Will not hold volts. battery good,new ing. switch,new alt. only holds about 8 volts.on gauge it's a old police car

Try this! Hook up a volt meter to the battery while its running. the voltage when running should be at least 14.0 or a little higher. when car is off, battery voltage should be 13 volts at least. if not 14 volts or higher while running, there has to be a break from alt, or a bad ground to battery or the gauge is wacky. God-Bless you and your family.

dont ck the voltage at the car gauge use a multy meter at the baterie therminal and ck voltage you need 13.5 volts or 14

Probably.
The volt gauge turns the alternator on once the engine is running. There could be a short in the circuit board inside the cluster.

Check for a blown fuse, if I remember right Oldsmobile had a charging fuse in those years.

well are you sure that you have a good or it just shows the correct amount of voltage. which that should 12.6 volts for a fully charged battery. to actually answer your question, (assuming)
you do have a good battery your alt. shouldn't keep it from cranking unless it is locked up, but you would use a digital volt meter and select the d.c. volts setting touch the red (positive lead) to the red (positive post) on the battery and the ground ( black lead ) to the ground terminal on the battery (negative post). it should read very close to 14 volts. if not check to see if you are getting voltage to your alt. by using your volt meter to test the power wire going to your alt. it will be the one on the backside of your alt with a protective boot over it. with your volt meter still set on dc volts touch your black lead lead to a good ground and your red lead to your power wire going to your alt. it should have 12 volts there. if not check your fuses. if your car clicks when you try to start it then you may have a bad battery connection. dirty terminals with corrosion built up on them will do the same thing. also a starter that is bad will click because the bendix can't kick out to engage the flywheel to turn your engine over. well hope i answered your question and then some.

Its ether the battery or the alternator. The best way to check it is with a load tester. To check it hook the tester to your battery with your car running, red to positive (+) black to negative (-) if the alt. Is good it should read around 13.6 volts to check your battery shut off your car and toggle the load test switch hold for a 10caribbean count the meter on the tester will tell you if its good. If you don't have one or can't get one most parts stores. Will do it for free. If both check out good you have a dead short somewhere. In that case you just have to track it down. But start with battery or alt.

without being there, it could be the alternator gave out. with car off with nothing on. at battery there should be at least 12.6 volts. with car running at battery should be at least 14.2-14.7 if alt. is good. a couple of years ago my car just quit on expressway, and come to find out the positive wire on alt. was loose. if alt. is not charging then you will not have enough of volts to power pcm properly. good-day ! check it out !!

check & clean battery posts first.

How do you know alternator is good?
Put in a cheap volt gauge.
Should read 12.5 normal.
Down to 10 starting.
Back to 12.5 at idle.
Up to 13.5 when engine revved up.

But battery low on water will not hold charge.
Could also have internal short.
Volt gauge will show.

Could also have something staying on, like engine or trunk compartment light?
Pull one battery terminal, and see if test light lights up across gap. If it does, even weak, you have a constant drain. Could be alternator. When diodes go bad, alternator can drain battery. Disconnect things until light goes out.

that sounds about right . the voltmeter just tells you how many volts the battery is holding,if you turn your lights on high beam turn your wipers on and your heater all at the same time,as soon as the current being used is greater than the alternator can charge , you will see the needle on the voltmeter drop back that is normal and logical. if you are running spotlights etc you may need a higher wattage alternator a lot of vehicles come out with 55watt alts.and the owners change the alts. to 100 watt alts... NOW as long as yours is not dropping below 12volts it is not draining your battery so there will be no ill effects it just means the alternator is working close to its capacity-------------------- there you go ,I would not be worrying too much unless it flatens your battery Idony think you need worry "youdoit"