Solved Cutlass Voltage Problems
April 2016

(Sorry spoiler alert).
The External Voltage Regulator Sensing wire was connected to the wrong location and the Alternator output wires was too thin.

I have a 1971 CS built in Framingham Mass. When I got the car a few years ago it was in great shape and over 95% original. I noticed my headlights were always very dim at night and worse at Stop lights. I installed an analog volt meter on the dash and connected the wire to the "Horn Relay" mounted on the driver's fender (with an in-line fuse for safety).

Now I was able to see the issues while driving.
At Highways speed with the headlights on and AC running, the "Horn-Relay" voltage would be between 13.0 - 13.5. That’s a bit low, the ideal voltages should be 13.8 - 14.2.
At a traffic stop, the voltage would quickly drop down to 12.0 volts or less.

Most of the time, a low voltage issue like this are caused by a bad voltage regulator or alternator. Since my car still had the original mechanical voltage regulator and 63-amp alternator (10-DN series), I assumed they were the problem and I replaced them both. I also installed a new relay controlled Headlight wiring harness.

Since I wanted to keep the car stock looking, I installed a Clone 10-DN Alternator, with an output of 100 amps (in hindsight, the 80-Amp would have been more than enough). I also installed a new solid-state voltage regulator and re-used all of my existing factory wiring.

After the update, I still had the same problems. At Highway speeds I was theoretically making up to 100 amps, yet the volt meter would show 13.0 - 13.5 while cruising down the Highway. And at a stop light with AC and Lights on, the voltage would quickly down to the low 12's.

The problem was actually the Framingham Factory Wiring.
For some reason, the Framingham cars in 1971 with AC, had the Voltage Regulator Sensing wire connected directly to the Alternator Output connection. Normally the Voltage Regulator Sensing wire connected is connected to the "Horn-Relay". There are many web articles explaining why you should not connect the Voltage Regulator Sensing wire to the Alternator Output connection.

The short answer is:
You will not get an accurate measurement of the cars voltage / current requirements when connected to the Alternator Output connector.
As the Current loads increase, the Voltage Regulator is not able to get an accurate measurement of the voltage drop. This is because the Voltage Regulator Sensing wire is looking at the Alternator Output connector and not your main power distribution buss which has all of the loads (The Horn-Relay).
The second problem was the Gauge and length of the Alternator output wire. My 1971, CS Alternator output wire is a 9 Foot length of 10-Gauge wire. The length and Gauge of this wire would cause up to a 1.0 volt drop or a 1-volt difference between the voltage at the Alternator Nut and the Horn-Relay Nut (Assuming a 60-amp load).

Since the Voltage Regulator Sensing was attached to the Alternator Output Nut, it would fool the voltage regulator into thinking car had plenty of voltage of approximately 13.8, when in reality the Horn-Relay Nut was running at 12.8 volts. And the voltage at the headlights was even lower.

I replaced the Alternator cable with a new 9-foot length of 8-Gauge wire and moved the Voltage Regulator Sensing wire to the Horn-Relay.
Switching from 10-Gauge to 8-Gauge reduced the resistances losses and added about 0.30 more volts at the Horn-Relay. The 8-Gauge can handle more current. The 10-Gague wire was at its load limit.

The greatest benefit came from moving the Sensing wire to the Horn-Relay nut. Now the Voltage regulator sees a more realistic system voltage. And my headlights are much brighter!

 

Voltage Results before and after wire upgrades:

All 4 headlights were connected and running. The voltage was tested from engine Ground to Passenger side dedicated High beam contact. The Headlight wiring harness has been upgraded to a dedicate Relay system with 10-Gage wires for all leads and is feed directly from the battery. The existing car headlight wiring harness is just used to signal the new harness when to turn on the relays.

Note: Voltage Regulators are usually Temperature sensitive. On a cold day, its normal to see a higher charging voltage when you first start your car. After the Regulator warms up to 200+ F, it should stabilize for the rest of your driving. It best to test your VR when the engine is warm (same for Mechanical and Solid-state).

Installing the 10-Gauge wire and moving the Sending wire to the Horn-Relay did have a big improvement. Now the charging voltage is staying between 13.5 and 14.2 under most load/Temperature conditions.
The High-beam voltage increased from 12.00 to 13.20.

Before Upgrade:
RPM 1100 (hot engine)
Lights on High
Alternator Nut, 13.69
Horn-Relay Nut, 13.15
Battery Terminal, 13.13
High Beam PS, 12.00

After Upgrade:
RPM 1000 (cool engine)
Lights on High
Alternator Nut, 14.32
Horn-Relay Nut, 14.08
Battery Terminal 14.05
High Beam PS, 13.20

After Upgrade:
RPM 750 (Cool engine, Idle in park)
Lights on High
Alternator Nut, 14.00
Horn-Relay Nut, 13.75
Battery Terminal, 13.70
High Beam PS, 12.90



Highway Road test on a cool day #1:

After Upgrade, 60-70 Mph (2000-2400 RPM)
Outside air temperature 70F
Loads, Headlights ON, Radio ON, Fan speed-3, AC Off,
After about 20 minutes at highway speeds, I noticed the voltage at the Horn-relay nut had dropped to approximately 13.5 volts.
I believe this drop is being caused by a cheap voltage regulator, drifting when it gets warmed up by the engine.


Highway Road test on a Hot day #2:
Looks like I still have a Voltage Regulator problem, Too much drift when Hot.

After Upgrade, 60-70 Mph (2000-2400 RPM)
Outside air temperature 85F
Loads, Headlights ON, Radio ON, Fan speed-3, AC ON,
On hot days the voltage drops more, down to 13.20 at highway speeds.

Then I turned off most loads.
Loads, Headlights OFF, Radio Off, Fan speed-1, AC OFF.
The voltages at the Horn Relay stayed at 13.20

Highway Road test on a Hot day #3:
Replace my Solid State Voltage Regulator with my Third Model.
The VR#3 works much better.
Cold engine voltage is 14.10 - 14.20
Hot Day with a Hot engine and RPM over 1,000 the voltage will be approximately 13.80.

After VR#3 Upgrade, 60-70 Mph (2000-2400 RPM)
Outside air temperature 85F
Loads, Headlights ON, Radio ON, Fan speed-3, AC ON, Rear-window Grid defogger ON.
Voltage stable at 13.80 at any RPM over 1,000.

Then I turned off most loads.
Loads, Headlights OFF, Radio Off, Fan speed-1, AC OFF.
The voltages at the Horn Relay stayed at 13.80, at any RPM over 1,000.

 

Intersting facts about the original sensor wire.

Thanks!

- Eric