A little bit of voltage fluctuation at the alternator can cause the
regulator to produce some big charging current swings.
- Negative battery cable for corrosion inside the insulation (arcs over
and welds for start then corrodes apart). While you're there check the
battery terminal -- I had a bad battery terminal on my C5 that caused
these symptoms then it quit completely. Take a good look where the
negative batt cable grounds to the frame.
- Bonding wire between frame and engine block or transmission.
- Headlamp wiring harness ground to forward frame.
For the '80, Clymer shows two ground paths for the alternator. One
ground goes via a black wire to a connector in the forward wiring
harness thence to the frame (at the same point where the headlamps,
horns etc. are grounded.) Check the connector (it's paired). Then lift
the ground lug to the frame and clean up any corrosion or rust.
Another diagram shows that ground path but also shows a ground from the
alternator to the engine block then to the frame via the block or
transmission bond wire. Might have been a mid-year change.
After you do the usual ground checks, battery cable checks, and so on, take
the alternator to a real alternator repair shop (not an AutoZone, Advanced
Auto, Pep Boys, etc) who can check all three phases of the alternator.
If you have access to an oscilloscope, you can do it also. There are three
phases to the output, 60 degrees apart, so that rectified you have 6
positive wave peaks in a full cycle. What often happens is that a single
phase set of diodes will go bad and you lose a phase. The other two will
still charge the battery but at a reduced level.
Also, check for condensers (capacitors) connected to the alternator positive
line either near the alternator or away, like the fuse block/bulkhead
connector or the starter solenoid, that may be bad. They would be there to
filter the voltage peaks.
However, enough to see flickering sounds more like a diode pack is bad.
The chain stores with the quickie belt type alternator tester will NEVER
If you have one of those chain brands, and have the lifetime warranty, tell
them the diode pack is bad and you need a new one. They won't understand,
and you will have to insist, but the alternative is to suffer dead batteries
every few months and flickering lights.
I should add the two most common causes of alternator failure I have found
are jump starting another car and over-tightening of the belt.
The belt does the obvious, it overloads the front bearing and either the
bearing will go out and seize or it will overheat the alternator so the
electronics inside fail.
Jump starting is not so obvious. What I have come across is that there are
huge voltage and current spikes that occur just as the other car fires.
What happens is initially running/charging puts a heavier load on the
alternator. Then when cranking the engine, the load increases tremendously.
At the time it fires and you release the key, the load requirements drop
completely and suddenly all the extra current causes a huge voltage spike,
plus the second car's charging system has also just added to the voltage.
Yes, I know that voltage on parallel circuits will remain the same across,
however, there is much more current. This increase in current causes a much
larger voltage spike across internal components and blows out the regulator
or one of the diode packs. The diode packs are rated by current, so if the
current is too large, out goes the diodes.
Sometimes they short, so then instead of getting rectified DC, you continue
to get AC, which means during a half cycle you are discharging the battery
on one phase. this usually means that you end up with a dead battery in a
If the phase is blown out - no current in one phase - then you end up with a
weak charging system that after a hard start (long cranking in winter, left
lights on, etc.), the system may not recharge the battery completely in a
short trip and then you have the "dead battery" and can't start, but in
longer trips, you have no problem.
Tom and Key,
Thanks so much for all the great help.. alway better to have more brains
on the subject... (especially brains with knowledge about electrical
probs) Thanks again for all the advise..and will sure let you all know how
it all works out
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.