Hi Theo, thanks for your interesting reply.

I sourced and cleaned the grounds above the fuse board and under the steering column today; they had some corrosion on the wires and the ground points on the body were very dull. I cleaned them up and covered them after with the copper grease. They were a bit tricky to access.

I took pic's of the points I've done so far hoping they will assist someone else who may want to do this work themselves. I've enclosed them. Maybe you can put them in the same order as my write up?

Brief write-up.

Ground point maintenance on a 1990 Rhd 928 S4 with Cat. Tools required: 10mm socket and ratchet with flexible attachment and or spanner; 180 grit sand paper; small brush; copper grease.

  1. Disconnect battery. Clean battery ground strap under tool panel. While your there, good to clean up the battery terminals and the positive+negative connectors (including the small ones on the positive).
  2. Remove the plastic cover at the front of the spare wheel well. Inside is a ground point easy to access. Remove the 10mm bolt, Look closely at the ground straps for any corrosion and clean them up to a shine with the 180 paper. Do the same to the ground terminal on the body. It should shine when you've finished. Replace the bolt through the ground cables and screw it back in nice and tight. Cover the exposed part of the body ground and exposed cables with copper grease using the small brush. DON'T GET ANY INBETWEEN THE GROUND CABLE AND BODY GROUND POINT. Replace the plastic cover.
  3. Move around to the engine bay now and begin with the easiest to access; the ground located on the passenger side near to the headlight motor. Repeat above process.
  4. Now look to the driver's side behind the AC pipes; the ground is located on the inner wing. Access is very limited. Remove rubber seal and unscrew the wing nut close to the ground for best access. Using the 10mm socket with an extension and flexible attachment, access the ground bolt from above the AC pipes. Once you loosen the bolt to the point where it's nearly out, you must finish removing it by hand as to not drop it; very tricky but it's possible with the left hand. Repeat clean-up process as above. Replace bolt with care! If you drop it, it will be hard to retrieve.
  5. Now go to the ground above the driver's side coil. Remove the plastic cover and undo the jumper nut. Repeat clean-up/replacement procedure. Replace jumper nut and cover.
  6. Move on up to the suspension turret. Here you'll find a single cable ground attachment; look close to the reservoir for a reference point to help find it. Repeat clean-up and replacement procedure.
  7. Now go over to the passenger side and find the ignition coil. Forward of this towards the Rad is a small black plastic cover holding a brown and red wire. Remove the cover and undo the large hex-head screw holding the two wires. Clean them up and the lower holding bracket the large screws go into. Replace and protect.
  8. Move up to the passenger side suspension turret and find a single ground as on the driver's side. Repeat clean-up and replacement procedure.
  9. Move on to the passenger side fuse and relay board inside the car. The grounds here are located at the bulkhead above the fuse panel; there's two. Remove parcel shelf and any electrical boxes restricting access; CAREFUL WITH ANY WIRING! The number of ground wires on the right side is huge and the bolt is very long. Check the wires here carefully as corrosion can be difficult to see as there are so many wires and access is difficult. Repeat removal, clean-up and replacement procedure. Replace any parts removed for access.
  10. Go round to the driver's side. Remove parcel shelf and stick your head under where the brake pedal is and look back at the steering column; the ground point is there on the right side. Again, access is limited and awkward. Repeat clean-up/replacement procedure.

There's 3 more to do; when I've done them I'll add pic's/info! Hope this helps someone else new to this work. It definitely makes a difference. My volt meter on the dash was always at 12; now it's between 12/14.

Best, Dave.


MP 1 -- at the headlight tube, close to ignition power stage


MP II -- next to the top of the radiator at the passenger side, close to the transmission cooler lines



MP III -- hidden below the jump post terminal assembly. It provides ground for the ignition end stages.



MP IV -- driver side footwell behind the parcel tray, near steering column



MP V -- Above the fuse panel---


MP VI -- under the cover in the spare wheel well.


MP VII -- at battery ground strap


MP X -- at coil near the ATF steering reservoir


MP  XI -- suspension strut driver side


MP  XII -- suspension strut passenger side


Hi Theo,

so now I've completed the work on my grounds can you please add the following info to my post on your website.

So, I've finally accessed the 3 remaining grounds on my right-hand drive 1989 S4!

The 2 main grounds on the top of the engine and the main engine-chassis strap. Here's what's required to do the work.

1) To access the grounds on the top of the engine you need to remove the air cleaner housing, MAF, fuel regulator and fuel damper and their fuel lines. Then you have the four way vacuum splitter and some wiring located on the passenger side. Even with all this out the way getting your hands on the ground points/wires is not easy as they're deep down in the engine bay!

So, a 13mm socket with extension from a ratchet is the way to go undoing them. Then, I used a chisel to clean the block where the ground wires attach (you want them super shinny for maximum grounding). Then, a flathead screw driver pushed down on some sand paper allowing me to scrub the block smooth and shinny. Not easy, but possible!

Using a thin/long screw driver I then pushed it through the ground wire connector hole and gently pulled them up to allow me to sandpaper them clean/shinny.

Then reinsert/tighten your bolt. BE CAREFUL not to drop them down under the manifold! Just in case, have some 13mm bolts spare! A trick I use to hold bolts I'm putting back in is to push some bluetack into the socket and then press your bolt back into the socket as tight as possible; it then holds the bolt in the socket instead of just dropping out.

Finish by painting copper grease over the top of the bolt and any exposed ground connection/block.

2) The main engine-chassis strap on my car was appauling; see pic. Heavily corroded alongside the connectors and ground points on the chassis/engine.

The engine should be cold. Ideally you need a lift/ramp to access the main engine-chassis strap as access is difficult (although I know some peeps have done it with axle stands). The main e-c strap is on the driver's side (right-hand drive car) just behind where the sway bar attaches to the chassis.

So, drop your engine under-tray/trays and then you will see it just behind the sway bar running from the inner chassis near the exhaust manifold and to the engine block. If it's never been serviced it will most likely be corroded and covered in grime; not what you want at all on the main e-c strap as corrosion+grime=electrical resistance=problems with your car!

Removing the strap just requires a 13mm socket on an extension to reach the engine bolt and the same again without an extension on the chassis. If you want to, it's possible to clean the original strap with a wire brush and/or Coca-Cola (yeah, that stuff is super corrosive and people drink it still!). To clean the connectors rub hard with sandpaper until bright and shinny.

Then clean the grounds on the engine block and chassis. I shifted the grime with a chisel and then sandpapered them bright; see pic. Do the same sandpapering to the washers connected to the bolts; you want optimum contact on this cable.

Now you can replace the old, cleaned up cable or buy a new one here: https://www.rosepassion.com/en/cat/piece/P28660
The Porsche part number is: 928 612 011 00

Again, finish by covering the ground contacts with copper grease. Thereafter, include this cable/grounds in your 928 maintenance routine as due to its location near the exhaust manifold it gets a rough ride.

Your car/electrical system and therefore everything else on your car! will benefit greatly from this work.

hope this helps other 928 owners.

The 928 myth: '928s are complicated and hugely expensive to run/maintain'; not if you do as much maintenance as you can yourself and make sure the basics are all up together!

928 truth: '928s are awesome if well maintained'!


best, Dave.