88 s-10 blazer alternator won't charge

I have a 1988 chevy s-10/blazer 2.8l and the alternator won't charge. Replaced alternator, battery, and battery cables. Alternator won't charge the battery but they both test good. The car is drivable for a couple of days but then you have to hook it up to a battery charger. Need to know where to go next! Does anybody have a wiring diagram for this? Please help.

this last time i saw something like this happen, it was a bad ecu.

make sure you have voltage coming down the signal line to the alternator while the car is running. that's what tells the regulator to start working. also make sure all grounds and cables are tight

did you replace the cable from the alt to the battery?

I have a 1988 chevy s-10/blazer 2.8l and the alternator won't charge. Replaced alternator, battery, and battery cables. Alternator won't charge the battery but they both test good. The car is drivable for a couple of days but then you have to hook it up to a battery charger. Need to know where to go next! Does anybody have a wiring diagram for this? Please help.

Need to know which alternator you have C-S or S-I and with gage or red/yellow alternator light?
4 wire plug in for regulator or old type 2 wire plug in?

First thing to check is for good batter voltage on the big wire to alternator.

Second thing to check is for battery voltage or key on voltage to one wire to regulator plug in.

Next thing to check is alternator dash light feed or gage feed to the other wire on regulator plug-in.
Does alt light come on key on and stay on engine running or go off?
The alternator has to have battery voltage on big wire and key on or battery voltage to regulator.
And the it has to have a energize from dash light or gage to start it charging.
After it starts charging it does not need the entergise to keep on charging.

Post back on voltage on wires to alternator and the wiring on it.

The ECM is not needed for these vintage of trucks to operate the alternator, instead the ECM just monitors the voltage output from the charging system to make certain adjustments for optimal engine operation.

Also this is, I am almost 100% sure its a CS alternator.

Basiclly what viggy and MT said, if the connections and wiring are all good. The alternator's field current is is not being energized or "excited" by the excitor wire. The excitor wire is connected to the back of the alternator up to the dashboard to the alternator warning light. If the connection is broken, damaged, or simply if the battery light is burnt out, the alternator will not get its field winding excited, and will not produce any or very low output.

If the dash light battery light comes on and goes off when you start the truck ,unplug the back alternator connector and probe for battery voltage, if you have no voltage, you have to back probe the wire and find the bad connection.