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1990 mustang charging & starting problem

danshenberger danshenberger
New User | Posts: 3 | Joined: 11/10
Posted: 11/26/10
06:53 AM

I have a 90 mustang 5.0 that has an unusual charging and starting problem. Ever since I have owned the car, the battery slowly drains. After a full trickle charge, the car can sit for a week or two, and I can still start it, but after that time period, the battery is dead. This is only one half of the problem however. After a full trickle charge, I can start and run the car just fine, but if I shut off the car it wont immediately start back up. It's as if the battery has suddenly lost all of it's charge and the starter will barely crank. However, if I wait several hours, I can start the car as if the battery is fully charged. I recently used a multimeter to check the battery voltage after a trickle charge, and while the car was running. The voltmeter read just over 12 volts at the battery. I increased and decreased the engine revs a few times during my check and noticed some minor increases and decreases in voltage. When I shut the car off, the battery voltage dropped immediately to 10 volts. I tried starting the car, and as usual, it wont start. Any suggestions?  

waynep7122 waynep7122
Addict | Posts: 4545 | Joined: 08/09
Posted: 11/26/10
10:15 AM

with a digital volt meter........   set to 20 volt DC scale...  turn on the headlights for one minute.. then turn them off and test the battery voltage...

12.65 is a full charge..
12.45 is a half charge.
12.25 is a quarter charge..

you need to have over half a charge to properly to the tests..  this only takes 2 minutes ..  i have done it many times..

get the engine started....  turn on the headlights...

set your digital volt meter to 20 volts DC...

1.   test between the battery posts...   should be about 14.1 to 14.6 volts

2.   test from the negative battery post to the engine block..    0.04 volts is expected...

3.   test from the negative battery post to the body..................0.02 volts is expected

4.  test from the engine block to the firewall...........................0.02 volts is expected.

additional tests...

5.   test from the positive battery post.. to the starter solenoid stud...    0.02 to 0.04...

6.   test from the positive battery post..  to the alternator wiring if you can punch a tiny hole in the larger wires sticking out the side...


7...  engine off...  headlights off...   test from the negative battery post to the engine block...  have somebody crank the motor while you watch the volt meter..    you might get upwards of one volt .. but post the volt reading while cranking..  i would think that anything over half a volt would indicate a ground cable problem between the engine block and the battery negative.

8..    does your digital volt meter have a 10 or 15 amp DC setting?????  if so...  engine off.. everything off.. drivers window down..  disconnect the negative battery cable....  test from the disconnected battery terminal to the disconnected battery cable.....   if you get more than 0.050 amps DC.. you have something in the car as a parasitic drain...    be sure when done.. to move the test leads back to the normal locations in the volt meter so you don't damage something when testing later...

why do this
tests 1. through 6 are done with the engine running.. and the headlights on.... if there is resistance in a connection..  there will be different voltages when measured...  this is the first test that mechanics learn in class... but many people just don't understand how important it is...    and it has to be done with a digital volt meter...

when you get a 0.02 volts DC or 0.04 volts DC measurement .. that shows a normal voltage drop on an working circuit..

when you get 0.00...  you might not be doing it right...  

when you get more than above...   you have found the problem...

remember on your mustang...  all the power to the various parts of the car is supplied through the starter solenoid post..

your mustang might also still have the original plug in the wiring harness to the alternator.. it will have 2 large wires and one small wire.. see if a new one has been crimped into the harness..  the original ones failed.. and caused many problems...

dirty battery terminals.. loose battery cables   dirty corroded connections at the starter relay... will cause increased resistance and burn out the alternator...   be sure to try to get an alternator with a life time warrantee from someplace.. they do fail again and again...  if anything is not right..  that alternator also DOES not like to charge a deeply discharged battery .. or jump start other cars...   if you are letting the car to sit for a week between starts...  you really want to install a battery cut off switch...

like this...

post what your volt meter readings are...  

prostock426 prostock426
New User | Posts: 21 | Joined: 08/10
Posted: 11/27/10
01:33 PM

You have a parasitic battery drain. Open the link below and follow the instructions exactly.  
The earth was created in eight days and not seven. On the eighth day GOD realized he needed help, so he called in THE UNITED STATES MARINE CORPS.

danshenberger danshenberger
New User | Posts: 3 | Joined: 11/10
Posted: 12/20/10
09:36 AM

Thanks a million for the info and the help. I went over the car and found several problems.
1. The original alternator wiring was still intact. I found the upgrade kit at mustangs unlimited. I'll be ordering that soon.
2. The positive battery cable was corroded. I couldnt see the corrosion at first, because it was underneath the insulation at the battery connector. I simply bent the cable slightly at the connector and could see lots of corrosion inside the insulation. I cut the cable back to an area where no more corrosion existed, and I replaced the connector with a battery disconnect switch.
3. The negative cable had some corrosion too. I cleaned the connector and reinstalled it.
4. All the electrical tests appeared OK after I cleaned the negative connector and replaced the positive connector. I'm going to replace the alternator wiring in a few weeks and re-test to confirm the results.  

waynep7122 waynep7122
Addict | Posts: 4545 | Joined: 08/09
Posted: 12/20/10
03:18 PM

the alternator wiring harness is available at EVERY parts store if you know the part number to ask for...

this is the autozone or wells part number

Duralast/Connector - Alternator
For your 1990 Ford Mustang 5.0L FI HP 8cyl
Part Number: 232

For Ford alternators
Part Number: 232

here is a link to the image of it..

post what auto parts stores you have nearby.. i might be able to find you a part number...

from some stores who sell borg warner...
BWD - Alternator Connector
Part Number: PT237

you also want to add a tool like this to your tool box.. for crimping the wires properly..

these cost about 25 bucks...   there are imported versions for 8 to 10...   OSH hardware stores have an excellent version in OSH brand with green handles...

almost as good as the channellock 909 crimpers... i bought 3 pair of the osh versions when they were onsale..

you won't believe that you never had a set of these before..  

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