My 1.8 L-jet will sometimes have the idle fluctuate down from a nice 800-900 rpm to 400ish (doubtful the tachometer is super accurate at that engine speed). I first thought it was a vacuum leak as the L-jets can easily develop problems from that. However, after replacing most of the hoses and clamps as well as the intake runner gaskets and short hoses(name?) it didn't seem to change. This problem was intermittent and seemed to fluctuate with the temperature.

To my surprise (well, not really extremely surprising) I noticed last night upon arriving at home that if I shut off the head lights the problem when away and the idle returned to normal. That explained why I first thought it was some sort of ambient temp thing (it is colder at night and wee hours of the morning when I would have had the lights on).

Related, I'm sure, is the voltage I measured from the positive connection to the coil being around a constant 8 to 9 volts (no lights on, idle at a nice constant 850 rpm) when I was performing a tuneup last week.

So my guess is that the generator is going bad. I am just curious if anyone else has an opinion of the issue that I may have not thought of.

Sorry for the long story. This has been a source of irritation for the past 6 months with the indeterminacy of the issue.

Try your voltage measurement again with the lights on and off. What is the difference? If it is a large one, then take a voltage measurement at the battery when running, both with lights on and off. What is the difference?
If there is a large difference at the battery, you have either a going bad alternator or a bad battery. If the voltage levels seem to remain fairly close at the battery and much greater at the coil positive, you have a connectivity issue in your circuit somewhere. Check the obvious first, the red wires at the battery that feed the power to the key switch and fuses. Check for corrosion and broken strands of wire at the soldered connection where they bolt to the batt +.
Tom

It's doubtful that your engine is idling at 400. The very bottom tic mark is 700 rpm, and the OEM tach has a mechanical stop which keeps the needle from from traveling beyond the last tic mark.
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How did you measure the ignition coil with the car running?

I'd believe that. The responsiveness of the tach needle is poor at best. I really only believe what I measure from the dwell meter's rpm setting. When it does fluctuate it seems to vary from small (- 20 to 50 rpm) to large (burying the tach into the mechanical stop). Surprisingly, I've only seen it stall once while doing this.



I was measuring the voltage coming from the positive wire (spade connector) going from the coil to the distributor points. My dwell meter has the standard dwell, rpm, voltage selector and I measured it while doing a tuneup on the car.

I'll have to investigate what Tom suggested this evening. I appreciate the suggestions of what to look at.

As further description of possible related symptoms... The FLAPS tested the battery with their instrument and said that the battery tested as good but under charged. He said the alternator was going out (voltage regulator). My understanding is that I have a generator with a separate voltage regulator (small metal box on the side of the circuit board, left side of engine bay). At the time I switched out the voltage regular with a used one from a previously running car. Previously running because I bought it as a roller from which the previous owner sold the engine separately.

Also, there is a voltmeter gauge installed under the dash (I have no idea where it is actually connected to in the wiring of the car, ignition circuit?) While the car is running or ignition is switched to "on" shows a constant ~12.5 volts (as accurately as a small VDO gauge reads and can be read).

You may want check your voltage regular on the relay board as well.

What is the proper way to do that? I have a multimeter btw.

So you measured the voltage across both terminals on the ignition coil, while the car was running? If so, this is your problem. Try measuring the voltage again but this time, put the positive meter lead on the (+) side of the ignition coil, and the negative meter lead on the ground battery post.

There must have been a miscommunication on my part. The way you describe as correct (pos coil to ground) is the way I have always done it. I don't think I could get it to show proper idle and dwell otherwise and would have many other problems.

You do not have a generator. Its an alternator

With the procedure I described above for measuring the voltage at the coil, would make it impossible to check dwell.

You're measuring 8 volts, because I think you are measuring across the 2 terminals on the ignition coil with the car running. The (-) terminal on the ignition coil is not a true ground.

You either need to rephrase that or you have somethig hooked up wrong.

The Coil + needs to go to IGN and is always hot (+12V) when the key is on.
The Coil - needs to go to the points/condensor.

If you measure voltage on the Coil + (meter neg on engine GND) with the car running you should see a solid +12-14V (14V IF the alternator is working).
If you measure the voltage on the coil - (meter neg on engine GND) you should see the lower voltage because this is a pulsing signal.

AND be careful when connecting a meter or an oscope to the coil - when the engine is running.
That terminal spikes to ~400 Volts each time the points open.
You can easily fry a cheap meter or oscope.

The confusion was my fault. I spoke incorrectly when saying the wire ran to the points. There is a spade connector going from both the positive and negative sides of the coil into the distributor but that is only because I have an electronic ignition installed. If it were points I would be wrong in my description as they should only go to the negative post.

I believe I also got the positive and negative posts of the coil mixed in my head when describing what I was doing. Now that I think about it I was taking measurements from the negative post of the coil to ground (which consequently should be the correct way to measure dwell as well). Sorry about that.

Measuring with a multimeter from the negative post of the coil to a ground showed 8 or 9 volts with the lights on or off. I'll have to do another measurement from positive to ground later.

Battery positive post to ground shows constant 12.5 volts.

Turning on the heat booster fan causes the same idle problem as the lights.

It could be resistance in old wiring. Or very likely the ground circuit

The battery measuring 12.5 constantly is not a good sign. When running, the voltage should go up to about 13.5- 14.5 volts. This is the output of the alternator. If you don't get an increase from what the battery reads with the engine off, to when the engine is running, something in the charging system is not working correctly. If that is the case, and I don't think it is or the battery voltage wouldn't be 12.5, it would be steadily getting lower, then when you load the battery with additional loads, the coil positive would decrease. That would cause a weaker spark.
I really need to have you measure the coil + to ground when running and then when running with lights on. If there is a difference of more than 1 volt, you have some issues we need to track down. Probably some bad connections somewhere.
Tom


EDIT: If the car is running off of the alternator due to a weak battery, the loading of the alternator when turning on more electrical things will slow down the engine. Our engines don't have a governor.

The FLAPS tested the battery as good a few months ago. I'll take those measurements when I get home today.

Maybe 8 months ago the positive connection came off the battery while driving after having forgotten to tighten it down. This may have damaged the alternator which is one of the reasons I suspect it.

It is possible that the alternator was damaged when the battery became disconnected. Doesn't always happen but is possible.

Does the red GEN light come on when you turn the key to ON before starting?

Belt is tight on the alternator, correct?

As others have said, with no electrical loads other than the engine management, the voltage at the battery should be >13.5V. Fix this first before examining the coil voltage and dwell.

You cannot measure the voltage there! That is where you measure the dwell only.

Gen light does come on with key to ON before starting. Belt is tight on the alternator.

I'll be leaving for home in about 20 minutes and I'll grab the multimeter when I get there to recheck that as well as make the correct measurements at the coil.




Yes . I already admitted I did that wrong.

I measured 12.5 volts at the positive coil post to ground with no electrical draw other than engine management and the stereo head piece. With the lights on it falls to 12 volts and with the blower motor running it falls to something around 11.8 volts. This was with the oscilloscope.

The multimeter I could find only measured in integer numbers. Battery positive post to ground with nothing on but engine management and the head piece fluctuated between 12 and 13 volts but read 13 volts more than not. With the lights turned on it fell to a constant 12 and with the blower on as well as the lights it fluctuated between 11 and 12 volts. I'll see if I can find my other multimeter which I think has better resolution.

Your electrical system isn't charging correctly.

Try this test to max out your alternator:

1) With ignition key in the off position, remove the voltage regulator.

2) On the relay board, short the DF pin to the D+ pin on the connector which the voltage regulator is normally connected. This will signal the alternator to put out maximum voltage.

3) Start the engine.

4) Measure the voltage on the battery terminal clamps. You should measure greater than 15V.

5) Shut off the engine, ignition key to off.

Your electrical system isn't charging correctly.

Try this test to max out your alternator:

1) With ignition key in the off position, remove the voltage regulator.

2) On the relay board, short the DF pin to the D+ pin on the connector which the voltage regulator is normally connected. This will signal the alternator to put out maximum voltage.

3) Start the engine.

4) Measure the voltage on the battery terminal clamps. You should measure greater than 15V.

5) Shut off the engine, ignition key to off.

Is this the correct method to short those connections?
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What you are describing here is showing a battery that is not up to the task of supplying enough current to run all of the electrical equipment without the voltage falling below a certain level. With the car running, the current draw is not that great ( less than 15 amps). As you add loads, the current in a good battery will drop a little voltage, not as much as you are seeing. Your battery sure sounds as if it is failing. Do you have another known good one you can use to jumper to your installed battery and see if anything changes? better yet, replace that battery with a known good one. Can you borrow a known good battery to use as a test?
The car runs off of the battery, the charging circuit is to charge the battery back up to what it should be. That is why it is called a charging circuit, not a running circuit. I am sure there will be disagreement on this, but it is what it is. A weak battery will exhibit these exact symptoms. As the alternator is loaded with new loads, the engine must supply enough power to the new loads, without a governor, this is not possible and the engine slows down.
Tom

Found my other multimeter.

Lights and fan off:
Coil: 11 to 12 volts (jumps around)
Battery: ~13.2 volts

Lights and fan on:
Coil: 10 to 11 volts (still jumping around)
Battery: ~12.8 volts

Jumpering D+ to DF gives 16.0 volts at the battery


Tom:

What you say makes sense. I have a good battery but it was tested good by the same FLAPS that tested my current battery as good (6 or 8 months ago). I'll hook it up and see how it goes. I guess I could always "borrow" my wife's battery. I'm sure she'll love that.

OK, so we know that your alternator can kick ass. Try this again and turn lights on and such and measure. See if you can still get 15-16V with heavy load.

Could be the voltage regulator.

Going to pick up a new voltage regulator after work. A local place has the plastic ones for $30. Anyone have any stories about those?

I'll also test the other battery as well.

The plastic regulators suck. They are not adjustable, and usually fail.

I would suggest that you contact a reputable Porsche parts house and get a Metal canned regulator. They work much better than the cheap plastic

I picked this one up at Autozone. It was in stock . Now battery voltage is solid at 14V.

New voltage regulator (the one from autozone Spoke shows above, thanks for the recomendation) seemed to increase voltage reading across the board of about 0.2 to 0.3. The car will idle nice at ~850rpm with nothing on but engine management but will still at best drop about 100 rpm idle with lights and heater fan turned on and at worst the idle starts to fluctuate between 750 and 850 rpm at a frequency of what I would estimate to be 0.75 and 1Hz.

I also tried with my extra battery jumping the car without much change. This isn't a new battery so it might not be indicative of anything.

Besides testing the batteries again or getting a new one my next thought would be to make damn sure I have the tuning set right.

As a side note to the idle fluctuating problem due to the increased electrical draw: I set the idle to about 1000-1050rpm a month or so back and it seemed to MOSTLY go away. I was not aware of the correlation of the electrical draw at the time so I was not observing its tendencies with the lights on or off.