914 Alternator Charging issues Options

[kostritzer]

I've been working through a charging issue with my new to me 74 1.8.

This was a car that had been sitting for over 20 years. The guy I purchased it from went through the brakes and the fuel system over the last year to get it drivable, but I don't think he realized there was a charging system issue since the "gen" light was not on when the car was running.

What I found was that although the light would come on when the ignition was on, it would turn off after the car was running. It wasn't until I drove it and saw that the voltmeter in the center console was at 12V did I realize there was a charging issue.

The car came with a new battery that tests well.

Engine running, the light is off and there is only battery voltage at the battery terminals no matter the RPM. Shorting the DF and D+ at the regulator connectors only increased the voltage to about 12.5, so I decided to remove the alternator and get it tested at a local rebuild shop. The rebuilder tested the alternator and said it was good, so I purchased a new alternator harness from 914 rubber and reinstalled the alternator with the new harness and a new belt.

Fired up the car and it's still not charging. Shorted DF and D+ again, and now the voltage increased to about 15V. Assumed that the VR was the culprit, so I looked around online to find a suitable one, and it appeared that the ones available do not fit very well. I did find one thread where someone used a VR650 solid state regulator, so I figured I'd try one of those since I could get one locally pretty quickly.

After installing the solid state VR, the alternator was now charging the Battery at 13.5V, however now the GEN light is only when the engine is revved up. I popped the left gauge out to check voltage at the GEN light and inspect the bulb. There was a 1.2 watt bulb in place. I swapped the oil pressure light bulb with the GEN bulb since it still had a 2 watt bulb. No change. I checked voltage at the red/white F9 feed wire to the bulb while the engine was running, and it's about 13.5-13.8V. Voltage on the blue wire from the alternator/VR was 15-18V depending on RPM!

Now it makes sense why the light is coming on with RPM, but why the high voltage? At this point I'm assuming that the Solid State Voltage Regulator is the problem. Anyone else ever run into this issue?

[Spoke]

Here's a simplified schematic of the 914 charging system.

[kostritzer]

Here's a simplified schematic of the 914 charging system.

Thanks for the fast reply Spoke. These are my measurements I took back probing the Alternator connector at the relay board engine running:

With the Bosch VR that came with the car

Battery Voltage: 12.3V
D+: 13.5V
DF: 6V at idle, 8V at 2500 RPM
D-: 0

Gen light comes on with key on, does not come on with engine running with this VR

Solid State VR
Battery Voltage: 12.87 at idle 13.9 @ 2500
D+: 15-18V(idle to 2500)
DF: 8 to 4.5V (idle to 2500)
D-: 0

Gen light comes on with key on, is dim at idle and gets brighter with RPM with this VR

I also did a voltage drop test from the starter lug to the battery positive terminal and that was good. Car seems to crank easy. Battery is pretty new.

Side note, do you know where the ground strap from the transmission to the body is supposed to be located?