Electrical Problems I was working with the Aux.Battery and the OE isolator was changed with a Ford Type starter relay. the charge was not working and I had moved the wires around and hooked the lead from the fuse block to the fuse block and got 12.3 volts to the aux bat. but I am not sure what caused the problems but volts from the alternator are 14.9 at the Main Battery I show 12.3 with the engine running , turn the head lamps on and it dies. the check eng. light wont light up and cant get code to flash when grounding the ALDL Terminals A & B but the light will show when grounding only engine running but its a fast flashing which only means not with engine running lol... I have taken the Aux Battery out and disconnected the wiring. I can start and get the lights on without killing the engine but its at random. the volts are still dropping from alternator to the battery approx 4'of 12 gage wire. what are the possible causes and damaged parts for the fix??? I have checked the light switch. and all the fuses are good. the ign.switch has not been checked but the wires under the dash are all ok non burnt. If anyone has had this or close to please suggestions will help. Thanks Del B

Make sure battery has a full charge and battery connections all good. Check fusible links where applicable. Basically, you check voltage and grounds for the system. Check the fuses in the fuse box for voltage, use a test light. Some fuses are hot all the time, some hot with the key on. If the problem is ignition feed, meaning it goes hot with the key on, you may have to check feed to and from ignition switch. I can only help with testing.

If it was the PATS - anti-theft system you would not have any clicking at the starter . You have connection ,cable or starter problems , most likely . Testing the starter electrical circuit doing voltage drop testing .
Starter Voltage Drop
Do you know what a wiring diagram is ? Free wiring diagrams here
http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model and engine size . Under system click on engine then under subsystem click starting . Click the search buttom then the blue link. Looking at the wiring diagram I see fuse's at the top ,starter relay in the middle an at the bottom the starter , PCM - this is the engine computer . The PCM will energize the starter relay only if it receive's the correct coded message from the PATS module or which module is in charge of that . So you would not even get a click at the starter ,because the relay wouldn't even be energized . You could possibly have a transmission range sensor problem or positive battery cable to tthe starter or ground cable .
How to voltage drop test starter motor circuit
Test don't guess .

Hi,try putting the live on first then the earth.

Check to make sure all the wires are connected, the whole electrical system gets power from the starter connections to the battery.

Ok it seems you isolated and checked the usual suspects and some that I wouldn't have looked at except one. Did you check the neutral safety switch and Alignment on your gears for the gearshift lever. If it is not aligned properly it may not be hitting the neutral safety switch when in neutral and park. Also if the switch has failed. It may not be allowing your system to start when you move the key to the start position. I'm also wondering if this model has a fusable link in the starter cable but you say that you've isolated and tested both the starter and the relay. So I would really suspect that neutral safety switch. Hope this helps.

If it is just the chassis battery that is not charging, then the battery isolators if bad. Check the output at the back of the alternator and if it is puttin out, then the problem should be at the battery isolator which is somewhere on the vehicle (most likely under the hood). Follow the alternator output and it will go to the battery isolator. At the battery isolator the lead coming from the alternator should read at least 13.5-14.5 volts and if it does, then check the other two leads coming off of the isolator and they should also read the same. The two leads coming off of the isolator should each go to a different battery and they both should read the same but if not then the isolator is bad. If the two leads coming off the isolator do read 13.5-14.5 volts, then there is a problem between the isolator wiring and the battery somewhere. Check the battery to engine ground, engine to chassis grounds and make sure that they are good. The chassis battery is more than likely a chassis ground so make sure that is has a clean connection. Coming off of the starter solenoid are several fusible links and they could be open(blown). There could also be other fusible links on the vehicle so check around the starter and along the wiring harness. Check the harness for frayed/broken/corroded/ or discolored wires as they can cause problems. If the charging system is working the batteries do not need to be new in order to read the correct charging voltage.

Double check the battery positive connection wiring carefully as it commonly has connection problems when the battery is changed.

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Your vehicle has a starter relay that could cause that problem. But it could be the starter/solenoid also.

I don't know how whatever accessories you have are hooked up but,

When I would run two batteries for my stereo in my car I had it connected to the auxilary battery. The isolators kinda **** because they are to send more power from the charging system to the battery that needs the most charge. They do not "isolate" as in makeing the system use one battery while not draining the other.

What I did was to hook a solenoid (mine was aftermarket but I think the old style Ford starter solenoids that bolted to a fender will do the same) inline of the battery cable so when my key was off one battery would be cut off from the other. That way I could play the sterio without draining the battery that started the car (the radio was on a rocker switch for when I needed to kill power).

plug the lug wire (Bullet connector into the starter (Bullet connector) from relay and you should be set. the square thing is the starter relay and wehn you turn the key to start mode it powers the relay that then starts the motor. this is a safety feature so the starter don't get stuck trying to start the car all the time. If you need help post to this replay and if helpful please rate this answer.