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What's the voltage gauge measuring?

 
Old 10-28-2013, 03:04 PM
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sethwww
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Unhappy What's the voltage gauge measuring?

Hello all. I have a 1980 5-speed with 113K.

When I turn the key, the volt-meter reads way low-off the scale.

It starts and idles around 700, but won't rev up for a couple of minutes. If I give it gas, the engine stumbles and dies.

After a minute or two, I can get the RPMs up to 1400 or so. Then, the voltmeter jumps to 13.8 and the engine revs freely for the duration.

Even with the voltmeter reading low, I'm getting about 13 VDC on the battery and the thick red wires on the fuse panel. I have good continuity from the battery to the fuse panel. I found the battery had a bad cell, and replaced it, but the problem persists.

My questions:
What is the volt meter measuring?
Am I just facing a bad regulator?

Many thanks. Seth
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Old 10-28-2013, 03:11 PM
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Tom in Austin
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Gauge shows battery voltage ... assuming loads, grounds and wiring don't throw it off.
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Old 10-28-2013, 03:40 PM
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auzivision
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Hi Seth, I wouldn't spend too much time trying to correlate engine performance to gauge voltage. Your problem sound like it's due to a cold engine warming up. Your system is different than mine, but I believe you have something called a "Warm Up Regulator". You might want to research it's function... something in the system will enrich the fuel mixture during cold operation.

As far as your initial question and as Tom mentioned, the gauge is suppose to display system voltage, but can be off for a variety of reasons. It should display battery voltage prior to starting and then battery plus alternator voltage after. The fact that your battery starts your car makes me think you're ok in that department... even though the gauge might not be reading properly.
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Old 10-28-2013, 03:52 PM
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joejoe
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It also sounds as if the excitor may be bad (no charging until increased rpm) Does the low voltage light come on when key is turned to 'on' position? (not started) If no light it needs to be replaced as it completes charging circuit.
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Old 10-28-2013, 04:01 PM
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dr bob
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Look carefully at the alternator, and the dash bulb indicator for charging. When you turn the key to on but not yet started, does that bulb light up? If not, there's likely a break in the exciter circuit between the key, the bulb (and its parallel resistor), and the alternator. The alternator needs an initial bit of exciter current to get it started, Once excited and as long as it's turning, it will continue to generate power. That bit comes from the key in run position, through the bulb and resistor, then on to the alternator. If that loop is open, the aternator won't start charging until it's seen a couple to three thousand crank RPM's. It suddenly starts charging then and continues until you shut off the engine.

Another possibility, especially if the bulb works but it still doesn't start charging until higher RPM's, is that a later alternator has been fitted. IIRC, your orih=ginal is a Paris-Rhone. Later cars came with Bosch alternators (a couple different flavors), but all need a little more exitation current to get them to light off at idle. There's a retrofit of a different resistor around the bulb to support later alternators if that's the problem.

The dash area has a few important chassis ground points. Firewall avor the driver's toes. one near the CE panel at the passenger's feet, and one on the right side of the pass footwell IIRC. Those all need to be cleaned before you get too deep into your sleuthing. Visit the 928 Specialists (our forum sponsor) website at www.928GT.com Navigate your way to Wally's World area, and look at the annula electrical maintenance section. Cleaning grounds has 'solved' an incrediible number of mysterious issues.

There are many posts and threads here about diagnosing charging problems. Search is your friend.
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Old 10-28-2013, 04:50 PM
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MainePorsche
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What joejoe said.
Also, servicing all grounds is a good idea on this car at all times.
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Old 10-29-2013, 07:35 PM
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sethwww
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Default The follow-up

Thanks very much everyone for the insights. Here's what I saw:

The red low-voltage warning light is on when I turn the key. After it starts, while the engine is idling, it stays on. As soon as the engine revs up, it goes out and stays out. I put the scope on the main power lead, and it reads 13.4 volts with the engine off and 14.5 volts with the engine revving at 800 or more, with under 1 volt of electrical noise. (Thought it might be bad slip rings.)

I don't think it's the warmup regulator, because it drives well after it revs, but it's still cold. It seemed more like the ECU trying to run on reduced voltage and going into 'limp home' mode. Also, the oil gauge and volt meter bounce a little when the turn signals are going.

The car sat out for two years before I bought it, and has a leaky hatch, so it had a lot of corrosion around the fuse panel and under the seats. I spent our first summer cleaning the fuse panel, replacing relays, and bypassing burnt wires. I cleaned every ground point I could find, but evidently missed the one on the steering column bracket. I cleaned that one tonight , and the car started and revved right up, but it was warmed up at the time, so I'm not convinced, as the gauges still bounce when the signals are on. I'll check it again in the morning.
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Old 10-29-2013, 08:23 PM
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dr bob
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I suspect that you have a later replacement alternator installed, since the light is on but it does not excite until RPM's are up. There's a factory note on the excitation loop resistor on the back of the instrument cluster that needs to be changed. Per the bulletin, the replacement is a 68 Ohm 5W piece. If you don't already have the workshop manuals, get the full tech set on CD through Roger at 928srus (a site sponsor), with the workshop manuals plus a lot of rare tech publications. The rest of the update includes changing the cooling shroud to match the later Bosch replacement alternator too.

The resistor replacement requires that you remove the gauge cluster from the instrument pod. The resistor is soldered onto the rear of that assembly. There's a long list of while-you-are-in-there's, including replacement of the broken odometer gear, plus replacement of all the light bulbs. Some folks have upgraded those illumination and gear-indicator bulbs to LED's with good results.
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