5Volts reference putting out 12 volts with key on 18 volts with car on(overcharge)


New Member
Oct 15, 2019
Los angeles
I noticed my tps and other sensors like iac and others are giving a 12 volts read on my multimeter with key-on instead of the usual 5 volts
it goes as high as 18.5 volts with car on!
I'm guessing those ("missing") 5 volts show up eventually when I turn the car on? But wait , how can that be...

I replaced alternator/battery/drivebelt/power steering pump and problem still happening
Any suggestions on where to look next I'd definitely appreciate it? PCM wires maybe? T.I.A!!!
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SN Certified Technician
Aug 14, 2009
Houston Texas
I give up. What model year Mustang are we dealing with exactly?

What does the voltage read at the alternator main B+ terminal with the motor running?

You also need to learn about voltage drop testing and WHY a VOM isn't always the best way to test car electrical. WHY? Because the VOM has such high impedance that it won't "load" the circuit. That is why for many applications a test light is a better testing method because it "loads" the circuit thus testing the circuit's ability to carry a current.

Note, for the 1999-2004 Model year Ford uses a mix of 5 volt VREF and 12 volt reference sensors. This is a Ford (not a GM). Wiring diagrams can help you to determine which sensor is powered from which source.

Also, if you are testing the 5 volt reference for accuracy THEN you need to use the GY/RD wire as the ground reference.

Also. The IAC is powered via the 12 volt engine buss. So it would be expected to read 12 volts.

Howto perform charging system voltage drop test
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New Member
Oct 15, 2019
Los angeles
At the brink of giving up but I just thought heck maybe there's still some hope for it. It runs great other than the fact that it's overcharging and can pretty much damage something (if not everything)
It'sa 98 3.8 mustang
Alt B+ giving 18.5v with car on and multimeter to Battery + and - terminals
With car off, using my test light on B+
[I tested the three wires connected to voltage regulator and noticed the middle ground wire (white/black) and green/red wire and alternator case lit my test light up except for yellow/white which connects to fuse box but does so with test light on battery - ]

I traced the green/red wire to charge indicator on instrument cluster and no problem within wire harness
But my guess is that's also ignition wire? (Neutral?)

Still haven't performed voltage drop test or checked GY/RD as ground reference but will do so later today

I'll post results and let you know


SN Certified Technician
Aug 14, 2009
Houston Texas
The usual cause for over charging is from a voltage regulator "issue". Are you certain that the correct alternator has been used for this application? Ford does use alternators that are similar with different internal regulators. The difference can be very subtle and hard to notice from the outside.

Note, for a non PCM controlled alternator (like this model year), the high voltage isn't likely to be from an external signal to the alternator. EXCEPT for the 1996-1998 model years there is a W/BK jumper between two "S" pins of the alternator connector. Does your alternator have this external jumper?

But it would be wise to CONFIRM that all of the grounds supporting the charging system are solid and can carry a real current.

It also might be a good idea from you to remove the alternator and have it "bench" tested at your local auto parts store.

I have helped people that in the quest for a nice engine bay have powder coated the alternator. Only to find out that the powder coating affected the alternator's ground.

If this were my car I would be very concerned about the over voltage hurting the PCM.
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