1993 850 Why is battery light coming on when it's rainy?

Post by **JimBee** » 01 Jun 2014, 17:34

Through winter all worked well. When the weather warmed, on rainy days, my battery dash light comes on just after starting the engine and when I first accelerate. Letting off the accelerator to shift, the light goes out, then bringing up a few rpms in second it's on again very briefly. Usually by third and fourth gears it stays off and doesn't come again while the engine is still warm.

Suggestions welcome!

Post by **kahl** » 01 Jun 2014, 18:39

Slipping belt? gets wet then slips, may be worn.

Post by **abscate** » 01 Jun 2014, 18:42

I would guess a faulty ground is indicated. Have you tried wipers and lights on a dry day to test load?

Post by **JimBee** » 01 Jun 2014, 18:53

The wipers have worked normally when the light comes on, and I think the headlights are on (the switch is always on and I see them on the car ahead of mine at the curb).
The slipping belt idea—hmmm. If the serp belt tensioner isn't quite up to its spec's, that might be a possibility. I replaced that with a new one from FCP just a couple of thousand miles ago, though, so I wouldn't expect it to be weak. I haven't noticed a problem with my p.s. pump, but if it was dragging, that could slow everything down. Then, wouldn't I hear the belt squealing?

Post by **jblackburn** » 01 Jun 2014, 21:05

If I drove through a big puddle and made a big splash, you could hear the serp belt squealing on my S70 - alternator light would come on as well for a few seconds.

Post by **misha** » 02 Jun 2014, 17:04

I would say that you have a bad ground or short somewhere which is more present when area gets wet.Like all of us know,water is great conductor.

Post by **JimBee** » 02 Jun 2014, 21:12

To check out the bad ground theory, I suppose I'd start with the obvious: ground straps to firewall. From there, I'm not sure how to proceed. I appreciate any suggestions.
I should clarify that it only happens when the car has been sitting, like overnight and it has been raining. When I start up I notice it, just bringing up the rpms through first and second gear. When I let off the accelerator between shifts it goes out. By third gear, it's usually off. Though if I stop at an intersection and start up again, it will go through the same 1st gear, on, off between shifts, 2nd gear on, then off as I shift up to 3rd and 4th. Wouldn't a bad ground be continuous? Or would it? I'm puzzled.

shegarty · Post by **shegarty** » 03 Jun 2014, 09:37

The "battery" is really an MIL for your entire charging system. I would suggest putting a meter across your battery with the engine running to see what king of voltage your alternator is putting to your battery. If this is less than 14-14.5V you may need to replace the brush pack or the whole alternator. What you are describing sounds like an alternator issue of some sort as the light is only triggered under load. When you are testing the voltage output try it under load as well. You might find it helpful to run some jumper leads into the cabin so you can read it while driving. Otherwise you can try giving it throttle by actuating the cable under the hood. Make sure to check all the connections to the alternator as well, especially the heavy positive back to the battery. Even if the cable appears to be in good shape you might also want to do a voltage drop test on that line to make sure there is no internal voltage loss. I would also be inclined to do a load test on your battery if it is more than 4-5 years old, any autoparts store can do this as it requires a somewhat specialized tool (you can sometimes find them on sale).