Alternator?

[redcoupe86]

I have an ‘86 turbo with a relatively new battery.

I hook my car up to a battery tender since I only drive a couple times a month.

Before I leave for a 15 min drive the tender says 100% charged.

Car running I get 13.86v at the battery on the VM.

I take my 15 min drive and when I hook the battery tender back up it says 83%.

Now in between shutting the car down and hooking up the tender the cooling fan runs for 30 seconds or so. Seems it wouldn’t drain the battery to 83%, right?

Also while car is at idle if I put the window down till it bottoms out that load causes the rpm to drop as long as I have my finger in the button.

Any thoughts?

[944 Ecology]

Use a real battery tester... not an electronic one... I have had mine for 40 years and it has not failed me yet.

https://www.amazon.com/Battery-Tester-Voltage-Automotive-Batteries/dp/B0BFW6Y919/

[Monkey Wrench]

try leaving headlights on 1/2 hr, if it wont take that, battery is weakor not beimg charged up properly.

also check your battery to frame ground, everything electrical depends upon that connection and its easy to think its ok. similar with engine ground and alternator ground, ground for headlights.

cleaning all your grounds up well is free, parts cost extra , yes the problem can be elsewhere but start with whats free.

there is nothing on the car that uses power and does not use the ground points. often the issue is not complete abscence of a ground but a weak (dirty, rusted) thus high resistance ground point

when the battry charges by voltage applied to its red terminal , it still needs a good ground. in other words a bad ground can also affect charging capabilities. also if alternator itself has a bad ground it can look like a bad alternator, as the result can be the battery is not charging up properly.

you can remove your alternator and have it bench tested woth a load applied, if it has a bad ground terminal youll put it back , likely rebuilt and see similar issues IF the ground is your fault point. dont negelct to examine the external ground fron the case of the alternator to engine and frame as your fail point .

chances are if you remove a alternator you may remove a ground from the altenrator and neglect to check if that lead in itself has a good ground.

your charging system relies on the ground lead of the alternator ot chassis or engine, and the battery to chassis ground as well. those connections just as important as any hot lead, and easily overlooked and missed.

often as ground points fail they can surface as all sorts of unique circumstances often confusing the situation. it can cause poor cranking when the starter is fine, it can cause poor charging when the alternator is fine , it can make you think your battery is weak whan its fine, so start there..

as i clean I like using this stuff, to keep oxidation from coming back quickly, If you use that one, shop around the price varies a LOT. and there are also a lot of competing products. so you can study and decide what product you feel is best.

this stuff is non conductive,it is not a conductive paste that can bridge gaps. it is used to prevent new oxidation of alrrady cleaned connections.

the Porsche has a lot of alunimum and one property of aluminum is that it reacts easily to corrosion by way of electrolytic reaction , the 944 also has a lot of zinc coating or galvanization, alunimum and zinc are low on the scale of noble metals, in short, easily part of an electrolytic reaction caused by dissimilar metals in contact or in close proximity..

aluminum has a lot of good properties but it's ability to corrode easily is generally not one of them so watch any electrical connections that involve aluminum parts with a bit more caution.





https://hubbellcdn.com/specsheet/BRDY-P8A-SPEC-EN.pdf



not doing so often results in the cost of unnecessaruly replacing parts that are still OK.

[huskerick1]

do yourself a favor and replace the battery cables... I spent two years troubleshooting electrical gremlins on my 89 951 and as a final attempt, replaced the cables. Magically, ALL of the gremlins disappeared too.
Also, as noted above, do ensure all the grounds are cleaned too...
But, if the cables are full of corrosion like mine were (and hidden), nothing else will resolve.

[huskerick1]

and, if you need an alternator, I rebuild the OEM Bosch units... I have one on hand that I could send for testing.

[TibetanT]

Voltage Regulator needs to be replaced??

[redcoupe86]

An update. I did look at grounds (and clean them), the battery is relatively new.

I went ahead and bought a rebuilt alt. Now I’m getting 13.9v +/- and when I put an electrical load like holding the down on the windows, the voltage doesn’t drop like before. So so far so good.

[Jfrahm]

You might have only needed a voltage regulator as Ed suggested, and in any case you might want to get a higher quality one than they usually give you with a rebuild, even a Bosch rebuild. I have been using Hitachi regulators lately but the Hellas might also be good.
-Joel.