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Hi Guys-- Odd situation. Last night driving around town, the low voltage warning light came on. But the alternator gauge needle was in its usual place with the lights on, right below the 12.
The light got brighter as I accelerated (and the needle moved up, as per usual). The light was barely visible at idle. Headlights got slightly brighter with the pedal.
I cut short my trip and was on my way home when the light went off. Not faded out, just went off.
I am thinking that because the gauge needle was in the safe zone, that's it not actually the alternator, but a short/grounds. I can confirm by checking the voltage across the hot post tonight or tomorrow. I was contemplating a new 14 connection harness already. The one in the car is original, and there are some times where I feel like the car is running slightly better than others.
This sounds like your alternator to me. I would pull it off and service it by cleaning and putting Deoxit on all of the contacts. This is want I did with mine and the function improved marketly. My gauge use to bounce around the 12v range. Once I serviced the alt the needle is now stable at idle at about 12.5v. That being said, the dash gauge is not very accurate as I have a additional volt gauge installed which reads a constant 13.7v at idle. I have LED headlights also which make a drastic difference in idle voltage with the lights on. Before I installed these headlights the old lights would pull the idle volts below 12, now I run a constant 13.4 - .6 at idle. All this with the original harness. If you find you really need a new harness I have a brand new one from 928MS that I have been holding on to. I'll give you a deal on it as I dont need it right now. Just PM me if interested.
Besides the alternator itself, a bad (old) engine wiring harness contributes to a lot of mis-diagnosed alternator problems.
As the engine wiring harness grows in impedance, the alternator gets less and less exciter voltage with which to generate power. So output of the alternator drops.
Since the voltage level seems low at idle it is most likely a problem in the alternator regulator.
Obviously its intermittent now, but it will likely return and get worse over time...
Investigate the alternator & test jump post voltage with a DMM also check these areas - (14 pin connector, alternator connections, engine wiring harness)
NB Once the alternator has started generating - it's generating its own exitation voltage - higher impedance in the exciter line will then tend to keep the bulb off not make it brighter.
Higher exciter line impedance will usually compromise alternator startup - you didn't actually describe that so far...?
Carl is on the right path. Resistance in the exciter circuit is more likely to cause the alternator to not get adequate excitation to kick-start self-excitation. It's also a reason for the light not to work at all. Regardless, most aready-used alternators have enough residual magnetism to kick-start themselves if the engine is rev'd high enough. Folks report that to be somewhere north of 3500 RPM.
Meanwhile, put a real voltmeter on the electrics and see what the system voltage actually reads. The dash gauge reads from a CE panel source, so consider connecting your meter with alligator clip jumpers to the 30 bus at the top of the CE panel where te red wires from the jump post are attached. That way you can see the same voltage that your electrical accessories see. Diagnose any voltage issues from that knowledge.
Meanwhile, unless there are some cross-connected wires in the harness or the CE panel between the dash indicator light and the alternator, you need to look at the alternator itself as the probable cause of the light glowing.
60-second clinic:
The dash indicator, with a resistor in parallel, provides excitation current to the alternator to get it to start charging when tthe car is first started. It is not a direct indicator of system voltage as one might assume from its location near the voltmeter. It only indicates whether there is current flowing between the regulator and the bulb circuit. The bulb should light when the key is on (engine run position) but the engine is not yet running. Once the engine starts and the alternator self-excites, the regulator potential comes up to match the battery connection potential, and no current will flow through the bulb.
The indicator usually comes on to show when the alternator is not making enough voltage to supply the excitation loop, essentially telling you that the alternator isn't charging. But the bulb circuit only looks at the difference in potential, so it glows if current flows in the other direction too. Like if the alternator is making more potential than what the CE panel and instrument cluster see via the wiring from alternator to jump post, jump post to CE panel, through the bulb circuit, and on to CE panel ground. This is where Carl's suggestion comes in. Resistance in any part of that loop may cause the bulb to glow slightly, based on current flowing the otherway through the bulb. Voltage at the CE panel buss, thanks to that resistance and electrical load art the panel, is enough lower than what the alternator is making to cause current to flow through the bulb.
What to do: Start off with the voltage reading at the CE panel with a real meter. Engine running and accessories on, the same configuraion you were using when you noticed the glowing lamp. Write down the meter readings with the ground side connected to the panel ground and also to s secure chassis ground point. Move on to take a similar reading at the jump post, then at the battery with ground to battery negative and also to secure chassis ground. Wite down all the readings. If you can get to the starter positive post and take a rrading there, write it down too. These readings will help you/us identify where voltage is lost due to resistances in the wiring and connections.
Basic annual electrical maintenance has you cleanning the battery terminal connections and the major supply connections like the jump post. The ground strap is a known weak spot if it's original. The ground points on the car near the CE panel and the one for the instrument cluster need careful cleaning. Deoxit is recommended as a maintenance tool for some of these connections, but they need to start out clean and shiny before spray-maintenance will be enough.
Wow, thanks for all the really helpful replies. I should note that I am a newbie, and the one thing I do not have practice with is electrical diag. Ground strap and battery are about a year old, all grounds in the engine bay were cleaned but will go over all of the grounds with Deoxit. I've got a good multi-meter.
Engine wire harness is certainly in my future, if only because its original.
I might have some further questions about the thorough diagnosis procedure dr. bob was kind enough to outline. Will report back this weekend, and again thanks.
Just a quick update. Light has been off, so I didn't think it made sense to do any tests. Today, the light on the gauge has been on consistently. Symptoms are a little different than I thought, the gauge needle does not read correctly. It varies with acceleration. So at highway speeds, it is in the red near 16 and the light is bright.
Anyway, I got home and was able to test the voltage at the hot post in the engine bay. It reads 12.5 V at idle (much less than the gauge reads). I am guessing that this means the issue is not between the battery and the post, right?
I will be able to check the CE panel tomorrow. Where is the positive "terminal" on the CE panel? Do I have to take it off?
I am thinking I should just order a alternator. It is original.
16V is very bad for your electronics (and battery)... It has to be the alternator/regulator. I would remove it and take it in to have tested at a good (auto electrical) alternator shop - it may just be the regulator which is much cheaper than a new alternator - but have them evaluate everything that needs to be done.
This is not a service you can get at Pep Boys, O'Reilly etc... if you really don't have a good old world shop - consider a new or well refurbed replacement.
Thanks Alan. Do you think the gauge is reading correctly? Lights aren't getting bright with the petal. And the voltage reading at idle is reading lower than I'm getting at the post.
Regardless, I will be removing the alternator tomorrow. I found an auto electrical repair shop near me.
Well the only thing that could be an issue is the DMM - do you know that the battery in it is OK? They can read very inaccurately when the battery is almost dead. Otherwise you have an overvoltage condition and only the alternator can be at fault for that.
Measure voltages all over:
Battery, Jump Post, CE panel power pins (the 3 central red wires on the face).
You talked initially about readings of under 12v - if real? this means the battery is discharging and the alternator not generating (much...) - so whilst that may have seemed normal to you - it really wasn't.
Just an update. Pulled the alternator today because it looked fairly easy and was. Toughest part of the job were the little nuts on the back of the shroud. I figure, eliminate the alternator as an issue since it appears to be the most likely culprit anyway.
It's an original Paris-Rhone as I suspected. I have my belly pan, so the pully/cooling fan assembly looks good.
Will they be able to rebuild it, or just test it? What's preferable? They also appear to offer "convert a low amp to higher amp service", which is something I'm interested in. Will the Paris-Rhone be a candidate for that?
Engine wire harness is certainly in my future, if only because its original.
If you get someone else to rebuild or install one , ask to see the old one. When you cut the old insulation off you may be shocked!
IMO an essential ground zero job before you spend too much time chasing electrical gremlins.
btw my car started and ran perfectly and gauges worked pretty well , even so , there's nowhere to run or hide when it comes to ageing electrics!
Final update: alternator was crispy. Below is a picture of my voltage regulator. Wires are really frayed, and it appears that one of the retractable prongs is broken right off. Alternator looked ok from the outside, and the shroud was still on and I have my belly pan.
I took it to Guaranteed Rebuilders in Passaic. For anyone in N. Jersey having alternator/starter issues, I would recommend it. The owner is an older guy who seems really competent and has lots of experience. Didn't blink at my Paris-Rhone. Which brings up a related point: the Paris-Rhone alternators cannot be tested on the standard testing rigs at Autozone or Napa. The Bosches can, but apparently the Paris-Rhone's are wired in a non-standard way.
Anyway, got the alternator back on the car Saturday. Really one of the easier jobs I've done on the car. The shroud is fiddly, and of course the tension adjuster is more complicated than it needs to be, but everything bolted up pretty easily. Measured over 14 at the hot post, gauge is right in the middle at idle, but I suspect that the gauge is reading low.
Good News! If you don't mind me asking, how much was it to service your unit?
Thanks!
They charged me $175. That included at least two new parts, the voltage regulator and what I think he called the "instigator"? Probably the exciter? At any rate, the moon-shaped piece on the back.
A little pricey, but I doubt I would get a rebuilt one for much less, and I felt confident if I had any trouble I could return it and have him check it out.
08-20-2014, 10:22 AM
Low voltage warning light on, gauge reads normal
