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How to diagnose electrical problems part one

 
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leadfoot  



Joined: 11 Dec 2002
Posts: 2222
Location: gOLD cOAST Australia

PostPosted: Wed Jun 07, 2006 12:47 pm    Post subject: How to diagnose electrical problems part one Reply with quote

WARNING: Electricity is dangerous and can KILL... always excercise caution when testing this system on the car, if you don't feel confident ask for assistance from someone who has experience, always inform someone else of your whereabouts when working on your car, a friend ,neighbour,husband etc and have a first aid kit ready on standby...

Dead Battery
Charge the battery for at least one hour. Check for clean connections at the battery terminals, starter and grounds. Use the starter to crank the engine over five or six times. Attach a voltmeter to the battery and watch its reading as someone cranks the engine several times. The voltage should stay at 12 volts when the engine is not cranking. If the voltage drops below eight or nine volts while cranking, or the engine won’t crank any more, suspect the battery. If you suspect the battery, and it’s not very old you may have a slow drain or defective battery.

A battery should only be topped up with distilled water or a mix of hydrochloric acid/distilled water... anything else will ruin your battery. Fill up the cell to the top of the plate inside, there is usually a plastic marker on the inside that serves as a reference point for this. If the battery is topped up , charged overnight and still reading low voltage (11V or less) the acid levels need attention. You need a specific gravity tester from the auto tools supplier that has three discs in its main body, they float at different levels of gravity, so suck up some acid from one cell at a time and it will indicate which cells need attention. To do this a required mix of hydrochloric acid and distilled water must be added at the correct ratio, do a search on the internet for this... Wear safety gogles whilst topping up the battery and be careful as battery acid can ruin paintwork, clothes and your day...
Always wash your hand straight after handling the battery as rubbing your eyes or skin can cause serious problems. Most batteries last for around 2/3 years and even longer if maintained regularly.

Slow Battery Drain
If the battery is draining overnight or over the course of a few days, some device is still turned on and draining it. To find the cause, disconnect the negative battery connection. Use your test light to jump the negative battery cable to the negative post on the battery. If the light glows, something is turned on. Disconnect fuses and/or circuits one by one until the light goes out.

Trace the circuit that was causing the light to glow to find which device is still on. Dome lights, trunk lights, alternators, and non-factory accessory circuits are common causes of such drains. Radio memories and dash clocks usually are not drains and will not make the light glow for this test.

Alternator Over- or Undercharging
Attach a voltmeter to a good ground and a good positive lead. (Usually, the battery works best for this.) With the engine switched off, the battery voltage should read 12 volts. With the engine running, the voltage should read 13.5 to 14.5 volts.

Below 13.5 volts usually signals a non-working charging circuit. Check for a tight belt, and clean connections at the alternator and the battery. Also make sure the engine is properly grounded.
Above 14.5 volts usually signals a bad voltage regulator. Either way, the solution is usually a new or rebuilt alternator. Replacing the regulator is quite easy and cheap ,the part number is # and the procedure is as follows...

Remove the negative positive terminal from the battery, remove the belly pan, jack up the front end, remove alternator belt,ground cable and the altenator should pivot loosely from one bolt with a 6mm allen head. Remove this bolt whist supporting the alternator as it will drop away once the bolt is removed. the bushings and curved washers will fall at the same time don't loose them. Check their condition whilst your at it, they will oval and crack with age. Remove the back cover from the alternator and undo the two bolts holding in the regulator. Fit the new regulator and reinstall. Inspect the wiring that it isn't cracked and that the resistance is around 4 Ohms, anything more and the cable is starting to loose its conductivity which is either caused by, oxidisation at the connectors or fatigue and cracking of the cable itself. If re terminating the cable doesn't fix the problem the cable must be replaced.

Crank Starter, Everything Goes Dead
Sometimes everything will seem just fine until you crank the starter, then nothing will work, not even the dome light. Starting with the battery terminals, remove them and give them a good cleaning. Then clean the ground strap to the body and to the engine. Then clean the positive connection to the starter. One or more of these connections is corroded. The load of the starter causes arcing at the corroded connection, which weakens the connection. Since these connections are the main power connection for the whole car, they shut everything else down when they get too weak.

Wiring applicable to the N/A
Above the starter is the exhaust manifold, a very hot place yes indeed... you should have a shroud protecting not only the starter but also the wiring between the alternator and starter and the starter to battery. It is a must and removing this will cause very premature loss of the starter and alternator.
Using adhesive heat protective tape place som on the outer side of the shroud and also wrap the wiring previously mentioned. A roll of tape is around $30/40 AUS, $20 US and worth using as a preventative measure. The guage of wiring used for the Alternator/Starter should be gauged to carry the right amount of amperage, and inspect your cabling to ensure this.
Leadfoot
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Thu Jun 08, 2006 9:36 pm    Post subject: Reply with quote

Cross-reference - How-To understand wiring diagrams...
http://www.924.org/techsection/elect_diag.htm
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El Chato  



Joined: 22 Aug 2006
Posts: 205
Location: El Paso, TX

PostPosted: Tue Dec 02, 2008 1:43 pm    Post subject: Hi, big question Reply with quote

I had a short cirquit, and now I've notice that if I remove the negative lead from the battery while the car is running it runs as it should but when I remove the positive the car immediately stalls, strange, I tried what you said earlier about using a light tester, and the bulb lights up dimly, have you seen this discrepant before? I will start to play with the fuses and locate the source, not to say I was surprise there is voltage from negative to negative, great how to
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poorsha924  



Joined: 13 Nov 2007
Posts: 143
Location: Kansas City Mo

PostPosted: Tue Mar 03, 2009 12:50 pm    Post subject: eletrical diag Reply with quote

Hey El Chato
sounds like what you have is there is just enough ground thru the body & altenator ground cable to complete the ground side curcuit. When you disconnect the positive cable the altenator should take place of the battery Battery current flows back to the battery thru the negative side. sounds like you have a bad altenator.
good luck!!
poorsha924
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El Chato  



Joined: 22 Aug 2006
Posts: 205
Location: El Paso, TX

PostPosted: Tue Mar 03, 2009 2:07 pm    Post subject: Reply with quote

Thanks, I had the alternator checked seemed to trow the proper amps, it took me a while but I found a short that is somewhere in the internal ligths, so i isolated it coming out of the fuse panel, took one short which did not let turn the healigth motor or have brake ligths, then another short is still there but not giving me any problems, as like you said yeah the car should keep running after I take the battery of, seems like its the two red cables that also come out of the positive terminal, when I take these out it dies, I know one goes to the pump feed but I forget the other one, so ill check the back feed of the alternator,

I recently changed the voltage regulator, but no luck, I defenetly will check all the alternator wirering.
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Tue Mar 03, 2009 10:24 pm    Post subject: Reply with quote

You should NOT disconnect the alternator while the car is running. It WILL destroy your voltage regulator, no ifs-ands-or-buts about it.

One other thing I encountered recently on this topic was an article in Panorama. It was discussing the slow drain problem on late 70s and early 80s Porsches. One of the causes it cited was old and failing relays that do not properly open the circuit they're on when the car is turned off. This can cause annoying and difficult to find battery leaks. I am almost positive I have this problem on my 1980 Euro 928. Later this warm season, when I put that car back into commission, I am going to spend some time working on the electrics.
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El Chato  



Joined: 22 Aug 2006
Posts: 205
Location: El Paso, TX

PostPosted: Wed Mar 04, 2009 6:10 pm    Post subject: Reply with quote

Sweet, u also got a 928, cheers, thats defenetly one I also like to have, saw one in ebay in super mint condition with 1.4 K miles on it for 70K, maybe in 5 years ill come true. Electronics is almost like medicine I feel, you gotta keep lurking till everything can make some sense to diagnose the problem.

It sounds very true, relays gathering humidity and then getting stuck, contact cleaner or replacement would be a better way to eliminate a discrepant, so far the battery keeps its juice, right now just got 2 months left to finish college, and then I can tackle all the cars needs to restore it. among them are clearing all eletric faults, suspension, hacth sqeeking, A/C, dashboard, carpets, all uphostery in ensecense, deoxidefication and a new paint job. I come to love this car so much. That I hope after restoration I can keep it for many years to come, and just maybe reach that million mile waranty on the K&N filter.
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ideola  



Joined: 01 Oct 2004
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Location: Spring Lake MI

PostPosted: Wed Mar 04, 2009 9:31 pm    Post subject: Reply with quote

El Chato wrote:
and just maybe reach that million mile waranty on the K&N filter.

I like that goal
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El Chato  



Joined: 22 Aug 2006
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Location: El Paso, TX

PostPosted: Thu Mar 05, 2009 8:32 am    Post subject: Reply with quote

that got me going, what is the most miles you goten out of the engine and transs, or the most someone has gotten from them?
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Nobbi  



Joined: 24 Aug 2006
Posts: 1379
Location: Germany

PostPosted: Thu Mar 05, 2009 8:18 pm    Post subject: Reply with quote

Ideola is right with the relais, an easy way to find a hanging-relais is simply touching it! They are warm or even hot when stuck!! Some relais like the one for the fuelpump have extra fuses which can fail too.Its worth to replace all of them frequently.......
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Grenadiers  



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PostPosted: Fri Mar 06, 2009 4:35 am    Post subject: Reply with quote

Real life tip as well: My fans were working on an intermittent basis on the 931. I could push on the fan relay and they'd kick in. So, finally found enough warm temperatures outside to get a look at it. The upper left dinkoid female terminal was cracked and loose in the socket. Must have gotten boogered from pulling the fuse block out a few times. Wired in another terminal from my n/a parts supply house (the garage), and now works like a charm. Also, since it's a brown wire, must be a ground wire. I solved my leaky battery with that fix as well!
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