ecm gets power thru the ign switch via the ecm-ign fuse.
to properly measure voltage loss, you have to measure voltage drop across each component in the circuit, both positive side and ground side.
1. Put your meter on Volts.
2. Put one probe on the alternator case with the engine running 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)
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 on the POSITIVE side.
The voltage on the meter reflects the voltage that is being lost due to a bad connection or wire. The meter shows 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. Having the lites on, the fan running, etc. also will 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 at the switch...one of them my GN which always had a big gap between scanmaster and alternator and the problem was here.
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.
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.
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.
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 scantool 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!
Your initial readings appeared to show a big drop under the hood. Your last comments did not provide any information with regard to drop that I could relate to the first.
Remember that the alternator will put out a bit more volts when first started and cold than afterward so let things stabilize and 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.
If anyone comes in and tells me there is no voltage on ground wires, I have some experiments I would like to run on you before you go start an alternator business.
