Okay so I've never posted to a forum before because I can usually figure everything out by fiddling with stuff or a simple Google search. So this is weird for me. Here's the back story on my 1990 turbo miata.

I bought the car from a guy with the turbo kit installed and long story short, he ripped me off. It had a 1993 engine in it, stock internals, Garrett turbo, and I later found out it needed more maintenance than he previously stated. It needed a full rebuild. So instead of trying to rip someone else off or trying to sell it for a loss, I rebuilt it. Forged internals with a brand new Garrett turbo and I went and got it Dyno tuned.

So here's where my problems start. When the car warmed up, I would be only getting about a 13.1V reading from the ECU to the tuning software (Hydra nemesis 2.7). I always drive with the software open to periodically check on everything. The TPS (BMW TPS convertion) was acting up and would sometimes give no throttle reading. Replaced that. Replaced alternator and no change in voltage. Driving down the highway the alternator belt came loose and I tried to make it home and it completely came off. That was completely my fault due to stupidity. Barely made it back. I fixed the belt issue and it's all good now. But the battery charge light is now on. I used a multimeter to measure Voltage coming off the alternator with the car was at idle and it is 14.6v. Voltage on the battery is the same. Readings from the ECU were still 13.1. So I went through some grounds and cleaned them and replaced some. Voltage now between 13.5-13.6 at idle and doesn't change with any throttle. That's more than the cars ever had while I've owned it but the battery charge light Is still on. I'm stumped on what could be causing that charge light to not go away. I also would love to know what could be causing me to not get optimal voltage to the vehicle when the battery and the alternator are reading good.

Please help me out. I absolutely hate dealing with wires. Thank you.

 

The charge light doesn't detect "optimal voltage." It just detects whether the voltage at the battery is higher than the voltage at the alternator, which is a bad sign if the car is on. Since this circuit involves a diode, the delta must be larger than the forward voltage across the diode and still allow enough current to flow through the bulb.

Three main possibilities:

• The diode in the cluster has failed "shorted." This will cause it to light up under normal operation.
• The small bundle of wires going to the plug on the alternator is damaged. There could be a short that allows current to flow from the wire originating at the cluster directly to ground.
• Your alternator is damaged in such a way that the external field activiation (which comes from the charging light) can still consume current while the alternator is charging. This would be from internal shorts in the alternator, and is unlikely.

Another thing to look at for the ECU voltage is to probe the ECU power input while the car is running, but referenced to ground. The Hydra (or any ECU) will just report the voltage difference between its power and ground pins. Therefore, reduced voltage can be caused by poor grounding of the ECU. Unstable TPS readings are also a potential consequence of poor ground setups (not just cleanliness) for aftermarket ECU installs.