bronto

I removed all fuses, one by one, and none caused the circuit tester bulb to dim in any noticable way. I was reduced to a simple bulb style tester because it blew two fuses in my cheap multi-tester.

Suggestions?

Dennis Wilson

Use your bulb tester to determine which fuses are always live (battery circuit) by connecting one end to ground and touching each fuse. Make a list of which ones light up the tester. Next go back and check each by removing the fuse then connect the tester between each fuse contact. Now if they light up the tester, you have draw on the battery. Some, like the radio, will have a small draw to maintain the program but others like the brake lights, cigarette lighter, interior lights should not be on unless you have the switch activated.

Dennis

bronto

Good suggestion. Here's more to chew on.

I just pulled each plug and relay in the panel, and none except the main leads caused the bulb to dim. I am assuming that what I am talking about are the main leads. These are the three large guage single wires going in to the panel near the center of the 3rd row, the row that has all the multi-wire plugs in them. The bulb did not dim unless *all* of the 3 power leads were unplugged.

Dennis Wilson

The three wires are battery feeds to the relay panel. That has eliminated a short in the battery positive cable and in the loom coming from the jump post.

Dennis

bronto

Doesn't it also confirm that it's not an unfused circuit, or something like the alternator as others have mentioned?

bronto

Fuses 14 through 23 are all hot. Fuse numbers 15 & 23 light up the tester when the top and bottom contacts are connected with the fuse removed.

While doing this, I realized that the bulb for the glove box was burned out, so I wouldn't know if that circuit was live before now. I don't have a replacement bulb handy , but I connected the tester between the bulb contacts and it operates fine, and reacts correctly to the switch on the glove box door. The switch breaks the circuit at the top of it's travel, so I really doubt that there's a switch problem there.

I don't know what #15 does, but #23 seems to control the interior accessories like lights & clock.

docmirror

Well, you've got some tedious work ahead. You'll need a good digital meter, with at least a 2 amp, preferably a 10 amp DC range. Fluke model 73 or equivalent.

Next thing is the work shop manual with the circuit diagrams. Also, you'll need the PN of all the relays. I would check to make sure you have all the right PN in all the right sockets. The 79 looks to be pretty basic, but there's a bunch of stuff to check. You may have more than one or two circuits adding up to a high draw, and you'll need to work these separately.

Connect the DC ammeter in series with the - (negative) terminal of the batt and the batt lead. Plug in every fuse and relay, close the door, key off and get a measurement. That is your resting current draw, and like Alan said it should be less than 15-20mA. I know it'll be higher but make a note of the amount. Now take the fuses out one at a time, and note if the reading goes down after any fuse pull. Once all the fuses are out, start on the relays. If you still have a high current draw, you'll have to dig into the supply wires where you've already been.

Work methodical, and make notes. Each load can be traced back to some circuit with the diagram. Aftermarket amps and alarms are notorious for drawing current and being unfused. Good luck, it takes time, but you can get it. There's guys in Socal that can lend a hand in person I think. Recheck the relay part numbers, cause the wrong one in the socket can cause your symtoms.

Doc

bronto

How much would I expect to pay a Porsche mechanic to figure this out?

SharkSkin

Quite a bit, really... and they might not find the problem. Just ask Ron_H...

the flyin' scotsman

Do you mean that both top and bottom contacts have power to them without the fuse installed?

If so then you have a dead short across these fuse positions; pull the panel and see if there's a metal object bridging the contacts or fused wiring.........yikes!

bronto

No. I did the test you are referring to (by mistake, before reading Dennis's instructions more carefully) and none of them have power to both contacts without the fuse installed. Dennis's instructions (if I'm reading them right) are to clip one end of the circuit tester to one fuse contact, and touch to the other fuse contact with the tester pointer. So the point is to find the circuits that are live all the time.

the flyin' scotsman

ok...........lets make absolutely sure your doing the test correctly..............the circuit tester should be grounded at one end and the other used to test for power. When checking the fuse panel with the fuses removed you should see power only on one of the contacts and not the other. If you do find power on both contacts you have a short somewhere in the circuit.

That being complete and found to be ok we can get back to the problem.

docmirror

Well, that's just the problem, right there what you said in the question. "Porsche Mechanic". These guys are break-fix kind of folks. They diagnose broken parts, remove and replace them. The circuitry of any 928 is a bit of a challenge, and when it comes to diagnosing loads, it can get costly very quick, with no guarantee of complete success.

The shop will tell you to set a time, or price limit to work with. If that's five hours @ $100 per that'll be $500. Will it find all the unusual loads? I don't know, and neither will they. Shops love this kind of work because it's open-ended. Mechanics generally hate it because they almost nver have enough time to do the job "right".

I could envision a Porsche shop starting the discussion with 'we will remove any aftermarket equipment and wiring first. Then, we will work per-hour to resolve any PORSCHE related issues'. Maybe the mechanic is good at electrical troubleshooting, and maybe he's just learning the ropes on your dime. A good electrical diagnostician is valuable. A bad one can make a modest problem much worse.

I've seen such horrible work from Porsche shops, that I would be searching for an auto electrical specialist, not associated with Porsche. Bring them all the schematic prints, and let them have at it. They have the tools, know-how, and experience to find this kind of stuff. you can also give them ideas on the top probables, like the wiring of the dome light, relay issues, and possible alarm/stereo crap.

Doc

bronto

Yes, that's correct. With the "clip" end of the tester grounded, only one fuse contact had power - lit up the bulb when touched with the testing probe.

bronto

I have two possible routes for "work done". I originally had the car checked over by the most reputable independent Porsche shop in town last month. I think they did a pretty good job, although they didn't detect the battery drain. The battery that was in it was quite old, and they just said it needed a new battery because it wouldn't hold a charge. At the same time, I don't think I'm their typical customer; Subject to the limits of time and working space (I live in a condo) I'd prefer to do as much work myself as possible. It's been about 15 years, but I used to do a *lot* of auto work on my own cars. Their typical customer (I suspect) owns the Porsche for prestige and does not want to get his hands dirty. As evidence of this, they fixed a few things on the car without asking that I was planning on fixing myself. This is not just a car for me, it's a hobby. The other route is that I've discovered an independent mechanic who has his own shop, factory trained by Porsche - when 928's were new, in fact - and has owned several of them himself. He has worked for the dealer as well as the aforementioned independent shop. I don't know him personally, but we have mutual friends and I've chatted with him by phone. This might be the first work I bring him. He is enthusiastic about accomodating my hobby and being an advisor.