I have been dealing with this voltage drop issue for months. It is intermittent which has let me continue driving but now that battery is worn out, the car can no longer be driven. This car is an '82 Volvo with a '94 Camaro Z28 powertrain, LT1/T56.

At startup, alternator is charging at 14.5V. During warm up the voltage drops to 10.5-11V at which point the car is running on battery voltage. After going round and round on this, mostly focused on cooling fan turn-on and whether that circuit is wired correctly, I now believe the alternator voltage regulator is turning off. Perhaps due to a loss of signal on the 18 ga red wire that connects to the alternator voltage regulator and that the alternator is getting PCM signal to stop charging.

Also attached is the scan report using Scan 9495 in case anyone finds that useful. Below are a few screen shots of the Scan9495 software, two that shows voltage drop with fan turn on though I was able to run another test where the voltage dropped before fans were turned on.



voltage drops with fan1 circuit turn on

voltage drops with fan1 circuit turn on


Using an ampmeter, I checked current (A) on the main circuits of the harness:

Alternator- 35.4 (without fans on)
Ignition- 3.8
Fuel pump- 12.9
Fan1- 11.5
Fan 2- 6.8
Battery- 27.2 (without fans on)

The difference between alternator and battery is a concern. I also read that the single 18 ga wire that connects to the alternator voltage regulator should have 12V and 470 ohm while engine is running. I have 0.5 V and 35 ohm with battery disconnected and/or ignition circuit off. If I start the engine, the circuit read -OL ohm (negative overload?)

Going back into the factory manual now to understand how this single 18 ga wire should function to the alternator, but thought I should get your input.

Am I on the right track with this wire? Is it possible to switch this engine to a single wire alternator? Is it possible to jumper or dummy the voltage regulator to an always on condition? Is that current draw under running conditions normal for this motor?

Thanks for any input you might have!

 

Appears from previous posts this setup has been running OK for several years. And from the description above, while “months” old, this would be a problem that suddenly appeared. And you do not indicate that you changed any part of the electrical system about the time the charging problem appeared.

Have you had the alternator tested?

Since you seem to feel fan on/off may be contributing to the problem, which of the two 1994 fan on/off strategies did you incorporate in the transplant - 2-relay Fan 1/Fan 2, or 3-relay Fans Low/Fans High?

I'm not aware of the PCM controlling the alternator/regulator on a 94 LT1. Not shown in the wiring diagrams. That feature was added with the LS1 engines.

I'll see if I can get the resident electrical engineer to look at this.

 

Thanks Injuneer!

This problem did appear in association with a relay box I built to put the main engine relays in one place (fuel pump, AIR, cooling fan 1, cooling fan 2) a few years back actually. The stock harness was modified by someone 15 years ago and worked fine. I have tested each of those relays and also confirmed this issue occurs with all of them disconnected.

There is a single relay for controlling fan 1 via the PCM circuit A11. The secondary cooling fan is controlled via a switch on the dash.

I am focused now on the small gauge wire that connects to the voltage regulator on the alternator. It is not listed in the PCM wiring diagrams but it is posted on shbox.com indicating this wire initially comes from the instrument panel (5V or 12V?) w/ 470 ohm resistance: 95 charging schematic. The circuit is indicated by an L.

Below are pics of my factory manual which refers to this harness connector as the "PLIS" terminals. The L terminal is referred to as needing to be stepped down.



So at this point, I need to check voltage and resistance on this wire that sends signal to the alternator voltage regulator to turn on. Trying to find electronic resistors in case I need to rewire it. Hopefully, wiring the I terminal to the ignition circuit will work as a backup. In any case, I think this wire is broken or this circuit is in some other way defective and the alternator is not getting signal to maintain charge. If I disconnect this harness connector, the alternator does not charge.

Does this make sense?

 

The PCM wiring diagrams do not show the alternator wiring, because the 1994 alternator is not controlled by the PCM. In the LS1 years the circuit is the same up to the output from the instrument cluster. But instead of the red wire going directly to the alternator, is goes to the PCM, and the PCM is able to turn the alternator on and off through pin L. In Shoebox's 95 ignition wiring diagram (identical to 94 factory manual diagram) the 12 volt power for the alternator turn on starts at fuse #9, and is only powered with the key in the RUN, BULB TEST or START position. The 12 volt power is routed through a solid state device in the instrument cluster. The solid state device contains the 470 ohm resistor. That resistor is "stepping down" the voltage that reaches pin L on the alternator.

You indicate you measured the thin red wire to the alternator "with battery disconnected (disconnected from what - the alternator, or the power distribution system?) and/or ignition circuit off.". Doesn't work that way. You should have 12 volts on the thin red wire with the key in the RUN position, the battery connected to the power distribution system, AND the wire off the alternator. The resistor only steps down the voltage if there is current flowing in the circuit. To check the resistor, turn the key to RUN, engine off, wire off the alternator, measure the resistance between fuse #9 and the end of the red wire. That's when you should see +/- 470 ohms.

You can't measure the resistance with the thin red wire connected to the alternator (unless that is where you disconnected the battery). The measured resistance is altered since you are measuring a complex circuit with two parallel paths.

When you transplanted the engine, did you install a 470 ohm resistor?

If the resistor internal to the alternator is 470 ohms, it may be possible to connect switched 12 volt power directly to that unused pin. But I need to check.

I'll download the Scan9495 log, and see if I can find anything affecting the voltage. I'll ask GaryDoug to take a look, because you are always better off getting electrical advice from an electrical engineer, than you are getting advice from a mechanical engineer .

 

I found two events in the data log where the alternator seems to just shut down. In both cases the engine is idling. The first at 999.78 second log time is in closed lop, no AIR pump. After a while you appear to shut the engine down, and when you restart, the alternator appears to be recharging the battery.

The second event the engine has been restarted, the AIR pump is on, in open loop, and at scan time 2638.35 the alternator appears to shut off. Again, there are no changes in the data log sensor and parameter values before, during or after the shut down. In this case, you shut down the engine, restart, but the alternator never shows signs of recharging the battery.

You didnt answer one question from post #2 - have you had the alternator tested?

I passed a link to this post on to GaryDoug.(author of Scan9495, and an electrical engineer) Hopefully he will have time to take a look at this.

 

Everything that Fred has described is right as for the electrical. The single small red wire going to the alternator/generator is just to enable the alternator when the ignition is turned ON. When ON, the small red wire terminal at the alternator should be about 12v. If it is there and there is no alternator output, the alternator is defective. The pcm was only used to control the alternator for the V6 engines and in 1999 for the V8 engines. I recommend using the 470 Ohm resistor to minimize the chance of damage to the alternator or ignition switch. If the resistor is not used, when 12v is applied directly to the alternator and if the alternator is shorted, a fuse will blow.


p.s. the other terminals on the small connector on the alternator are not normally connected to anything external. Only the red wire at terminal B is used.

 

Thank you both for your time, most appreciated. Things have been busy this weekend and I have not had time to test 12V on the red wire that is used to enable the alternator. My next test will be to confirm a loss of 12V on that wire at the same time as voltage drops out. I did see 0.5V on that wire during the low voltage condition, so I am hoping that is it but I need to repeat the test with a proper back probe on the wire under engine running condition. Injuneer, thanks for going through the data log, I am also unable to associate any PCM function to this voltage drop out. The alternator was replaced as part of this troubleshoot and now needs to be swapped out again once this is wrapped up.

Just ordered the resistor I need to properly rewire this circuit. More soon, thanks again.

 

Oops, I forgot to update on this. Just so folks are aware- the problem turned out to be the single wire circuit that runs from the stock instrument panel to the alternator. Thank you to Injuneer and GaryDoug for your input on this. I stripped back this wire and re-wired it to ignition 12V with a 470 ohm resistor inline. No more voltage drop out. Car is running great, thank you very much for your help.