufficiently, it will be damaged. Hopefully the BT was able to keep the voltage up enough so that this didn't happen.

The descriptions you give continue to not make sense though. If you are measuring system voltage and you read x voltage, and then you disconnect a load (by pulling a fuse), the voltage should not go down. If anything, depending on the size of the load and the source, it should remain the same or increase.... maybe I'm not following the thread closely enough?

By the by... you are checking these voltages with the BT disconnected? Correct?

steely · 08-17-2014, 12:02 PM

Quote:

Originally Posted by mrdieseldude

The lit trunk lamp could easily be your problem. ....

If you are measuring system voltage and you read x voltage, and then you disconnect a load (by pulling a fuse), the voltage should not go down. If anything, depending on the size of the load and the source, it should remain the same or increase.... maybe I'm not following the thread closely enough?

I agree with Mr. DD.

This is the setup (below - if it comes thru and is readable) I thought you were interested in, I am not sure what you are doing. (I suggested Current, not resistance or Voltage). It all may be moot if the light was stuck on and not switched off.

Karter18 · 08-19-2014, 07:58 PM

Yeah I did the voltage test with the negative terminal disconnected and that was when it dropped before I fixed the trunk light. Hopefully the battery is ok as I took it to autozone and had them charge it and they checked it under load before and after the charging and it tested ok. I will try it out this week and see if it's ok as it is relatively new in case I need to take it back. The voltage dropped from 12.7 to 12 when I pulled the fuse on a new voltmeter digital which is way easier to read.

Thanks for everyone's help so far! Here is the obligatory photo os the subject car. New tires, shocks, turbo tie rod ends and it drives excellent and hopefully the battery starts her up this week.