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I have a 1980 VW Dasher. Most of the electric systems perform noticeably weakly, e.g. the A/C fan, interior lamp, dash panel backlighting, windshield wipers, and headlamps. These symptoms have been present for as long as I've owned the car.

I measured about 8.5V on the headlamps with a multimeter.

Since multiple systems are affected, I'm wondering if it's the entire electrical system, or just some circuits.

The battery is good, and I have observed the same symptoms with previous batteries.

What are some things I can do to investigate the root cause further, and what are some common causes of undervolt conditions?

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    What is the voltage measured at the battery with/without the engine running?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 1, 2016 at 15:39
  • Engine off: 12.4V, engine on: 12.2V
    – Troy
    Commented Mar 1, 2016 at 16:00
  • There's your problem, the alternator is not charging the battery. Either the connection is bad, or the alternator is not putting out enough voltage to charge it. You should see 13.8V at least in order to charge that batter.
    – cdunn
    Commented Mar 1, 2016 at 16:03
  • Do you keep needing to recharge the battery off the vehicle?
    – HandyHowie
    Commented Mar 1, 2016 at 17:09
  • 1
    You should voltage drop test the ground side of the battery circuit. Connect one probe to the battery negative and the other to an engine ground. If you see 0.25 volts or more than you likely have a bad ground. The alternator on the Dasher should be putting out 14+ volts so you may want to check the wiring there as well. How many volts are on the field wire? You should expect to see ignition voltage.
    – Ben
    Commented Mar 1, 2016 at 22:50

1 Answer 1

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There are several things you can look for:

  • Complete paths
    • Electricity requires a complete path from positive to negative.
    • In a car with that kind of age, I would be checking that you don't have bad grounds or corroded connections. Especially where the ground straps connect between the body and engine block.
  • Slice and dice the problem
    • First thing to figure out is which systems are effected
    • Then go back to the electrical diagram for the car and see whats common to those systems. That's a likely place to find the issue.
    • To help isolate the problem, remove / turn off everything you can. See if turning something off cures the problem.
  • If the problem seems to be in every subsystem you look at
    • This sounds like either the battery is not grounded, or there is a partially bad connection from the battery positive
    • Measure the current coming out of the alternator. It should be within norms for idle with everything electrical turned off. If it's not, there's a partial short somewhere, or some subsystem is broken and drawing a lot of current.
    • Measure the current flow in the battery. If there's a lot of drain on the battery then it will drag the voltage low. But the amount of current required to drag it down to 8.5V would mean warm wires coming off the battery.
      • Measure the voltage at the battery with the cables connected, and again with them connected. Repeat the cables connected test with the engine running, and with the engine stopped. That will tell you if the alternator is charging the battery. Big thing is what's the voltage on that battery when disconnected from the car.

I hope that helps!

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