MrAdequateWrench

My '82 928 won't start. It has always been reliable, I drive it almost every day. I moved it in my driveway, but when I went to move it again, 5 minutes later, there was no power to anything. Nothing happened when I turn the key. Horn, hazard & dome lights do not work. The battery is fine, there is 12V between the + battery terminal & the chassis. Is there a 'usual suspect', or do I need to start tracing wires?

j.kenzie@sbcglobal.net

These don't fit perfectly for universal no power with 12 volts from batt + to chassis:
Consider battery ground strap or ignition switch.
Good luck,
Dave

MrAdequateWrench

Thanks, there doesn't seem to be any power getting to the fuse box. It it all sourced through the ignition key?

j.kenzie@sbcglobal.net

By memory, power goes from battery to starter to alternator to jump post to fuse block. It does not all go through the ignition switch, or even the fuse block. You could see if you have 12 volts at the jump post under the hood.
Good luck,
Dave

dr bob

No.

Power flows from the battery through a large cable to the starter. Cable runs along the left "frame" rail, makes a right turn towards the starter. That's in the middle of the car under your knees give or take. Power flows from there up to the alternator, than on towards the jump-start post on the right side of the engine bay against the fender wall. From there, it flows through a couple smaller conductors to the central electrics (fuse and relay) panel under/forward of the passenger's feet.

Test for voltage under load (headlights on) at that jump post connection, as that will help you isolate the problem to before or after that point in the current flow. Then diagnose forward or back in the faulted section until you identify the cause.

If you haven't done so already and somewhat recently, go through the recommended annual electrical maintenance protocol. A copy of this is available at the 928 Specialist website the "Wally's Tip" section. 928 Specialists is the Forum Sponsor so you can click on their banner. Or go to www.928gt.com and follow the menu to find the guidance needed. Wally's protocol includes cleaning and maintaining the grounds, the cables, and some of the critical main power system connections. None of it is hard, but it takes some time and diligence to get everything done correctly. OTOH, that time is well spent, and often solves a slew of casual electrical symptoms along the way.

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From your symptoms, I'd start at the battery end of the system. Clean and secure the main battery post connections plus the minor connections on the positive terminal. I'd put a good charger/maintainer directly on the battery while the rest of the work is being done. If the battery is more than a few years old and/or has been deeply discharged in its life, it should be tested for specific gravity and charge density after it's been on the charger long enough to get it completely charged again. Look carefully at the ground strap from the battery. There's a copper braid wrapped in shrink sleeve that runs from the battery negative terminal to the rear "frame:" rail behind the tool cover. There should be a large wingnut there at the tool panel end. Inspect the ground strap carefully, as fumes from charging the battery get inside that sleeve and corrode the copper. Many of us have found that there's enough corrosion inside there to keep the battery from charging completely even though, at least from the outside, the strap looked just fine. New heavy-duty straps with tinned conductor braid are available from our regular 928 sponsors/suppliers. My own annual electrical maintenance ritual includes testing the specific gravity of the battery electrolyte. The tester is under $2 at exotic places like WalMart, and they are almost always available from regular parts stores. Clean the connections and tighten them adequately, then coat the connections with Vaseline to help keep the corrosive battery fumes out of the connections. I'll speculate that full-charged good battery, with shiny new connections for the power and ground cables, will solve your problem.

MrAdequateWrench

There is power getting to the starter motor main post and to the alternator terminal. I think I'll start digging out the ignition switch.

MrAdequateWrench

The battery is less than a year old. I'll clean the terminals, but I'm getting 12V at the starter & the alternator. I will next
check for power at the jump post, (which I did not know about, so thank you!) next. I cannot check for voltage under load, since nothing will turn on.

MrAdequateWrench

After seeing 12v at the starter and the alternator, I checked the jump post and it had power, too. Then I noticed my dome lights were lit, so I tried the horn. It worked! But when I tried starting it, it all stopped. So there was a little juice trickling through a bad connection somewhere. I cleaned the battery terminals & the end of the ground strap, but still nothing. Next is cleaning the terminals at the starter & the alternator & the jump post, I think.

j.kenzie@sbcglobal.net

The battery grounding straps are known to corrode internally. You could try a jumper cable from the battery negative post to the frame stud where the grounding strap mounts. That may temporarily solve your problem, and tell you that you need a new ground strap.
Dave

dr bob

Waste no time digging out the ignition switch.

Your symptoms point directly to a bad battery connection or a bad/discharged battery. the ignition switch did its part just fine as it tried to engage the starter. Not enough current available at the starter to crank the engine, the load drains all available current to ground via the starter and solenoid.

Follow the guidance I shared above. Download Wally's guide. Put a charger/maintainer on the battery as you work on the rest of the stuff. Inspect the ground strap at the battery, and replace it if it's the original. Clean All The Connections between the battery and the battery cables, the ground strap and the ground point for that strap on the rear apron.

Still true! Your problem is very simple. Don't make it tougher by dinking with things not related to your problem.

Chucks928

Dr Bob knows his stuff... seriously!

dr bob

I referenced Wally Plumley's Annual Electrical Maintenance guidance. Then I went to look for it on the 928 Specialists website and didn't see it in the Wally World section as I had suggested. I happen to have an archived copy, and share it here as PDF. All credit goes to Wally Plumley, a long-time 928 community supporter.


Here it is:

j.kenzie@sbcglobal.net

That's a great resource. Thanks to Wally, and thanks for posting, Dr. Bob.
Dave

MrAdequateWrench

I cleaned & tightened the battery terminals. The battery is 9 months old, fully charged, and is of a sealed design. I will not tinker with the ignition switch. The battery ground strap has been changed recently to a flat, braided, uninsulated style. I cleaned up the contacts at the jump post and when I applied voltage there, the car sprang back to life. Tomorrow morning, I plan to slide back under the car and clean the terminals at the starter & the alternator. Wish me luck!

dr bob

Glad you at least isolated the problem to the connections between battery and jump post!

Be sure to add an insulating layer to the bare ground strap, so that lifting the end at the rear rail will completely isolate the battery from the chassis. Without that the bare strap will still risk contact with the body. You would have to get into the battery well and lift a battery terminal anytime you decide to work on the electrics, as you propose for cleaning the starter connections for instance. You can use heatshrink for this to duplicate the factory look. But even a layer of Gorilla tape will do the job in a pinch.