Scanmaster voltage reading?


Senior Member
May 24, 2001
Kinda curious, as I am trying to trace down a fluctuating voltage problem in my car.
Where does the scanmaster take its voltage readings from? Does it read what the computer is seeing as far as voltage, or is it reading from the power the unit is getting itself?
Right now I have it hooked up via the ciggy lighter socket, and Im getting wild readings from 12.0 to 13.3.
Since my t link laptop crashed I cant check with that, I am just wondering if thats what the ECM is getting as well.
The Scanmaster does not measure voltage, it displays the data sent by the ECM.

The ECM measures the voltage feeding the ignition module and injectors.

Varying voltage can be caused by a few things.

ONE time I have seen a bad terminal in the fuse box do this but the voltage
was dropping to 10 or so but only at higher RPM at WOT.

Your alternator is probably kicking on and off. A bad alternator, loose tensioner
or something like that could cause it. But the alternator gets the power it needs
to operate thru the VOLTS lamp in the dash. The circuitry that connects this is
old and flaky. (My car was doing this too.)

My fix was to clip the wire going to the alternator plug and connect it thru a 1K ohm
resistor to the fuel pump prime connector just behind the alternator.

My VOLTS lite no longer functions, but the volts displayed on the Scanmaster
doesn't drop out and act funky any more.

Hope this helps

Turbo bob,
Can you be more specific in what you did. I have had my dash off several times to clean the terminals and get it to seat good, and I still have problems with the volt light making good contact. I would like to hot wire it the way you did, and be done with the problem.
The plug in the back of the alternator (4 pin, 1 wire used) feeds power to the
internal regulator circuits, the power comes thru the VOLTS lite in the dash.

I had an extra connector, so I just unplugged the original and tucked it out of the way.
(Caspers electronics has "repair ends" like this)
I wired the "L" pin in the plug to a few inches of wire, soldered on a 1 K ohm
half-watt resistor, then another piece of wire, then a 1/4" male spade crimp terminal.

I plugged this into the fuel pump prime connector that is behind the alternator
(gray wire/black plug I think).

This circuit essentially replaces the "power feed thru the lite bulb" stuff in the dash.
My VOLTS light does not work any more, but the voltage displayed on the SM
is steadier and doesn't take an occaisional dive like it used to.

Hope this is clear....

Well, that sounds easy except....

Since my motor was a transplant in this car, there are a lot of things that are rigged up, one of them being the fuel pump. Right now it is hotwired but off of an ignition switched source. My prime connector is non functional.
So my question is...

1. Is the lead supposed to be constant power or switched power? Im assuming switched??

2. Its essentially substituting a resistor for a light bulb right? But then if thats true then why isnt the "volts" light on all the time, seeing as the lead is a power and its going through the bulb in the dash to the alternator.???

I will try this if I can understand whats power, whats ground and when its on and off.
And if I remember right, normal auto parts places carry the alternator repair leads.