Electrical Car Died On Highway - Ac / Electrical Issue

GT99BLACK

Member
Apr 15, 2012
31
1
9
Quick history and current issue:

My 99 GT had a battery drain when I purchased the car. The previous owner was more clueless than me (bad tune, bad engine ground, did not care about battery drain) so I got a good deal on the car and quickly fixed the engine ground and found the drain (with the help of members on this forum, especially wmburns). I have not had any electrical problems since that drain other than slight rpm reduction/lights dimming when I put windows down. Fast forward to this week when I talked my wife into taking a road trip in the Mustang. I rarely use my AC so I had it on full blast on the highway. All of a sudden my battery gauge went to 0 and soon I was rolling to a stop on the highway with a bad alternator and dead battery. I started by having the battery checked and fully charged by Autozone, no problem there. I had the alternator tested at Advance Auto and it was bad so I bought a remanufactured alternator and installed it today (thanks to this forum again). When I was installing the new alt, I noticed the coolant temp sensor was toast (picture below). I thought it might not be a coincidence that the alt went and the coolant temp sensor went at the same time. After I installed all this, I took the car to have the battery and charging system tested at Advance Auto. Advance said the battery is good but the alt is at 12.4 volts without headlights on and 12.8 with headlights on. The guy there said I might need a different new alternator and that it is weird voltage went up but he had no clue why. I should have had alt checked before I installed but it was the only one there. Also, when I turned the AC on, the car slowly stalled and died (I had this issue before but my tuner fixed the issue). His idea is that my AC clutch/compressor is bad and it is causing massive drag, enough to completely kill my car. There is some good news since new alternator was installed (although with voltage where it is, I'm not sure if that's the reason). When I used to turn my lights on or roll my windows down, there would be an RPM reduction, now the RPM doesn't change at all when I have headlights on or put windows down. I have searched the forums and checked connections but was wondering if anyone has any ideas.

Thanks so much. (Pics below)

Old Ground/Tray:
oldground-tray_zps5d450cae.jpg


New Ground/Tray:
newground-tray_zps141943b3.jpg


Old Sensor:
oldsensor_zpse3056c7d.jpg


New Sensor:
newsensor_zpse00c9b21.jpg


New Alt/Sensor:
photo_zps6d3c3500.jpg
 
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COramprat

Drone Driver
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Mar 2, 2003
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If it is killing the car with the AC on then it is likely that is the culprit. You said your tuner fixed it before. How? Does your battery light come on? Look at the alt plug and make sure the wiring is good. I'm having an issue with my car where the wire has a short and I have to manipulate it a bit to keep the alternator charging. I've ordered a new harness and have to wait until it get here to fix my problem.

These reman alts are junk as far as I'm concerned. I've all all sorts of issues with them.
 

GT99BLACK

Member
Apr 15, 2012
31
1
9
Thanks COramprat, I am not a tuning expert but I believe there is a setting for the AC when a car is tuned and I believe he increased the idle with the AC on. The battery light does not come on. I took the alt to the store today and had it tested, all good. I also had a battery under warranty so I replaced it with a brand new battery that I also had tested and is 100% good to go. I might address the AC compressor next or just need to get it up on jackstands and look for ground/wiring issues. I appreciate the advice and will take any more.
 

trinity_gt

10 Year Member
Jan 31, 2003
3,125
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I might be misreading things here but I don't believe the electrical system for the A/C compressor would be capable of drawing enough current from the car's electrics to cause a complete system brown-out and stall: The wiring gauge simply in the compressor coil and feedwires is simply too small to sustain such a current and the circuit is fused anyway so you're only going to draw maybe 15A before popping a fuse anyway.

It sort of sounds like a mechanical problem: Perhaps the front bearing on the A/C compressor is seizing...
 

GT99BLACK

Member
Apr 15, 2012
31
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Trinity, so the A/C could be blowing ice cold but the compressor could be bad? Would the seized bearing make the pulley cause drag? I'm just trying to figure out how the alt went bad and the battery drained from the seized compressor. My car was running strong on the highway and then all of a sudden sh*t the bed. Thanks for the input.
 

trinity_gt

10 Year Member
Jan 31, 2003
3,125
80
99
Canada
Trinity, so the A/C could be blowing ice cold but the compressor could be bad? Would the seized bearing make the pulley cause drag? I'm just trying to figure out how the alt went bad and the battery drained from the seized compressor. My car was running strong on the highway and then all of a sudden sh*t the bed. Thanks for the input.
It sort of sounds like you've got or had a couple of separate issues. The initial sudden death of the car on the highway -- voltmeter to zero, everything dead -- sounds like a catastrophic internal battery fault. Sometimes you see stories of people that drive their car to the store, come out a minute or two later and the thing is completely dead. The battery develops an internal short and drains itself in seconds.

But I don't see how an A/C clutch solenoid can draw enough current from a large lead-acid battery, through relatively thin conductors and through a fuse, to kill the electricals of the car. That aspect of your problem(s) -- "when I turned the AC on, the car slowly stalled and died" -- sounds a bit like the A/C compressor itself being a large mechanical draw on the engine. You might try removing the accessory belt and running the engine without it while cycling the A/C compressor. If the problem is electrical it would seem that the engine should still have trouble since the solenoid is still active. If the problem goes away there could be a mech issue in the compressor. (Don't run the engine too long without the drive belt; keep an eye on the temperature gauge...)
 

GT99BLACK

Member
Apr 15, 2012
31
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Trinity, my plan is to check all the pulleys while the alternator is out. I already can tell that the upper idler pulley is a bit wobbly and I may need a new tensioner pulley (missing the bolt to release tension, had to use a temp bolt). Once I have checked/replaced the pulleys, I'm going to decide whether to get a new compressor or run the car without AC. I was trying to connect the dots on the AC/Alt going bad and complete battery drain but based on what you're saying they may not be related. I just think it's weird that I never run my AC but when I did on the highway the alt went bad. I have been daily driving the car every day to work (short distance & not using AC) for the past 9 months or so with no battery/alt issues. Anyway, starting fresh with new battery, new alt and pulleys should be a good start but I think I need to start searching for a short in the electrical system.
 

COramprat

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If the bearing were going bad on the compressor, generally, you would hear it. Bearings do not go quietly from my experience. I wouldn't rule out something electrical in the compressor clutch although I agree with what trinity says...but if the clutch were bad it could cause enough drain on the system to shut the car down. These cars are real finicky when their power isn't near perfect.

Check the pulley on the compressor and see if it is solid...mine was falling apart right before I changed it. It's a moderately difficult job but can be done under the carport. There is a thread on the replacement of the compressor clutch in the 4.6 tech thread in this section.
 

GT99BLACK

Member
Apr 15, 2012
31
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If the bearing were going bad on the compressor, generally, you would hear it. Bearings do not go quietly from my experience. I wouldn't rule out something electrical in the compressor clutch although I agree with what trinity says...but if the clutch were bad it could cause enough drain on the system to shut the car down. These cars are real finicky when their power isn't near perfect.

Check the pulley on the compressor and see if it is solid...mine was falling apart right before I changed it. It's a moderately difficult job but can be done under the carport. There is a thread on the replacement of the compressor clutch in the 4.6 tech thread in this section.
The pulley seems smooth. The idler pulley was wobbly though and tensioner was missing a bolt so I picked these up:

image_zpsd26ad1b6.jpg
 

COramprat

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Rereading your issue, if the car is shutting down, then yes there is an electrical fault somewhere. It may just be coincidence that the AC was on when it shutdown. The plug to the coolant sensor might be a clue. I'm surprised you aren't having fuses blow...but then again maybe not. I had an issue with my 96. Driving down the road and it would just shut down. The last time it happened I had to have it towed home. At this point it was blowing a PCM fuse. One of those big 40 amp boogers. It was also setting off the theft system and counting the theft light blinks it was telling me no communication with the PCM (obviously). I looked backed on what had been done to the car recently and I had the transmission repaired after I broke a shift fork. So I crawled under the car and found one of the O2 sensor wires was rubbing on the tray the supported the connector. It had worn to bare wire. I taped it up with electrical tape, replaced the fuse and it fired up with no more issues.

You may have an intermittent short similar to what I had. Movement of the car will manipulate the wire causing the short one minute and it will be fine the next. Think about any work you had done recently and work your way from there. I'd pay particular attention to the coolant sensor wire 1st.
 

GT99BLACK

Member
Apr 15, 2012
31
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Thanks for the info. Ill def check where some recent work was done. Got the new battery, alt and idler pulley in, battery tested at 12.94 volts, 13.97 with no load and 13.9 with high beams. AC definitely an issue so not worried about it until I fix it. Thanks for the help everyone.
 

wmburns

SN Certified Technician
Aug 14, 2009
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If looking for an explanation how the AC could cause the motor to die, consider IAC/Idle trim. The PCM maintains two idle trim settings. AC on and AC off. After a battery disconnect (or dead battery), the PCM needs time to re-learn new idle trim values. Therefore it would be normal for the idle to be poor after a battery disconnect.

If the AC is low on Freon this may make the AC short cycle. If true, this will PREVENT the PCM from re-learning new AC on idle trim values. The result is an idle that's too low when the AC kicks in. The symptom maybe a car that runs fine until the AC is turned on.

IMO this may be another example of why s STRONG battery and charging system is vital to today's cars. That's why I believe that every electrical problem should start with a inspection of the battery and charging system.

If looking for a "quick fix" work around, consider putting a temporary jumper across the AC low pressure switch. This will force the AC to run even though the pressure is too low. Let the AC run for a few minutes with the motor hot. This should allow the PCM to re-learn new AC on idle trim values.

Remove the low pressure jumper so as not to freeze the AC coil.
 
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