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Old 10-06-2015, 10:16 PM   #1
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Default Intermittent Electrical Gremlins

Everyone favorite car problem to solve.

Reading some of the other threads got me thinking and has given me some ideas, but I wanted to start my own to get the conversation going because I can't find anything quite like my problem.

History, '92 NA Miata, had dead alternator when I bought it and a voltage gauge installed. Battery charge light was on but it would run. Replaced the alternator, charged at 14.xx on the gauge, but the battery charge light never went out. Mid July I swapped the voltage gauge for a Wideband. Occasionally the WB would drop down to 9-10 afr and the car would run like garbage but it would be fine a few seconds later. One day the check engine light came on, then it wuold only come on intermittently, not always when the AFR's dropped, but sometimes. The code was a water thermistor code. Figured it senses cold air, dumps fuel, explains the rich afr sometimes. Must be intermittent short we caused when we jiggled those wires during wideband install.

Fast forward recently it appears to be developing some type of short, or something else that's causing the voltage to drop to 9v ish when engine is on, and drop even more when engine revved or any additional load is applied to it like brake or headlights. I'm having the alternator checked tomorrow because I think I may have killed it, but there's something more going on here with the charge light being on with a good alt and going off the way it did. Also, the dead alternator when I bought the car, another dead alternator now 7 months later, and a volatage gauge in the car when I bought it leads me to believe theirs a root cause as to why the alternators are dying.

Any suggestions on how to go about finding the true issue?
Steps for tomorrow:
Clean engine, ppf, and battery ground
Have alternator tested
Remove 80A main fuse and check it and wires leading too it

Any help appreciated. Any if anyone in SW Wisconsin is bored I'd love the help.

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Old 10-07-2015, 12:32 AM   #2
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Originally Posted by Plasmiata View Post
Any suggestions on how to go about finding the true issue?...
Get a multi meter and start measuring the voltage at different spots. That's the best way by far to troubleshoot an electrical issue. A volt meter will tell you the voltage difference between whatever two things you touch the probes to. So touch them to the battery + and -, you should see 12V. Touch them to the engine block and chassis, should be 0.00 or maybe 0.01V since they are both grounded together. If they showed 0.3V for example, you would then know they are poorly grounded.
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Old 10-07-2015, 11:51 AM   #3
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replace the refurbished alternator that was never good to begin with.
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