Help with charging problem
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Junior Member
Joined: Jul 2000
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From: Norfolk, VA
Help with charging problem
I purchased an 85 Z28 with 305 TPI and I'm having problems with the charging system. When I first start it I get 13.8 to 14.2 volts but after running a while, it drops to 13.1 to 12.8 volts. If I shut it off and restart it, it will idle as if it is a cold engine (1100 rpm). I think this is being caused by not enough output by the alternator and the ECM is trying to get more
voltage. I am not sure what size alternator is on it but what size is suppose to be on it or where else might I look for this problem?
voltage. I am not sure what size alternator is on it but what size is suppose to be on it or where else might I look for this problem?
Member
Joined: Jul 2000
Posts: 461
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From: Warrenton, VA U.S.A.
The ECM will NOT adjust engine speed to achieve a specific battery/alternator voltage. Alternator voltage is maintained by the internal (or external if it's an old or specialized alternator) regulator.
1. Are you taking the voltage readings at the battery terminals?
2. What is the battery voltage with the engine running and headlights and fan (on "HI") on? Does it drop as before?
3. Do you get more voltage when you rev the engine (1500-2000 RPM)?
13.8-14.2 VDC is normal. 12.8-13.1 VDC sounds like the alternator output at idle while trying to provide more amps than it can. At least it's above the float voltage of the battery, so we know that it's putting out SOME current.
It could be a lot of things from a bad battery to a bad sense wire or regulator. Is there any way you can check the amperage output of the alternator while its running? If not, try this:
With the engine running,
1. What is the voltage from the output terminal of the alternator to the positive terminal of the battery? (It'll be a very small voltage in the millivolt range such as 50 millivolts (0.050) - you're checking for voltage drop, which will be in proportion to current).
2. Now, what is the voltage (from the same points in step 1) when the output drops to 13.0 volts?
If the voltage obtained in step 2 is higher than step 1, then it's putting out MORE current when the voltage drops from 14.0 to 13.0 volts. The big question is WHY? Bad regulator? Did something turn on (a fan or fans?)?
On my '91 Z-28, immediately after starting the engine (the alternator should be putting out more current now to charge the battery since starting), I have 0.050 (50 millivolts) from the alternator output to the positive terminal. With one electric fan on (12 ampere current draw), it jumps to 0.090 volts due to the increase in alternator output (and the associated increased voltage drop across this wire due to the increased current passing through it).
Let me know if you have any questions on the above.
1. Are you taking the voltage readings at the battery terminals?
2. What is the battery voltage with the engine running and headlights and fan (on "HI") on? Does it drop as before?
3. Do you get more voltage when you rev the engine (1500-2000 RPM)?
13.8-14.2 VDC is normal. 12.8-13.1 VDC sounds like the alternator output at idle while trying to provide more amps than it can. At least it's above the float voltage of the battery, so we know that it's putting out SOME current.
It could be a lot of things from a bad battery to a bad sense wire or regulator. Is there any way you can check the amperage output of the alternator while its running? If not, try this:
With the engine running,
1. What is the voltage from the output terminal of the alternator to the positive terminal of the battery? (It'll be a very small voltage in the millivolt range such as 50 millivolts (0.050) - you're checking for voltage drop, which will be in proportion to current).
2. Now, what is the voltage (from the same points in step 1) when the output drops to 13.0 volts?
If the voltage obtained in step 2 is higher than step 1, then it's putting out MORE current when the voltage drops from 14.0 to 13.0 volts. The big question is WHY? Bad regulator? Did something turn on (a fan or fans?)?
On my '91 Z-28, immediately after starting the engine (the alternator should be putting out more current now to charge the battery since starting), I have 0.050 (50 millivolts) from the alternator output to the positive terminal. With one electric fan on (12 ampere current draw), it jumps to 0.090 volts due to the increase in alternator output (and the associated increased voltage drop across this wire due to the increased current passing through it).
Let me know if you have any questions on the above.
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