Summary of issues:

Alternator light goes on erratically after engine replacement
Alternator light goes on at low RPM after fixing prior issue
TPMS light blinks after hard acceleration, but maybe at other times too


For a few weeks I've been dealing with an intermittent alternator light going on and off. This started happening in conjunction with misfiring which led to an engine replacement at 52k miles (2004 RX8). After the engine replacement the light intermittence got worse and worse which led me to find a loose alternator output cable and a loose alternator belt which were both the dealership's fault given that the engine had just been replaced.

Let me add that up until now I was measuring voltage with a DMM at the battery terminals and using the hidden screen on the GPS to see the battery voltage while driving.

The cable going from the alternator to the fuse box was toasted on one end and when bent I could hear it cracking inside so I decided to replace it without buying a new engine harness ( PN F151-67-070F ). I think the original wire was slightly bigger than the gauge 10 I was able to fine so I replace the single wire with 2 paired gauge 10 wire which should be good for 100A or more according to a table on the wire's packaging.

The terminal on the fuse box side was recovered and reused with great effort. The terminal on the alternator side was replace with one I found at napa auto parts. For this terminal I had to get creative... I purchased a copper one made for a huge wire, then I cut a T shape with a small dremmel into the top part of where the wire was supposed to go in so that I created a U shape I could bend down onto the copper wires. Imagine curling down each side of the U to make it look like a B laying on it's back. So... we went from a O shape to a U to a B and each wire was in one of the B's holes. Of course I soldered everything in place with abundant solder.

I was hoping this would fix the issue but the light, although less frequent, always comes on at low RPM of the engine. Over time it looks like it is becoming more frequent.

I purchased a cool application called TORQUE for my Android phone and an ODBII (ODB2) Bluetooth adapter from eBay for less than $30. I logged RM+PM and voltage reported by PCM and voltage reported by the ODBII adapter. All I was able to notice what that when the AC kicks in there is a small downwards spike on the voltage, but other than that I could not see any correlation between voltage and RPM.

One thing I noticed is that the cabin voltage measured by the adapter is about 1V lower than the voltage measured by the ECU. The GPS also measures about 1V less than the voltage measured by the ECU… my DMM confirms the voltage measured by the ECU so it appears there is a 1V voltage drop between the battery/alternator and the cabin.

Can this be the root of my problems? Should there be such a voltage drop?

The main issue is I do not know how the car determines whether the alternator light is supposed to go on or not. If the voltage powering the gauges has anything to do with it then this drop may be part of the issue… if the light is controlled totally by circuitry in the engine bay then the voltages there are fine.

Please help! I’d like to avoid taking my alternator to be rebuilt if I can. Oh and the battery was found to be bad even though my car starts without a hitch by Pep Boys so I replaced it with a top of the line Bosch battery.

Last but not least, my TPMS light starts blinking randomly. The tire pressures are ok, plus an issue with pressure is a fixed light and a beep, not blinking. Is this issue also caused by voltage issues?

All these issues were reported to the dealer when the engine was replaced but they were unable to even replicate them.

 

I purchased a 2007 generator on ebay that had half the miles mine had and installed it. This seems to have resolved my alt light problem... but the TPMS issue is still there if anyone has suggestions.