Volts lower with car running .


Well-Known Member
So my volts dropped to 9.9-10 change the alternator still the same a was getting 14volts or so at the alternator andvjust a little less at the battery but between 13-14 volts.before it started acting up but the scanmaster always showed 12volts even though the alternator was putting out alittle over 14.like 14.48. I know you get voltage drop do to wires and travel to the scanmaster.so i a was getting at least 12 volts at the scanmaster i new a was good so long i was getting the 14+at the alternator and battery.the funny thing is car not running i get 12 +volts at the battery and the same at the back of the alternator mybe just little less.then when i turn the car on at the back of the alternator its11 volts same at the alternator and at the scanmaster.9.9-10.....any ideas as always thanks in advance ...ps also the check engine light does come on while cranking then turns off volt ligjt also comes on and off .
if you had 14 V at alternator why did you change it ?
im trying to follow what you posted but sounds like you expect to have 14v at alternator when not running .. engine off it will show battery or a little less due to the wiring from alt to battery

scanmaster issue not showing voltage as high as you have could be the ignition switch on column
Sounds like a intermittent connection issues either at 1 wire plug on the alternator or the cluster plug in the dash.
If that connection to the volt light is lost, the alternator will not charge. That's really all there is to it.
It happened with the one already in thr car .that i rebuild with with the better parts .had it on for about a year or so. Before this started to happen yesterday. So i put one in that a got from advance the first start was the same thing just underr 12 volts it got dark so closed the hood .this morning when i started it 12 at the scanmaster and 14.48 at the alternator and battery.then turned it off and back on problem started again ..the one funny thing is seems to have residue power cause the scanmaster stays lit for about 10 seconds after i chut it off.
14.48 at the alternator and 12 at the scanmaster means the alternator isn't the issue.

What does the orange ECM wire read right before it vanishes into the firewall?

And if it's low,check at each side of the plug, and right at the battery terminal.

Also, while the car is running (14.48 at alternator and battery) what does the cigarette lighter read?
If you would run the SM ground wire to the battery instead at the dash, you should get a more accurate value of voltage.
Gonna run that ground to the battery to get the more accurate reading cause that bothers me to see that much loss of voltage reading bbbuuutttt.....problem mostly solved about it being intermittent so had the car running took off my cluster pushed on the dash where the volt light is it flickers then boom 12 volts at the scanmaster. And cigarettes ligher had 12.54 volts key on engine off.and the ecm wire has 12.54 before after and as close to fire wall man i hate to see that much drop in voltage at the scanmaster so going to ground it at the battery will post results thanks .for all your time i dont say this often to men besides my sons and brother and nephews buut man i love you guys.ha
That means the battery voltage is making it's way into the car just fine. And if your volt meter was using a ground inside the car, that means the interior grounds are in good shape too (with such a small load).

Now it's back to sounding like your ECM wire, or the ECM plug, has some resistance. I'm 99% sure when the scanmaster shows voltage, it's telling you the reported voltage from the ECM, not the voltage at the SM power feed.
Probe the orange wire that feeds the ECM.

Not a bad idea to pull the EMC plug on the box itself too and check for corrosion. (ESP if you have a T-top car).

The Scanmaster just displays data from the ECM. The ECM gets its voltage reading from the ignition input, which is pin A6 at the ECM. That power comes from the ignition switch, then through fuse "ECM".

I have seen bad/corroded fuses, and bad ignition switches, cause low voltage at the ECM.

IMHO someone is being just a little ..........picky? Like we are looking at a 30+year old car. Not all things are going to be perfect. Just my opinion.
IMHO someone is being just a little ..........picky? Like we are looking at a 30+year old car. Not all things are going to be perfect. Just my opinion.
if your voltage is down you should find out why , it can be down to critical components that affect performance . fuel is usually hotwired so not dependant on incar voltage.. ignition coil is something that needs all the voltage as well as injectors
IMHO someone is being just a little ..........picky? Like we are looking at a 30+year old car. Not all things are going to be perfect. Just my opinion.

Striving for perfection is[/is] an exercise in futility. 1/2 a volt or so is acceptable. More voltage drop than that isn't being picky, it's an issue that's outside of acceptable parameters. ''out of spec'', if you will.

To make things worse, a lot of us have doubled the load on some very important electronics on these cars (and the stakes are much higher during poor electrical performance), so it's worth it to do due diligence.
I had bad grounding on my painted tuff stuff alternator, added a ground from the alternator to body. Volts jumped from high 12s to high 13s low 14volts on my Scan Master.