308 Electrical Gremlin - Help!

Test the battery with a meter when the car is running, to be sure the alternator is charging it.

 

I had a similar problem before and it turns out to the alternator. Once the battery is jump, the care will run all day and all night without dying but once you turn the ignition off it will not start anymore as you're going to need a jump again. Somehow the alternator have just enough to keep the car run but not enough to start it. I had the alt rebuilt and it has been good since.

 

Unfortunately, the red warning light is only designed to detect a broken alternator belt. The red alternator (aka generator) light will often come "on" when an alternator goes bad, but not always. If you can get a multimeter, making some DC and AC voltage measurements at the battery terminals would be more conclusive.

 

Here's what I'd do... measure the DC voltage at the battery terminals with the engine NOT Running - just observe what the batt voltage is. Then, with the car running at idle, using the multimeter probes, check the voltage with the probes touching the top center of the battery posts, NOT touching the cable connectors. At idle you should see at least 13 volts. Then check the voltage with the probes touching the cable connectors. You should see exactly the same reading but if you don't, it indicates the connection is poor/corroded between the cable and the post and need to be cleaned. It is possible that probing the cable connectors shows proper alternator voltage while poor connections at the batt results in the batt not being charged. That's why it is important to check the batt voltage at the battery posts. Probing only the clamps can give you the wrong answer.

If neither reading produces 13+VDC,at idle (approx 1000 RPM) the alternator is not charging properly. It could still be connections at different points so it would be worthwhile to break/clean/remake the appropriate connections. It is very unlikely that an alternator failed while sitting unused for a week...

Another issue could be a defective battery but the voltage test on the charging system should detect that. If the alternator IS putting out appropriate voltage but the battery cannot start the car, that would indicate the battery is the problem Yes, new batteries are sometimes defective.

 

Nice breakdown, Mike.

 

It's possible that the car may have a phantom draw somewhere. Perhaps a door light? The radio antenna is constantly hot and so is the trouble light socket on the left hand side of the dash which seems to go unnoticed.
Now, if the battery is fully charged and is discharging, becoming flat on a 10 minute drive without anything else on, the issue may only lay with the battery.

 

Good advice posted here
Try swapping your relays around. Worked on my GT4 when it wouldn't start one day.
But the weak relay will possibly give probs somewhere else, like headlamp motor function, etc.
Worth a try to pinpoint what is going on.

 

I just came across a "SOLAR" battery checker which will display the actual CA and CCA of your battery by measuring the internal resistance through the plates. If you want to get a quick and easy readout of the health of the battery, this tester is awesome: https://www.amazon.com/BA7-100-1200-Electronic-Battery-System/dp/B0015PI7A4

It comes in a "BA5" which just tests the battery and a "BA7" which can also check the charging system as well. I have the 5... I'll be posting a video tomorrow on electrical gremlins in our Porsche 993 where I use the tester. I also used it in a video of a battery change on the 996. Neat bit of kit.

Franny

 

This.

 

also check for corrosion on the red battery cut off switch (next to the left /driver side) headlight housing

 

+1. In my 308QV the battery cut of switch made similar failure. I cleaned the conection and it works fine since then.

 

You really haven't provided enough info to tell what's happening. Then you have a no start issue, do the dash lights come on when you first turn the key? Do they dim when you turn the key to crank the engine or do you just get a click from the starter but all the dash lights remain relatively normal?