LT Texan

Thunderstorms up here on Friday. Coming home from the train station, headlights on, aircon and fan on (surely the aux airfan on), wipers on, radio on.

Volts showing BELOW the red zone on the gauge! That's less than 10v.

Get home and test the voltage with my multi meter at the jump post. I read 11.9v with everything on.

With the engine off, got just 12v at the jump post. Battery is new. Need to check voltage at the battery posts - but would expect 12.5-13V minimum there.

Time to bite the bullet and replace the + battery cable. Oh, I have a new wiring harness too.

Got a Delco alternator. Seems to work okay. Why do we struggle with this?

928porschemangreg

11.9 is not that bad, but i agree the meter lies to you. Batteries alone will read dead 12 nothing connected. Alternators put out 13-14 volts with the goal of keeping up 12v everything on. If you were at 11v everything on, I would then take action.

WallyP

There are a lot of connections between the battery and the voltage gauge. Check the running voltage at the jump start terminal and compare it to the voltage gauge.

Then, either clean every connection (Central Electric, instrument panel), or just try to not think about it.

Also - the alternator belt has to be TIGHT. None of this "one-half-inch deflection with a thumb".

GlenL

Doesn't this show up weekly?

The numbers on these 17 to 34 yo gauges mean little. Look for sudden changes in running level and having a higher reading when running than when off.

If there is any question check with a real voltmeter across the battery or (easier) the jump post. 12.5 V off and 13.5 V on means it's healthy. Your voltages, especially the "on" voltage, may vary.

RKD in OKC

On my 952 the stock dash voltage gauge had a calibration adjustment screw.

LT Texan

this is probable some great advice.

911tracker85

I posted a thread a few weeks ago with a similar experience. ironically it happened just after I installed a new/rebuilt BOSCH from Roger. so put the old ALT back in and same symptom.

one question I got on my post was whether my belt was tight, but no comment on how tight it should be. I was not sure how tight it should be so adjusted with the 'standard' 1/2" play. recently I installed a new battery as I am sure mine was getting old/weak.

have not had the issue for the past few weeks as the temps have moderated. the situation I experienced was when temps are 90+.

getting ready to put the 'new' ALT back in and will ensure the belt is tight, unless someone else jumps into this thread and makes a different suggestion.

911tracker85

PS. had installed a new belt too.

and as Roger asked when I called to order the ALT, "why do you want repl the ALT if the old was working fine?" because is was old and this was part of my final phase of doing as much mechanical refresh on the car that was reasonable.

jeff spahn

Need has no home here.

Alan

There is not much electrically to an alternator. If it works mechanically OK then just replace the regulator for most of the benefit wrt wearout.

If a battery at rest ignition totally off is less than 12.6v then it is partly discharged (this is not NOT key on - becuase then there is loading). At 12.0v it is significantly discharged.

You should not see a difference between voltage at battery terminal and at the jump post under non-running conditions.

There can be connection issues in various places on either power or gound sides, but grounds are more likely initially - so start there.

The Bosch 928 alternator does not do well when very hot - just a fact. Gets worse with age, gets worse without a shroud, gets much worse @ very high ambient temps. Its mostly a regulator thing. Some headers make it worse due to radiated heat.

Solutions are - better crank/alt pulley ratio => bigger crank pulley (maybe ~15%) and/or remote regulator - to avoid engine temp increase. As Wally noted the belt has to be much tighter than most people think...

Swapping to an alternate alternator is also problematic - the stock crank pulley still gives a slow Alt. RPM - slower than most expect.

Always measures voltage with a good DMM - at best the dash guage is for relative estimations..

Alan

Speedtoys

http://www.davebarton.com/AdjustableVoltage.html

??

911tracker85

yea, I have trouble distinguishing between WANT and NEED.

with the Shark, WANT always wins when the funds are available.

interesting posts. need to discuss w/Roger where I got the rebuilt Bosch ALT.

my past conclusion suggested the issue was heat related. AND, I installed a set of MSDS headers this winter too. seems OK as long as I am moving, it was in slow/stop-go traffic when 90 that I saw the issue in the past.

my current/old ALT is not 'correct' and does not have a shroud, so not original but looks pretty old. when I got the new ALT from Roger, got the shroud/hose for it.

from checking with a meter at the jump post in the past, know the gauge is reading about 1 volt low, so I do take it as a relative data point and knew it may not be accurate.

I started carrying my meter to check when the low reading symptoms happened. when it did, got about 11.8 at the jump post. when the volt reading more 'normal' got 13.8

all the info is piling up to tell me my car may have issues when really hot.