Saab C900 Rear Ground/Earth Point Reconstruction

August 2007 Update: The method has changed a bit since my first attempt in 2003 of solving the problem, but the result is basically the same, which is that the primary ground/earth connections into the two rear light clusters are bypassed. It still doesn't hurt to clean/fix the existing ground connections so they still work as intended, but sometimes the contacts and surrounding plastic is too badly damaged.

Background Information

Examples of Long-Term Water-Ingress and Ground Contact Breakdown
Effect of water ingress on light cluster board	Melted plastic around the earth/ground contacts	Central chassis member in boot/trunk
The left picture above gives an overall view of how badly the metal 'tracks' of the circuit panels (left-side one shown) can corrode over time due to water ingress. The middle picture shows how the plastic around the ground contacts has melted and the metal has degraded. The white wire was added by a previous owner to overcome the bad earth problem but it's not the best way to apply a long-term fix as solder won't adhere to steel very well, and a soldered connection can fail due to vibration, etc. over time.

You will find the rear ground/earth points at the very back of the central chassis member which is hidden under the false floor of the boot (or 'trunk') space right before the rear crossmember that forms the structural support for the back of the body joins on. The third picture above shows the boot/trunk space of a sedan-body c900 with the two false floor sections removed. Just out of the picture at the bottom (due to the angle at which the picture was taken) is the extreme back part of that chassis member where the ground/earth wires are attached.

These ground/earth points provide earth connections for the two rear light clusters, the fuel pump and fuel level sender, and optionally for the electrical connector for a trailer (if fitted). So they perform a very important role. There may be other items installed in the rear of the car (such as car hi-fi gear, phone/radio equipment, etc.) which could also be grounded at that location.

Getting Jiggy With It!

As supplied from the factory, there is usually only one screw fitted in the left-most hole but there should be two holes (the second hole might have a plastic 'foot' with a velcro-like arrangement held to the second hole by a rivet).

As part of the process, you'll need to remove the screw since you need to check all the lugs on the factory-installed ground/earth wires (thick ones go the fuel pump and sender, thin ones go the light clusters), and look at the state of the connection point on the chassis rail itself. Regardless of anything else you do, you will need to make sure that the area around the hole (or both holes if you want to connect extra earth wires to the other hole) at the top is cleared of paint, dirt/oil/etc. and any surface rust.

Use some wet-and-dry grit paper, a small flat-bladed screwdriver, and a small brush, to clean those areas and get back to bare metal for about 3 to 4 mm (1/8") around the outside of the hole(s) since metal-to-metal contact at that point forms the actual earth/ground connection to the chassis of the car which conducts current back to the front of the car (and into the battery via the big earth point behind the radiator).

After that, you can do one of two things. Either you can decide to re-use the screw if it's not corroded (or replace it with a stainless screw of the same gauge) and put it aside since you'll need it again, or find some suitable nuts and bolts. My first attempt (pictured below) which I did on my 1983 900S used screws that fitted the hole (I think they're 12-gauge stainless-steel screws from a hardware store).

With the 1983 900S, the old (single) screw in the left-hand hole was too loose and I believe that accounted for part of the problem which had manifested itself due to the poor ground/earth connections to the rear light clusters (not to mention the earth contacts in the cluster harness connectors on the rear light cluster boards themselves - see examples above).

First Attempt at Rear Ground/Earth Point Rebuild/Repair - 1983 900S 4SN
Rear earth points after reconstruction	Left side rear light cluster wiring	Right side rear light cluster wiring
Left-side light circuit board	Close-up showing earth bolt location	Notes:
		The 'ballast' resistors paralleled onto the +12 volt feed to the indicator lamp for each cluster (shown in the second and third pictures above were part of the initial trials of LED lamps conducted through my re-lighting project. The green wires with red-insulated lugs bolted to the cluster panels and screwed to the right-hand rear ground/earth point are the ground/earth contact connection bypass wires. The pictures at left show one spot where the ground/earth bypass wires can be bolted on (left-side circuit board shown).

In subsequent cars (my 1985 900i, 1983 8V turbo [restoration abandoned], and 1981 8V turbo), I have used stainless steel nuts, bolts and washers instead of screws to make connections to the rear earth/ground point(s) that are going to be more dependable in the long term. Using either spring washers and plain nuts, or plain washers with nylock nuts, will ensure that once tightened, the 'sandwich' of nut, lugs (with washers top and bottom of the 'lug pile'), and a nut/washer underneath (inside the chassis member which is hollow and formed from folded sheet steel at the factory), will not come loose and providing water doesn't get to the 'sandwich', the integrity of the ground/earth connection(s) to the central chassic member will be good for a long time.

I have discontinued the use of ballast resistors for the indicator light circuits since I found out about replacement flasher relays that do not depend on the current draw through the indicator lights to set the flash rate. See the LED lighting project for more info about that.

Light Cluster Earth/Ground Contact Bypassing

• - Suitable nuts, bolts and washers (5 or 6 mm stainless steel ones work well),
• - 'Eye' style insulated crimp lugs with a hole big enough for the bolts to go through and a crimp section matched to the size of the wire you'll use,
• - Tools for cutting and stripping wire and crimping on the lugs, and for drilling and cleaning up holes in the light cluster circuit boards,
• - Suitable insulated wire.

5 or 6 mm bolts (hex-head or allen-head) of suitable length (about 20 mm for the circuit boards, a bit longer for the chassis member screw replacement so you can get your fingers at the bottom to put the nut/washer on), with a combination of washers and nuts are what you'll need. Nylock nut with plain washer or spring washer with plain nut will do equally well.

Don't use anything other than stainless-steel fasteners as they won't rust, and that help with ensuring long-term reliability of the electrical connections they'll be forming.

In the initial attempt (pictures in the above table), I ran the extra wire for the left cluster with the factory wiring loom that goes under the rear chassis section, and up around to the top of that steel piece to the socket at the back of the light cluster circuit board, and the extra wire for the right cluster direct instead of following the loom to it (which comes from the left light cluster). In all the other cars I've done the change on I've run both extra earth/ground wires following the loom since I felt that was a bit neater and reduced the chance of the extra wire to the right cluster getting snagged or damaged.

Cutting the New Ground/Earth Bypass Wires

Before doing anything else, with the light circuit boards still clipped into the housings, work out the length of wire you'll need to run the new earth/ground wire from the ground/earth point(s) on the central chassis member by running out the wires from the reel (or however your supply of new wire is stored) one at a time and holding each against the existing wiring loom for each of the two light circuit boards before cutting to length.

It's a good idea to allow an extra 10 cm (4") or on each wire just in case you need to re-do any of the crimp lug connections later on. A slightly longer wire can always be accomodated, but a wire that ends up too short will need to be replaced as joining on sections of wire with additional connectors creates points of unreliability, and that's one of the primary things the ground/earth wires are meant to be dealing with.

Remember to MEASURE TWICE, CUT ONCE! You'll feel very silly if you find the wires are too short once they have lugs attached and are cable-tied in place along the existing wiring loom... 8-)

Drilling Holes in Light Cluster Circuit Boards

Disconnect the wiring harness connectors from both light circuit boards. If the ground/earth contacts are badly damaged they might be stuck together, and each half of the connector housing (esp. the left side one) might come out seperately. Don't worry - they're designed to do that! Just ensure that if that does happen, you label the two so you re-insert them correctly later (especially for the left-side connectors).

Unclip the circuit boards from the lense housings, remove all the lamps and if you're going to re-use any of them instead of replacing them with LED lamps, put them aside in a safe place. Then find a good place to work (on a work bench is best) and figure out using the tools you have available the best way to drill holes for new ground/earth lug securing bolts.

There are a couple of places on the rear light cluster circuit boards (which are actually identical on both the left and right side - they just install the other way up in the right-side light cluster housing!) where holes can be carefully drilled through the metal circuit 'traces' providing the ground/earth path for all the lamps via the new wires you'll make up in the next step.

I'll try to get some good pics to show the earth/ground circuit path on the light circuit boards to put in this location.

Set up to drill the holes for the bolts that will secure the lug on the cluster end of each new earth/ground wire and carefully drill the holes, making sure all the metal and plastic 'swarf' is removed from the area around the hole you drill in each circuit board.

Making the Ground/Earth Contact Bypass Wiring

Start to make up the wire assemblies by crimping a lug onto one end of each wire after stripping back a small bit of the insulation, and bolt one end of each wire lug to it's respective circuit board. Make sure to use a washer either side of the lug since that will be attached to the plastic (back) side of the circuit board with the head of the bolt on the metal side. When crimping the new lugs onto the prepared wire ends, I like to twist the multiple copper strands but some people apparently don't. Providing the crimp lugs are properly joined to each end of the new wires, they'll work as intended and won't come apart.

Temporarily re-clip the circuit boards into each respective lens housing, re-connect the main wiring harness connectors - it's a good idea at this stage to spray a good electrical contact cleaner/lubricant on the contacts of the light circuit boards and into each wiring loom conector before joining everything back up.

Run out the new wires along the loom from each light circuit board and use small cable-ties to strap the new wires to the existing loom back to the rear ground/earth point. Start with the right-side circuit board and when it's wire reaches the back of the left-side circuit board, continue on running both wires together around the existing loom and cable-tie them as you go.

When the wires reach the point where the wiring loom bends towards the front of the car and runs along the central chassis member, work out exactly how you want to crimp on lugs to join the wires to the ground/earth point at the back of the central chassis member, and the extra bit of wire you allowed for should come in useful here since you can cut a little off one or both of the wires to get the ends evened up, then crimp lugs onto them in preparation for attaching them to the central chassis member.

You might find that, especially if you use decided to use 6 mm bolts at the chassis grounding point, you'll need to drill out one or both of the holes (depending if you want all the lugs under one bolt or decide to use the second hole for the new wires). Make sure all the metal swaft is cleaned out as any tiny piece of unprotected steel will rust very quickly due to atmospheric moisture, and the spare tyre space is also somewhere that water will accumulate from time to time when it leaks in around seals, the antenna hole, etc.

Ensure the metal area around the hole(s) is fully clear of paint and any contaminants such as oil, etc. so that when you bolt up the lugs, the electrical connection is as good as possible. Refer to the Getting Jiggy With It! section for info on how to prepare the holes if you haven't already done it.

Ground/Earth Bypassing in 81/82 8V Turbo 4SN
Left-side light circuit board	Rear of left-side light circuit board	Right-side light circuit board
Rear chassis earth/ground point	Notes
	The rear light cluster circuit boards shown in the pictures abovewere originally used to replace the ones shown right at the top of this page in my 1983 8V turbo sedan which were massively damaged by rust, etc. due to water ingress around seals (what was left of the seals!). When the car was scrapped, I transferred them (including the yellow ground/earth bypass wires) to the 81 turbo sedan. The middle picture above gives a better view of the way the wiring loom plugs into the back of the left-side light circuit board and the yellow earth/ground bypass wires are easy to spot. The picture at left shows how I connected up the re-organised main chassis earth/ground connection point. In this case I re-crimped the yellow wires into a single larger yellow lug, and twisted together the bared back copper of each bypass wire and folded that back on itself so the new lug would grab the bare copper strands reliably. I used an allen-head 6 mm stainless bolt (which required drilling out the hole in the chassis rail slightly).

Time for the 'Smoke' Test!

Double-check all the wiring connections to make sure you haven't missed anything. Unclip the light circuit boards and replace all the lamps (noting that one of the brake lamps is a two-filament version and will only go into it's socket one way around), the clip the circuit boards back into the lens housings.

Check (again!) all the wiring connections, then re-connect the battery negative and try out the car's electricals to make sure that all the lighting now works as it should. You can check the brake lights by holding the brake pedal down with piece of wood pressed against the driver's seat so you can get out of the car and look at the rear lights, and the indicators are easy to test. You might need a helper to operate the transmission shifter/selector so you can check that the reversing (aka 'back-up') lights work. If your car has a switch connected to the rear fog light socket in the dash wiring, try that out as well and turn the headlight switch so that the 'running' lights come on if they aren't on all the time when the engine is running (depends on the build year and target market).

Concluding Notes

The spare wheel can abrade the insulation on the wiring loom if the loom isn't tied up properly where it runs underneath the rear chassis member from the left light cluster to the central chassis member - check the pics below to see what I mean, and you can also see some more of how I was running the ground/earth bypass wires (yellow wires in this case) in this car before I scrapped the car (and transferred the rear light boards complete with bypass wiring to my 81 turbo car). The plastic 'loom tube' shown in the third picture is something I put around the wiring - it's not a standard factory fitting. The wiring loom is unprotected normally.

Rear Wiring Loom Damage and Repairs on 1983 8V Turbo 4SN
Close-up of the abrasion damage	An attempt at repairing the wiring	Underside view showing new loom tube tied up

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