Old Car, Old Problem: Low Idle with Lights On

I’ll try to make this as short as possible…

I have a 1970 Porsche 914 that I’ve owned for about 25 years, you can see it at http://mem…/pbanders. It has NEVER idled properly with the lights on since I’ve owned it, and I’ve done some fairly heroic stuff to fix it. I’m not expecting anyone here to solve the problem, but I’d appreciate any new ideas, I’m fresh out.

The symptom is that after the car is fully warmed up, if I turn on the lights, the idle drops from a normal 1000 rpm down to 500 rpm, with significant roughness. Otherwise, the motor runs fine, it was rebuilt in '06 and is essentially a stock 2.0L with D-Jetronic injection. I’ll save you asking the obvious questions - cam is stock, and the ignition and FI systems are in perfect working order - I have checked everything imaginable.

I recently figured out what seems to be the root cause, but have no idea of how to fix it. After the rebuild, I had a problem with my charging voltage being too low, because I had the alternator mounting bracket powdercoated, which prevented a good ground contact from the alternator case to the engine block, which is connected to chassis ground. In the course of tracking down the problem, I charged the battery using an automatic charger, the final voltage with 12.75 V, which was above the charging voltage of the alternator, due to the crummy ground.

When I ran the car with the battery fully charged like this, the idle with lights on or off was rock-stable at 1000 rpm - UNTIL, the battery voltage dropped below the charging voltage, past which, my idle drop symptom returned. It seems that alternator loading was causing the problem.

I fixed the alternator bracket, and installed an additional 8 gauge ground wire direct to the chassis from the alternaotr body. I also installed a high quality rebuilt alternator harness, plus an additional 8 ga wire from the battery ground to the engine block, right next to the starter. New battery, fully charged, new alternator, proper belt tension. I’ll talk about the voltage regulators in a minute.

After this work, the car’s charging system works absolutely perfectly, and it spins the starter like a turbo. All lights bright, charging voltage is 13.8 V, and the battery voltage with surface charge removed is 12.75 V, exactly as it should be. Oh, and I STILL have the problem with idle - because if anything, I’m now loading the alternator even more at idle.

Bosch voltage regulators for this car are separate from the alternator, and the OEM part is mechanical in design. There are three terminals: DF for the field voltage, D- for the ground, and D+ for the output. See http://www…ystem.html for a detailed description. When I put an analog voltmeter on DF with the lights off at idle, I get a steady reading of about 4 V. You can see some wiggling of the needle due to the mechanical switching of the regulator. When I turn the lights on (and they’re stock rated 55/60W), DF goes all the way up to 12V, with significant variation - the alternator is putting out its maximum output, which causes the idle to drop due to the loading, which makes the regulator want to increase DF, causing more loading, etc.

So, I tried a newer Bosch electronic regulator. I haven’t done the DF readings, but I still have idle drop, though it might be a bit better. I’ve checked the current load with the lights on at idle, it’s about 22 A, which from what I can tell, is absolutely normal.


1. Any new ideas of what I can do?

2. Assuming that “that’s just the way it is, the alternator is undersized for the application”, any ideas on add-on idle stabilizers? I can’t find one that I can retrofit to this setup.

Any help would be appreciated. If I could fix this one problem with the car, it make it a lot easier to drive in city traffic.

thanks again,

Brad Anders

First, bravo to you for being so knowledgeable and posting such an informative problem description.

Let’s take a fresh look at this problem. Instead of thinking in terms of voltage and current, which seem to be OK, let’s think in terms of energy.

With all the new parts properly adjusted let’s assume the alternator, belt etc. are working with high energy efficiency.

That leads me to think the problem is the engine itself for some reason is not willing to deliver enough energy at idle. My guess is that the fuel injection is in fact not working perfectly. I think the engine is going lean when the RPM drops a little lower than normal, as evidenced by the rough idle. An emissions sniffer might tell the story. Does the D-Jetronic use a mass flow meter? There could be a leak in the air hose from MAF to the throttle body.

“Does the D-Jetronic use a mass flow meter?”

No, it uses manifold vacuum.

It sound to me like the alternator is simply putting a lot of drag on the engine when it is under load. Have you tried swapping out the alternator yet? My 2.0-4 never had that issue with a stock alternator, either with the D-Jetronic injection or with webers.

think only with lights on. could there be a direct short in the lights wiring. sucking all the volts to ground.