Ok, nomrally I would start poking away at problems myself, but I don't even have the foggiest on where to start with this one and have never seen/heard of it anywhere as well as searching.

All the voltage/resistance reading guages and sensors on my NB1 read slightly high. (Anything that reads a PWM or pulsed signal seems fine). They do this only when the alternator is running. On batter, they are fine.

For example, my dash temp guage reads at noon normally. If the alternator is not running, it reads at 11:30 like every other miata. The OBD2 temp sebsor does the same. Reads 195-200 with the alternator not spinning, and 215-220 with it spinning. Same with MAT, gas gage, all the sensors reporting through the OBD2 port. I am baffled, cause charging voltage is 13.8-14.2 just like normal.

I honestly have zero clue even where to look. It's bone stock other than stupid ebay/parts store intake header and exhaust put on by the PO. It did this with the original alterbator, and a replacement I put in.

Any ideas on what to even check?

 

I would start by rechecking the charging voltage. Assuming you still run the stock ECU, NB alternators are controlled by the ECU and any added resistance between the alternator and the ECU (yes, I'm looking at you, Main Relay) drives the ECU to raise the alternator voltage. I have seen up to 15.8V.

 

I will check again, but it was good when I replaced the alternator (first replacement I got was DOA). ECU is stock.

 

I have not had luck with Bosch alternators and their voltage regulators. I don't know what brand you got but I would definitely check it.

 

Alternator is some cheap one. The voltage regulator is built into the ECU on an NB1.

It also did this on what was the original alternator. That was working fine but had to be replaced because somehow the PO managed to bend the shaft in it and get the pivot bolt seized in a position where it was loose. The bushings finally loosened up enough from the shaking that started eating/throwing belts.

 

I forgot NB alternator. Ignore me

 

I would check the engine ground strap, as well as the ground connection at the very back of the PPF, by the diff.

Better yet, get a good jumper cable and run it from the battery negative to the engine block (or the PPF if you have a hard time reaching all the way to the engine) and see if there is a difference in gauge readings.
You can just hook on and remove the cable while the engine is running to see if the gauges fluctuate.

 

+1 on it being a bad ground.

--Ian

 

+2 on bad ground. If you have a multimeter lying around, you should be able to check the resistance (ohms) on the ground straps you think may be "bad"

 

Hadn't even considered grounds. I will look into that!

Brilliant idea on the jumper cable. I will give that a try and see how that goes. I keep forgetting the alternator grounds through the engine block.

 

Finally got a chance to try the jumper cable. Grounding the engine to the battery does nothing.