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Measuring Voltage Drop Across a Circuit

At times it may appear, from scan tool readings that we have less voltage than we would consider desirable.  We then need to find out if this indeed the case.  As I note below, I have seen some cars that consistently report a lower voltage from the ecm than we can measure with an independent meter.  Whether this is something in the A to D conversion by the ecm, or something else, I am not sure.

Yes, we can measure the voltage at various points along the circuit in the conventional way...positive probe on the point and the other probe on a ground, but this is not as accurate as measuring across points in the circuit and does not tell us if the problem is in the voltage feed to the point, or if is is in the ground side of the circuit.

Therefore to properly measure voltage loss and to determine the source of the loss, you should measure voltage drop across each component in the circuit, both positive side and ground side, in order to find the problem.  Yes, there is voltage on the ground side of the circuit and the condition of the grounds is just as important as the wiring off the positive side of the battery.  Trying to measure the resistance of a ground circuit is not very accurate as it can change when a load is applied.  Measure the voltage drop!

Remember that the alternator will put out a bit more volts when first started and cold than afterward so let things warm up and stabilize. Make sure voltage is not changing while you are making measurements.. .go back and reconfirm the beginning points once in awhile so you are not misled.

A good alternator will put out 13.9-14.4v at the back of the alternator when under load...even the stock 120 amp alternator should do this.  If you are seeing a good voltage directly at the alternator, but are not seeing a good voltage further down the line, the following can help you find out where the drop is located.  I would like to see voltage at the module, injectors, fuel pump, whatever be above 13.5v at all times.  Ideally, I would like to see no more than say 0.4v in between the alternator and the component being powered.  But, if you have 13.5v, or better, at that point at all times, then that should work fine.


1.  Put your meter on Volts.
2.  Put one probe on the alternator case, with the engine running/headlights on/blower fan on high (so there is a good load on the alternator), and one probe on the engine block.
3.  Read the voltage on the meter.

That gives you the voltage loss between the alternator case and the engine on the GROUND side.

4.  Put one probe on the engine block and the Negative terminal of the battery (not the cable connected to the battery but the actual battery terminal.
5.  Read the voltage.

That gives you the loss on the main battery ground cable.  (I bet that I have seen at least a dozen cars that people had either bought, or had worked on, that did not have the main ground cable connected to the engine or was loose)

6.  Put one probe on the Positive terminal of the alternator (not the end of the cable) and one probe on the end of the battery Positive terminal (not the actual cable)
7.  Read the voltage on the meter

That gives you the loss in the cable between the alternator and battery on the POSITIVE side.

The voltage on the meter shows the voltage that is being lost due to a bad connection or wire.  The meter reading reflects the "escape path".  Many times, if there is no load on components, the lack of current draw will allow the voltage drop to be low but when there is a a higher draw demanded, the resistance of the poor connection will then show up as a higher voltage on the meter.  That is why I suggest to have the engine running with the lights on. The fan running, etc. will also help show the problem better.

Be sure the probes are on a good connection point and not on a spot of rust/paint/grease/corrosion, etc.

You can check between the battery ground terminal and the inner fender sheet metal to check the quality of the sheet metal ground wire from the battery.

From the firewall back to the engine block.....we often have a ground strap between firewall and block...and/or a ground relocation kit that brings the grounds from the back of the head to a terminal box on the firewall which is, in turn, grounded by a strap to the block.

From battery positive terminal to starter post to check the loss across the positive cable.  Be sure the connection is tight at the starter.  Then the fusible links take the power to the bulkhead connector and into the car.  I have not seen too many cases of the problem being in the bulkhead connector as GM was pretty good at packing connectors with black goo to keep the moisture out.

Inside the car, check the ignition switch http://www.vortexbuicks-etc.com/ignition_switch.htm  I have seen two cars that had a major drop under load in the switch connectors...one of them my GN which always had a big gap between ScanMaster and alternator and the problem was here.  That meant everything else was seeing a loss that came thru this circuit on the ignition switch, not just the ecm.

Read across the fuse terminals on the ECM-IGN fuse...there should be no loss there...I have seen corroded fuse block terminals that went high resistance under load.  Also check across the CCCI fuse terminals to be sure there is no loss in the ignition module circuit (Type I ignitions like our cars use have two voltage feeds-terminals P & M http://www.vortexbuicks-etc.com/coil.htm).

Check the ecm connectors to make sure they don't have green corrosion (or any other color) layered on them.  And so on.

Check the ecm ground path as well.  You can check from the ecm metal case to the battery ground for the complete path.

Now, I should admit that I have seen a couple of cars that showed quite a difference between scantool and the alternator that I could not find a problem with and the voltage drop was minimal on both the plus and ground sides of the circuit.  We would be better served installing a quality voltmeter across the wire coming out of the ignition switch that goes to the ecm than relying on the scan tool reported voltage in many cases.

What really counts is that the injectors, ignition, and fuel pump are getting good voltage-and they were in the above cases.  Take your manual and check the voltage on the appropriate feeds to those three items when the car is under load.  If above 13.5v, then call it good and obsess over something else :)  Heaven knows, these cars will give you plenty to do so.

Most stores that test alternators do not do a good load test.  This means that they may say an alternator is good, but, it may not be putting out sufficient amperage to maintain sufficent voltage as the load on them increases.  Therefore if a store tells you it is good, but you see a big drop when the system is imposing a big load on the alternator, then it may not be, if all the connections in between check out good.

Don't insist on believing that your new super alternator is necessarily working right without proving it.  Many of them have cheap imported diodes in them.  I would rather have one out of the junkyard that was built with quality parts back in the day. 

If you need a new alternator, don't waste your money on a cheap upgraded unit.  Instead look for one of the below alternators which is a second generation of our original units.

In 1993, GM made some revisions to the CS144 series and added a new model rated at 140 amps.  This unit is capable of approximately 105 amps at 800 engine rpm (~2240 alternator shaft rpm) which some 30 amps more at idle than our original units.

This unit was installed on the following cars:

1996-94 Buick Roadmaster 5.7L, 1996-93 Cadillac Fleetwood Brougham (RWD) 5.7L, 1996-93 Chevrolet Caprice 4.3L& 5.7L, 1996-95 Chevrolet Impala 5.7L, 1995-92 Chevrolet Lumina APV Van 3.8L, 1995-92 Oldsmobile Silhouette 3.8L, and the 1995-92 Pontiac Trans Sport 3.8L

This alternator was also installed on police cars, and fleet vehicles such as taxis where normal operation may include lots of non-moving use with heavy radio traffic.

 It is listed as a Lester no. 8112 which may be helpful at wrecking yards, or such places.

This model is a standard stock item at major parts stores like Autozone and Napa.  It is also available new online from most reputable online dealers.  Prices generally range from $130-$170 although I have seen it for less.  It pays to shop around.

I like the batteries that have the +/- terminals on top of them because it makes it easy to make a connection to eliminate the cables without having to have long arms.

Note that the ecm gets its main power thru the ecm ign fuse.  Not the orange wire from the battery as this is the ecm memory that maintains the chip programming.  This has to be connected for the car to start and run, but, it does not provide the power that the scan tool reads...which according to the manual also feeds injectors, ignition...

Your manual is your friend when it comes to tracing out each leg of the journey!  The above technique can be tedious but it beats rigging things in hopes of making it work.

It works on anything such as power to the headlights, fans, fuel pump, window motors, whatever.....