1979 928

[Jamesocchipinti]

New to the forum, hello all.
Question regard the 928 electrical components? Everything works on my car but it works very weekly. I replaced with new Bosh battery. Windows work at half speed, lights are about half brightness?? I just brought the car back to life after sitting for over a decade but all harnesses a pear to be in good condition???

Any ideas?

[WillyDaP]

Welcome and does this occur with the car running? Just wondering as one of the most common things to fail in many cars is the alternator.
Glad to have you as a new member.

[Tom]

While we wait for @dr bob to school us on 928 electrical issues, I'd be inclined to start by checking the battery voltage across the battery to make sure it's at least 12.6 with the motor off and well over 13 with the motor running. Assuming that checks out, I'd check your battery-to-chassis ground cable. If resistance builds up on a corroded group strap/cable, it can make everything weak/dim/slow, etc. Using a multimeter, you can measure the ohms between the neg battery terminal and the chassis/engine -- should be under 5 ideally. If you are not a multimeter guy, use jumper cables to enhance the ground. Attach one end of the black cable to the negative battery post, and the other end of the black cable to a clean bare metal somewhere on the chassis. (Leave the red unattached.) If the lights get brighter and windows speed up with the jumper cable attached, then there's something wrong with the ground strap/cable...

[Jamesocchipinti]

Thanks to both of you. I checked with multi meter, all conditions at 12.6 or better. I will check all of the grounds.

Merry Christmas!

[dr bob]

I took a day off for Christmas...

Welcome to Carpokes and the 928 forum!

This may be a little long-winded, but bear with me.

Tom hinted at perhaps a favorite electrical issue with the cars, and that's the grounding. Let's start with that basic problem.

The original battery ground strap is a well-known weak spot. It's braided copper strap wrapped in plastic shrink tubing. At the battery end in the sealed battery box, the corrosive fumes from charging the battery conspire to corrode the copper inside the plastic. Mine looked good but not great, but turned out to be bad enough to seriously affect charging. I found a 2ga tinned-copper replacement strap, added some heat-shrink for electrical insulation for the run out of the box, no issues in the last dozen-plus years. The company I sourced the cable from, Waytek Wiring Products, no longer lists that cable. I'm sure you can find one with a little searching though. I bought a dozen, and offered them to local owners at a clinic day when we lived above Los Angeles.

Related: The original Group 49 battery in your '79 has provision for connecting an external vent tube. Install one, and pass the tube through the positive cable grommet towards the ground. I used a stub of rubber vacuum hose for the connection, and some handy 1/4" plastic drip-irrigation tubing out of the box and down. Saves a slew of corrosion issues at the battery connections.

The workshop manual offers a 3D location diagram for ground points throughout the car. You'll need to look at the diagrams for later cars to get that though. The "index" page in your '79 CFD's lists the ground references by number as they appear in the current flow diagrams, but gives only a text description of their locations. That may be enough, as there are only 5 points listed for your 1979 car. Contrast with later cars with a whole lot more places for them to hide.

The goal of course is to have clean lugs and eyelets under clean bolts to clean chassis ground points. For the battery you use a battery terminal brush for the post and clamp. For virtually everything else I use a brass detailer's toothbrush. On my car, some of the lugs are tin-plated copper, and the brass brush is able to clean the plated rings without stripping the tin plating off. For the bare-copper lugs and rings, you can get a little more aggressive with a steel wire brush if needed.

Use the same wire brush on the bolts and washers, and on the chassis metal where the connections are clamped.

If there's corrosion on the way up into the ground wire insulation, don't be afraid to trim back those wires a bit and replace the lugs/ring terminals. You want good copper in the terminal, obviously. You can buy both bare copper and tinned copper lugs and ring terminals from Amazon if you can't find them locally. Use the proper crimping tool please. In a pinch, I've pulled the plastic sleeve off of parts-store terminals to get a plated ring terminal.

I add Vaseline to cover the terminals and connections once they are tight and proven. You can use silicone grease like SilGlide for a similar effect. Vaseline washes out of clothes a lot more easily, so it usually gets the nod for the duty. With a little hot-day heat, the grease will wick up into the wire and around the connection, and protect the connection from more corrosion. All the cad-plated ground bolts on my car still look new and bright after over 25 years of this treatment. There are commercial battery-terminal connection spray coating, but are a lot more trouble than they are worth in my experience. For actual connectors though, the Deoxit contact cleaner and preservatives are very popular among the experts at this stuff.

Long-time 928 sage Jim Bailey preached relay-relay-relay when chasing electrical ghosts and gremlins. I suspect some of the results come from pulling and replacing, when the terminals were wiped of corrosion in the process. Still, relays are often cheap (excepts specialty relays like lights and some others) so are useful for a quick and easy elimination of possibilities while diagnosing. The original ceramic or plastic "bullet" fuses in your car have undoubtedly corroded over time. The elements are beryllium copper, as are the tabs that hold the fuses. Still, we often end up with some corrosion and resistance at those points. Were I starting a whole electrical maintenance program, replacing those fuses with the original style (not the glass with plated metal bullet ends) would be high on my list of PM steps. The process of removing and installing new fuses, and maybe even just spinning the existing fuses a few times in the tabs may be enough to get you going.

To your specific issues: The lights and window motors in your early car both receive current via the "X" bus and the X contact in the ignition switch. It's no fun trying to probe the ignition switch connector to test for voltage from the X contact, but you can more easily find the feed from the switch at terminal A3 at the central electrics (CE) panel and large black-with-red wire, and at the top of fuse 6 (16A) that supplies current to the wiper switch and the motor. You should see battery voltage at those points with lights and wipers running, key on. Since the X bus supply from the ignition switch is the sole common element between the lights and wipers, I would focus my troubleshooting attention to that after doing the ground and fuses PM tasks.

I will alert you to the existence of 'bargain' aftermarket ignition switches, so if you go shopping for one, consider buying from a known-reliable supplier who vets the sources. I've known the folks at 928 International since before I bought the car, and gladly pay a little more for known-good pieces vs. trying to determine the quality of a low-priced search-engine substitute. Your dealer, online genuine parts places like Sunset Porsche in Portland, etc., will also have legitimate parts.

Please report back your results.

[Jamesocchipinti]

Dr. Bob, I very much appreciate the detail. The guys at 928 international are only about 5 miles away from where I live so I have frequented them lately.

I will report when done.