Hi everyone, first post and have been through the forms but have not found enough info to help me with this issue.

87 928 s4 auto has been very reliable but lately has done this a couple of times. i'll be driving along and it, as if some one has turned off the light switch. i loose all electrical in the vehicle. checked at the batterery voltage is good but at the jump points in the front it shows 3 volts. now here's where it gets better, let the car sit for a while and i guess cool down and the voltage comes back. car starts again ok. can disconnect battery and vehicle will run no problem. if you try to jump the vehicle right away it runs horrible , misses and stalls.

Now two other things i've noticed some times when your driving and the fan comes on, on the climate control it sometimes but not always will cause a short or some thing along those lines as it will reset all the instrument guages and the stereo as if you tripped a breaker and then reset it in an instant. The other thing i noticed is that the volt meter will sometimes show 14-15 volts but when tested at the jump point it it's normal.

Now given if these things were consistant they would be alot easier to run down but they are intermitant, just yesterday all was back to normal again. started fine let her run for an hour solid to see if anything would happen turned climate on and off auto and manually and not a thing went wrong.

I'm thinkinking voltage regulator from what i've read but would like some more advice on what else to check and look at. alternator seems ok and proper voltage is there when running. will still have to check the multi plug near the jump point on the passenger side.

Thankyou in advance to those who take the time to answer.

 

I would have the battery tested at a good place - load tested on a carbon pile load tester - many places will tell you they have something more modern and better - go somewhere else that actually has one - its usualy a big box with a fan built in. It actually tests how well the battery perfoms under full load...

If the battery checks out well then replace the battery ground strap.

The voltage regulator does not control the system voltage below 12v (battery level) - it only controls the alternator output..between 12.7-~14v for the dash to reset you are hitting probably 8v... something is very amiss here.

It could be that the alternator is not charging and the battery has an intermittent connection... however I'm not sure I believe that...

Start here and report what you find?

I'm not sure if I understood what you said - but NEVER EVER run a 928 without the battery connected...
you can do lots of damage very fast this way...

Alan

 

thanks alan for your quick response. i've check the ground strap before and cleaned the ground points check the strap with a meter an it is ok. i will get the battery checked and let you know but it is only one year old ( i know means nothing)

Now i am curious as to why never run a 928 with out the battery and what damage it could do. As far as i know with my previous vehicles the the system should be able to sustain itself . i don't know of what damage could be done, but i am very interested in learning. By the way it only ran without the battery hooked up for about 30 seconds so i could test that i was getting proper voltage back to the battery to make sure the system was charging it.

thanks again.

 

It may check OK most of the time... that means nothing - check it when there is an issue...

Without a battery the voltage regulation will be awful (may not be obvious) - your ECUs are very expensive and very delicate when it come to voltage spikes... dont do it...

Alan

 

Rider if you look in the Work Shop Manual it clearly gives all of the warnings for the charging system, one of them is to NOT run the car with the battery disconnected from the system, as computer and or alternator damage will occur.
Most likely is that the regulator is now damaged, if its still running.
This also goes for trying to jump the car having a dead battery, the voltage spikes can damage the computer or alternator.
You could also check the hot post connections under the hood and the 14 pin connector above it, clean them with a pink eraser.
Also check the CE panel and look just above it for water tell tales, if water has been leaking on the CE panel then you should remove and clean every fuse, and also remove the relays and check the pins for corrosion, and corroded pins indicate that the relay should also be opend to check the internal contacts.
You can get a fuse/relay chart here www.928gt.com find page 2 then find tips and link and then fuse relay charts, print one off for your car
I would follow Alans advise he is the Elex guru in this neck of the woods, i hope that you get the car fixed

 

Hogrider,

" i'll be driving along and it, as if some one has turned off the light switch. i loose all electrical in the vehicle. checked at the batterery voltage is good but at the jump points in the front it shows 3 volts."

Nothing in the car except for a faulty connection between the battery and the jump post will cause your problem.

If you tested the voltage at the battery by checking from post to post, you checked the battery. If you check the voltage from the positive post to the body/chassis, you would include the battery ground strap, which is one of the usual suspects. If the ground strap or either of its connections is faulty, this will cause your problem. Clean both ends of the ground strap, the battery posts (both), the spot where the strap connects, and the positive cable end. A bad connection at any of these can cause your problem.

If this does not cure the problem, disconnect the ground strap, making sure that it can't fall onto the body and make a connection. Carefully jack and support the car so that you can safely work underneath. Follow the positive battery cable to the front of the car, inspecting it for any visible damage. Disconnect and clean the main connection at the starter, inspecting all of the cable ends.

Visually follow the cable from the starter to the jump start terminal, inspecting it for damage. Disconnect/disassemble the connection at the jump start terminal, inspecting the cable end for damage.

This should take care of your immediate problem.

The potential damage from running the engine with the battery disconnected is caused by poor voltage control. The voltage regulator in the alternator uses the battery to help control the voltage output from the alternator. The alternator is capable of putting out over 125 volts if uncontrolled. This voltage level can definitely cause serious damage to any electronics on the car, especially the engine ECUs. That is a very risky test. It is much better to use a voltmeter to see if the alternator is working.

 

Thank you all for your insight.

I see in the manuals now where it says not to do it. don't know why i didn't see this before, luckily it has not done any damage as she starts and purs away.

double checked the ground it's o.k. right now she starts and purs away as if there is no issue at all. it's the intermitentness of this that is driving me crazy. i will jack her up on the weekend and check all connections underneath ad see if i can find something ovious or if my meter tells me somthing else.

Now one thing that i can,t figure out is why when it happens i only get a 3 volt reading at the forward jump point? the battery is giving 12v. the voltage slowly comes back as if you lowered the resistance slowly. at first i though it could be caused by heat expanding and contracting a connection but the engine area does not always have to be hot when this happens sometimes it can do it right after a start other times it might be a few weeks before she does it again.

i did forget to mention when jumping the car i'm using proper jump start kits with regulators in them to control the voltage and they are connected via the front jump points when the battery was unhooked, so in essance i guess it took the place of the battery. probably why no damage occured.

Again i truly appreciate all the knowledge here and thank you for your help.

 

there is a ground wire thats connected to the right side of the blok and then to the frame.
I would also follow Wallys post regarding the starter wire, remove the wires and clean them ,then reattach.
Always work with the battery disconnected

 

I concur with the experts above(not surprising). This is a primary wiring fault. You will need to check the large cables carefully. Based on your failure mode, you have an intermittent open or worse, a short on the primary + side wiring. Start at the battery cables, and trace all of them forward carefully. Look for any part of the insulation which is discolored differently than the insulation around it, or insulation which is getting hard and cracked. Heat effect indicates the fault may be in the engine bay, but that's not always the case with the 928 since it uses a great deal of current throughout much of the car. At worst, if you can't pin it down, you may be stuck replacing all your primary wiring from the battery, to the starter, jump post, and aux hot post on the inside left fender near the PS tank. Not a fun job.

 

Try this when the car isn't running - flex the battery ground strap while a DMM (in voltmeter mode) is connected between the battery ground post and a seperate point on the chassis... does it ever read in the volts range....

Connecting another battery source (usually another car) to the jump post is not really a major risk in my view... its disconnecting it again that it becomes risky...

If your battery is just low on charge but basically good - this is low risk.

If on the other hand your battery is fatally damaged and basically useless - it will not act as a regulation aid for the alternator - so as soon as you remove the jumper to the "extra" battery - its the same thing as runing without a battery - bad news...

Sometimes its possible to judge this - sometimes not... if your battery can't even illuminate the dashboard - I would say don't even think about it ... its always better to charge the battery off the car or swap in a known good battery.. neither of these is easy in the middle of nowhere...

Alan