alternator is overcharging

[Sprite]

While installing a new battery and checking voltage with my multimeter I found the Alternator sending 17 volts. Not wanting to fry my ECU I took the cover off of the alt and put a new voltage regulator into the alternator. It is still overcharging at 15 to 16 volts. Is there something else wrong with the alternator? Diode Arrays? Should I just buy a new alternator? What am I missing.

I'm running a stock 94 Silvertop with the stock Toyota ECU with a glass mat Miata battery.

Thanks for any advice.

[jondee86]

That can happen when you have a bad diode and it lets some AC leak thru. Check to see
if you have any AC ripple at the battery terminals when the alternator is charging.

Cheers... jondee86

[Sprite]

Thanks Jondee. I'm guessing I would check for the AC ripple by setting my meter to AC and then any specific voltage range? Would that be a bad diode in the sensing wire coming off the alt ( I have installed one there) or would that be a diode in the alternator itself?

[jondee86]

Your alternator will have a 3-phase rectifier so usually six diodes, What you are looking for is
a "spike" of higher voltage in the rectified output, caused by the breakdown of one of the diodes.
Not all multimeters will be capable of detecting the spike, but if you have a digital meter you
can measure between the battery terminals, starting at high range AC and work your way down
to lower ranges.

A good meter may detect some small degree of AC ripple. That is normal. What you are looking
for is a spike or pulse that occurs irregularly. Here is a good article explaining what goes on
inside your alternator... https://www.scannerdanner.com/media/kun ... torial.pdf

Alteratively (no pun intended) you can extract the rectifier board out of the alternator and test
the individual diodes. If one has continuity in both directions, that is he bad one. Replacement
boards can be had from some repair shops, or salvaged from junkyard alternators.

Cheers... jondee86

[Sprite]

Jon thanks for the article. I did check the alternator for spikes and for ripple AC voltage. My multimeter is not super sophisticated but it showed a consistent AC cycle but no spikes. On an additional test drive the engine developed a bad miss fire under load. Years ago this happened due to a bad coil so I replaced the coil and it cured the miss. I then wondered if the coil failure might have been due to the alt over charging. So I purchased a new alternator also. I installed it and ran an additional ground wire to the alt case just for belt and suspenders fun.

But the new alt is still showing an over charge between 15.8 and 16.10 at idle. I'd rather see the expected 14.5. I know that modern ECUs actually regulate the alternator's voltage but I think this old 1994 ECU is not that smart. Plus my car has very little voltage requirements, the radiator fan probably draws the most but it rarely runs. I might run another ground wire to the coil rather than relying on the bracket to body grounding. Otherwise I'm stumped. I've been reading through the ECU pinouts but not seeing any alternator clues there. The alternator plug has a sense wire, I'm not sure what that function is.

[jondee86]

Ahhhh.... sense wire is important. Regulator can't regulate correctly without it. See here...

An alternator sense wire, often referred to as a "sense" or "reference" wire, is a wire connected to the battery or electrical system of a vehicle that allows the alternator to monitor the voltage level of the battery. This wire plays a crucial role in regulating the output of the alternator to ensure that the battery is charged correctly and that the electrical system operates efficiently.

Functions of the Alternator Sense Wire:

Voltage Regulation: The sense wire provides feedback to the alternator's voltage regulator. By monitoring the voltage at the battery, the alternator can adjust its output to maintain the desired voltage level (usually around 13.5 to 14.5 volts).
Compensation for Voltage Drops: If there are voltage drops due to resistance in the wiring or connections, the sense wire helps the alternator compensate by increasing its output to ensure the battery remains charged.
Improved Charging Efficiency: By having a direct connection to the battery voltage, the alternator can optimize its performance, charging the battery more efficiently under varying load conditions.

Typical Installation:

The sense wire is typically connected to the positive terminal of the battery or at a point in the electrical system where accurate voltage readings can be obtained.
In some systems, it may also be connected to the main power distribution point to account for voltage drops across the wiring.

Importance:

Using a sense wire is particularly important in high-performance or modified vehicles where electrical loads can vary significantly, or in applications where battery life and performance are critical. Properly configured, it helps ensure that the electrical system remains stable and that the battery is charged effectively.

Cheers... jondee86

[Sprite]

Ah thanks for that, very helpful. I didn't do the wiring on the car myself so I'm not sure where that sense wire is connected. I may just re-wire it temporarily directly to the positive on the battery to see if that does the trick. But first I'll try and track that wire. Some have said that center pin on the connector is the dash indictor bulb wire.

[jondee86]

Some general info here...
viewtopic.php?f=9&t=11437&p=76095#p76095

The charge light plays no part in the operation of the alternator. All it does is tell
you that the alternator is working (or not, if the light stays ON when the engine is
running). Similar information to that which you get from an "oil" light.

Cheers... jondee86

[Sprite]

After checking everything I could think of and running additional ground wires I was at my wits end. I finally decided to check the multimeter itself and found it was off by over two volts. I checked it against three of my brother’s meters that were all consistent. So the alt is putting out 14 volts just as it should not the 17 the meter had my believing. Don’t buy the crap Sperry meter that I bought at Home Depot. I wasted a lot of time because of that bad meter. Moral of the story whenever you buy a new meter check to see if it is in spec