Where should I start looking at this electrical problem?

[race bannon]

Hello, I have been driving my '91 for a year and a half trouble free years!
I had an issue this weekend that has me puzzled. Drove '91 XLT to coast(approx 70min drive) for the weekend. No issues while driving but when I got there and shut off truck, 5mins later I couldn't get power lock to open the other doors. I put key in and NO power at all on anything, even the tiny underhood bulb was dead. Jumped it off and it cranked up was driving it to town and it died as I was coming to a stop(braking), jumped it again and turned around made it back to house (1/2mi). Got my hands on another battery from a marine mechanic and it cranked right up. Put my battery on charge and it said it was at 10% and trickle charged it overnight.

With new marine battery in I again cranked it and pulled the battery cables off the posts and engine never missed a beat. With battery disconnected I sat in seat to do a quick assesment of accesory usage. I put my foot on the brake and it caused a slight rev in RPM's, gauges jumped and it killed the engine. I reconnected battery and did the same thing only this time I disconnected battery and drove down driveway and as soon as I stopped at street the brakes killed the engine. I connected battery and cranked it up, disconnected battery and without touching brakes, turned on headlamps only a slight twitch on the voltage gauge and back to normal. I turned off the lights and tapped the brake and it wanted to choke down but I released the break and it idled back up, I then turned on the headlamps and it instantly killed the engine. I repeated this with new battery connected and brakes or lights or both had no effect on it. I was able to drive 70mins home today with the borrowed battery and mine as a back-up if things went bad. I made it back OK with no issues. My voltage gauge on the dash reads 14+/- volts the entire time. What should I start doing to run this down?

 

[Iron Weasel]

Take your old battery in to Autozone or some place and have it checked - make sure they load test it. Initially, I'm thinking your battery crapped out.

Might also want to verify the alternator output with a multimeter as well just to make sure it's actually putting out 14v+.

 

[StryctNy9e]

There was a few stories on here about peoples first gens would not turn off unless they toggled the lights on/off. I know it sounds bogus, but they replaced thier headlight switch and all went back to normal. I would think maybe the headlights are ran parallel with the ignition so they are somehow tied together or connected in the wiring along the electrical somewhere. I would say replace the switch, but not sure if my reply hepled you. I am just shooting things out there.Good Luck.

 

[Roadrunner777]

Old battery did not charge on trip out... New battery and everything's good. Truck runs without battery, charging system is good. There could be an intermittent in alternator or cables we are not catching, but first pass, definitely have the old battery charged and load tested. I think it's gonner.

 

[race bannon]

Autozone load tested my original battery after I charged it Saturday Night at 12.X volts

Alternator checked at idle at 14.6 volts. I am now officially confused.

 

[Iron Weasel]

Start at the terminal ends on the battery and work back from there. Check for corrosion, worn insulation, breaks in the wires, etc.

 

[Roadrunner777]

Look hard at the cables, peel back insulation, make sure there is actual copper. If you are the systematic type, get a schematic of the starting/charging system (chiltons is ok for this) and start making running voltage measurements, at the alternator, then moving towards the battery. Wiggle connections as you go. You may get to one where you wiggle it and the engine suddenly loads down. That's your big clue!

 

[DV8]

check your grounds. had a f150 that did the same thing, and it was the main ground cable. corrosion from the battery had worked its way halfway down the cable, and there wasnt much making connection. replaced the cable and all was well... its worth a shot, and only costs about 12 bucks

 

[race bannon]

Will do, this is bizarre, but a shaky ground / cable would make sense. The brake choking it down has me puzzled.

 

[Tony H]

Everything pointed to a posible bad battery connection. I am with you as far as being puzzled. Not sure why the slight Bulb drain of teh Brake lights would cause the engine to Stall. The alternator should be able to run the engine and have you step on the brakes.
The alternator might be failing. Check the battery voltage with engine of 12.5V

Start the car, see if battery voltage is 13.5 or more. If it isn't then the Alternator is not happy. And te Fuel injectors can't run well on a low voltage.

 

[Roadrunner777]

Voltage was weak to start out with... brakes do cause a reduction in manifold vacuum, usually not noticable, but if he was on the edge, maybe.

 

[race bannon]

Update 6/10

My truck has been sitting unused since Monday AM at work.

I placed multi-meter on alternator at idle with pos lead (on nut diode with rubber cap on back of alternator) and (neg lead on the alum. body). It read 14.49 volts.

Back to battery read 14.44 volts, at idle.

Auto zone tested my Battery today as well and after a week it was still at 12.69 volts. With truck running and lights, radio, and defrost on high, alternator still checked out OK, 14.12V with all the accessories and then 14.38 with no load.

If I had the perfect corrosion on my batt. posts and connection that could have let my battery die. Once I took out my battery out and placed new one in I could have moved the terminals where there was no corrosion and it ran fine with new battery. After cleaning my terminals and posts (no visible corrosion) it seems to run fine as well. The only thing I can't figure out is why did it choke down without a battery in it or with a dead battery when I touched the brake and headlights if alternator is working properly? Or I might not have a problem and I could just be an idiot who didn't check for corrosion first.

 

[Roadrunner777]

I'm going with the corrosion answer. Why did the engine load down... I have two very questionable maybe's:

1. The ECM senses over voltage from the alternator open load and reduces engine speed to protect components from an overvoltage condition. If the battery has a poor condition, this could also appear to be an open load on the alternator.

2. The alternator uses voltage to set up a magnetic field, which is, in a sense, amplified by the rotary motion, and output. Often, there is an internal connection in the alternator so the output voltage is fed back and used as the input rotor voltage. In our case, the input rotor voltage is actually provided by an external circuit that goes through a fuse in the power distribution box. I think this is to protect the wiring against a shorted rotor.
If there were resistance in this circuit, it would reduce the ability of the alternator to create power in a no battery situation.

If #2 makes no sense to you, it's ok, it barely makes sense to me, and I still see some holes in my theory. I'm going with reason #1.