Electrical: Tailgate Wiring Solution - Once & For All?

[TheHungriestBadger]

Hi all,

So, bear with me here, it's gone 4am as I write this. Everyone is sick of the tailgate wiring issues, right? I was looking through the paperwork for my '90 245, and it's already been through three of them since 2000, and now it's looking for a fourth (not quite 100% rear wiper operation, no central locking on the tailgate... Probably borrowed time before the MOT-able things start to go as well).

How about I design essentially a 'megaRelay' that slips inside the tailgate (there's got to be room, surely? There's already a wiper motor in there after all), and all it requires is a single low-resistance switched positive, and maybe one or two other tiny wires for sending a data signal - I'm confident I could handle that wirelessly, but I don't necessarily trust the cyber-security of wireless over a wired equivalent.

This whole system would consist of two small circuit boards (which could probably be packaged down the line as relays with a ton o' pins if desired):

- One would probably end up under a kick-panel somewhere, near all the existing relays, and would have all the existing connections 'terminating into it'. This would turn the existing power connections into simple signals (i.e. the current-carrying rear demister would now be a binary on/off signal).
- The other would be in the tailgate and would route the proposed single switched live, as appropriate, to the relevant accessories. This would be achieved based on the aforementioned data wire(s).

Essentially, this would keep all the existing functionality, it would just condense the 8 or so wires that are currently mounted in the hinges, into only a couple of wires. Given these would have to be added in anyway, they could then be brought inside the car, and any section exposed to flexing be made pluggable at both ends allowing for massively increased serviceability if it were to go wrong.

I think I'll leave the conceptual rambling there for now, please let me know your thoughts and also please play Devil's Advocate for things I'm likely to overlook. Obviously, the first victim of such an experiment would be my own vehicle.

P.S: A rough block diagram is attached and linked, wiring colours should be fairly accurate, but this is only for demo purposes.

Link: https://drive.google.com/file/d/1XdQ...usp=share_link

[dingov70]

Morning , my solution would be to use solid state relays & AMP -TE connectors ( https://www.te.com/global-en/product...?tab=pgp-story ) , These connectors are self sealling against the elements and the relays are available from CPC Farnell . Hope this helps you

[TheHungriestBadger]

Hi Dingo,

Appreciate the notes, I'd probably shrinkify the whole thing to a custom PCB design using (probably surface-mount) transistors and microcontrollers with a semi-custom interface between them. The technical design I'm more than happy with, regardless of how complicated it gets.

Connectors could be a sticking point to be fair, so definitely appreciate the link to those, I've used that brand for large DIN connectors in the past and they are definitely good quality.

The amount of current through the proposed new switched live is the biggest issue - worst case one could have the wiper running, demister active, brake light on and numberplate lights on at the same time. I'm not sure how much current that is exactly, but I think the answer is something along the lines of 'plenty enough'. I'd be looking at running a length of multi-strand 10AWG wire for the switched live, perhaps even something thicker like 8AWG - I use this website as my reference.

[TheHungriestBadger]

So, I just took a look around in the back of my car and made a couple of observations:

1) TONS of room in the tailgate, no problems mounting stuff there at all.
2) Rear window demister has its own power feed (and presumably ground connection to the tailgate metal), which is super good news as that is by far the highest current draw. That wire is the standard 'oldskool telephone cable' affair, and comes from the top centre of the vehicle separately into the top of the tailgate.
3) My wiring harness is evidently frayed on (at least) the right-hand side (driver's side for us UK folk) - I didn't even need to open that tailgate to see this, I could just look down at the hinges and see where all the insulation was gone, and bare copper was exposed and had snapped.
4) Just looked through the wiring diagrams as well, and concluded that there are 6 wires that connect through to the tailgate (technically 7, but the rear demister wiring is separate and seems to be reliable enough). These are, in no particular order:

- High-Level/3rd Brake Light (BLUE/BLACK) - from bulb failure detector.
- Rear Wiper Motor Permanent Feed (YELLOW/BLACK) - from fuse #1.
- Rear Wiper Motor Control Feed (GREY) - from rear wiper relay.
- Tailgate Lock (YELLOW) - from central locking relay.
- Tailgate Unlock (BLUE) - from central unlocking relay.
- Numberplate/License Plate/Tag Lights (WHITE) - from fuse #15.

Can anybody spot the issue yet? Yes, of course, bulb failure circuit will always claim that there's a bulb out since the 3rd brake light would not be directly connected. This is easy enough to work around: just switch in a separate resistor whenever the brake light is illuminated. However, at this point, I'm essentially trying to defeat a safety-critical aspect of the vehicle's operation, and I'm not sure how comfortable I feel doing this, even if I was to add various liability disclaimers.

So, I have a few options now:

1) Carry on as I planned originally and try and fool the bulb failure circuit as mentioned above. Would involve drilling a hole in D-pillar and running 2 or 3 wires.
2) Kind of carry on as originally planned, but run an extra cable specifically for the brake light (since this is the only safety-critical aspect of the tailgate wiring, at least in the eyes of the MOT). Would involve drilling a hole in D-pillar and running 3 or 4 wires.
3) Change course completely, and essentially do as others have done: reroute the harnesses to run inside the car, but so that they both come to one side. Then, jump them across internally using a short DIN cable or something (needs to have a bunch of pins since all 6 original cables would be getting jumped across).

Again, thoughts please on how to proceed? I'm personally leaning towards option 2, but I could also see at that point how some people would prefer the low-tech path of option 3... Rats 😂

[TheHungriestBadger]

Apologies if people think I'm spamming here, I'm not: I'm just posting stuff as I find it out.

I was wrong on the demisting wiring: It DOES appear to go through the hinges as well. The big telephone-cable-looking thing is, I believe, actually just the hose for the rear washer jet with some slightly springy braiding.

And also, from further forum scouring, it appears as though the grounds also travel through the hinges as well? In which case, some serious current handling is going to be required. if I go for options 1 or 2 from above. Low-tech may be the way to go, and at least that way the current handling is spread over the wires a bit.

[TheHungriestBadger]

From what I have gathered through reading and preliminary inspection, there are two small wiring harnesses for the tailgate wiring:

Harness 1 travels down the right-hand (UK driver) side of the car and contains (I believe) the 'nice-to-have' bits: The tailgate lock & unlock wires, and the two wiper motor wires.

Harness 2 travels down the left-hand (UK passenger) side of the car and contains the important bits: Third/high-level brake light wire, numberplate lighting wire, and (least importantly) the rear demist function.

There is also a ground wire on each hinge - I believe the tailgate latch also acts as an extra ground path, although this is not reliable enough to be used as the sole ground path (loss of electrical contact over sharp bumps etc.).

This is good news to a large extent since it appears to be commonly agreed that the right-hand harness is more prone to failure than the left-hand harness, the latter of which contains the must-haves for passing an MOT/inspection. The reason for this is the increased number of wires running through the right-hand hinge, placing increased stresses on the wires since they have less space in which to flex and move.

Also, this does influence my decision of how I intend to implement a new loom, as will be discussed in the next post.

[TheHungriestBadger]

So the plan for this revised loom is as follows:

Drill - or, if one already exists in a suitable place, repurpose - a hole in the left D-pillar at the rear of the car. In this hole, the hope is to put an 8-pin DIN connector socket. An identical socket will also be placed on the left side of the tailgate itself. A short 8-pin DIN patch cable can then be plugged between the two, making the only part exposed to any flexing instantly serviceable. A separate ground would be routed either through a braided grounding strap (similar to those found in engine bays), or if that's a step too crude, it could be done using both wires in an XT60 patch cable.

This might sound overkill, but I am trying to do this properly here, so below is my rationale.

Connector - why DIN? Because it's a standard. That means that the parts are widely available, and that minimal confusion can occur. It's also a keyed connector - so you can't ever plug it in the wrong way by accident. It's also relatively compact for the amount of cabling you're getting here - think of the footprint of the 7-pin external amplifier cable on some Volvo CR head units, versus the 9-pin square connector for powering the thing. Finally, and perhaps most importantly, it will restrict any flexing purely to the replaceable part of the cable. Sure you could use a grommet and bullet connectors instead of the mounted DIN socket, but then there's a risk of the car and tailgate sides of the harness flexing and failing over time, rather than just the pluggable part... In which case you may as well have hardwired the whole thing to begin with.

Location - why the left-hand side? Because that's where the high-current paths are. The rear demister is by far the highest power device, drawing what I believe is about 10A at full battery voltage. This estimate is based mainly on a report of a 940 rear demister grid measuring at 1.3Ohms, and the fuse rating (16A, but also shared with the OD solenoid on autos). Also, common sense: a nominal 120W heater for a single pane of glass? Sounds about right. Thus you'll want to minimise the length of this wiring, which is already down the left-hand side of the car. The much lower current wiper motor and lock/unlock solenoid connections will cope much better with being run across the top of the boot area, just under the headlining.

Pins - why 8 of them? Sure, there are only seven signals (the six listed in post #4, plus the rear demister); however, 8-pin DIN connectors are only specced for 5A per pin. I want to play it safe and double up the positive feed for the rear windscreen demister to avoid stressing the DIN connector(s). Why? Because reliability. Again, why fix one unreliable implementation with another?

Grounding - why separate? Because it's carrying potentially a lot of current. ~10A for the demister + (guessing here) ~2A for the wiper motor + ~2A for the (21W bulb) third brake light + ~1A for the two numberplate lights (presumably 2x 5W bulbs? Although could be higher, in which case even worse). So that's an estimated 15A of current which would be 3-din pins... So why not just get an 11 or 12-pin DIN cable then? Because the datasheet specs only 2A per pin for the 10/11/12 pin DIN connectors, and then you're back to square one.

Grounding (again) - why an ugly braided strap? Because it's made of tons of fine strands, and is flat, unlike a cable. Remember how everyone says to only use the special fancy OEM Volvo harnesses because of the ultra-fine, ultra-flexible strands? Same goes here then, right? Also, they're cheap, and even the copper-coated aluminium ones will be overkill for this application so the risk with this solution is pretty minimal. Also, they have lovely terminations at the end which are just begging for a nut, bolt, and some washers to secure them to the car body and tailgate metal. No doubt you could even use self-tappers at a push. I guess you could even add this alongside the DIN patch cable and neaten it up with some corrugated plastic sheathing. An XT60 cable (with both wires used for grounding in parallel) could be used but this will be less physically flexible, bulkier, and more likely to break on the wrong side of a connector.

One final advantage with this is that you don't have to even touch the tailgate hinges at any stage - not when disconnecting the existing harness, nor when fitting or repairing the new harness. Just disconnect the battery and snip the existing harness and then forget about it. From this point onwards, any cable running through the hinges is just wire stuck to your car. It'll either pull out or just disintegrate over time, but either way, it won't be carrying any electricity so no risk of short circuits, fuses blowing etc. if left in place.

I think this is a bit more sane than my initial proposition of some crazy overcomplicated relay. In due course, I'll order some parts and start figuring out the exact routing of everything, once I do I'll be sure to post pictures, videos, etc. of the whole installation process.

[TheHungriestBadger]

Quick update using some actual numbers (both part numbers and some calculations). Components are going to be ordered from RS Components because it's substantially cheaper for me, although alternatives can be found from global suppliers like Mouser and Digikey, and undoubtedly a trusted local supplier. Trusted supplier is key here, the last thing you want is to melt your connector because you cheaped out and bought a generic part on Amazon, eBay, etc.

For the DIN connectors mounted to the car body and tailgate, I am opting for RS part number 2100260. This claims it has a 5A contact current rating (i.e. per pin), and also comes in a nice physical form - round (like the holes you'll be drilling), and relatively low-profile. Attaching these may be difficult, given the geometry inside the rear opening, particularly near the roof line, but if I can find a suitable spot, they can likely just be squidged in with silicone/mastic/your squidgable thing of choice.

For the DIN connectors used to make the cable, I am choosing RS part number 2100209, again featuring a 5A contact current rating. The only issue I can see cropping up, is if the cable 'gland' attached to it is too narrow for the subsequent wiring; however, this should be easy enough to either omit or remedy by force if required.

As an aside, I believe the contact current rating of 5A is specified for a 40°C temperature rise, so I believe there is quite an overhead left for additional current. The 'worst' load will be the rear demister, as stated previously, and presumably the ambient temperature will be cold anyway.

For the patch cable itself, I intend to use 8, 0.5m lengths of multi-strand 20AWG wire. 0.5m should be plenty to connect car to tailgate, and leave plenty of slack to avoid stressing it - potentially it could even be shortened, I'll have to see when I start mounting things up. The maths for the gauge selection is as follows:

• Let the maximum allowable voltage drop across the patch cable be 100mV (or 0.1V, if you prefer).
• From this table, the resistance per 1,000m of cable for 20AWG is 33.292Ω. This assumes copper conductors, if using cheap Copper-Coated Aluminium/CCA (hint: don't), double this value. You'll very quickly know if you're using CCA as it'll be much more difficult to solder to than pure copper. Also, I believe you can hold the strands over a lighter and if it's CCA they'll curl up and away from the flame whereas pure copper won't?

The resistance of each individual length of the patch cable will be: (0.5m / 1000m) * 33.292Ω = 0.0166Ω

Using Ohm's equation V = I * R, and rearranging for I, we get I = V / R
Substituting the above values: 0.1V / 0.0166Ω = ~6A

Power dissipation per length of wire is given by the equation P = V^2 / R
Substituting values gives: (0.1V)^2 / 0.0166Ω = 0.6W
Perhaps a little higher than I'd like, but it's unlikely to have that as an extreme case for an extended period of time - this will be tested before installation, and results given.

Now we know the wire is capable of handling that current effectively. Essentially, anything bigger will simply be derating the system (which is good). If the connector allows it, I will likely go with thicker conductors. The only issue is the thickness of the insulation, which makes the entire system considerably thicker. It also depends on the size of the contacts on the connector; again, I will experiment once the connector is in front of me.

Any extensions to the wiring in the car (i.e. the stuff that makes up the existing looms) will likely also be 20AWG, although, as before, if I can make it thicker I will. Similar story for the wiring in the tailgate itself.

[Angie]

Originally Posted by TheHungriestBadger

Hi all,

So, bear with me here, it's gone 4am as I write this. Everyone is sick of the tailgate wiring issues, right? I was looking through the paperwork for my '90 245, and it's already been through three of them since 2000, and now it's looking for a fourth (not quite 100% rear wiper operation, no central locking on the tailgate... Probably borrowed time before the MOT-able things start to go as well).

How about I design essentially a 'megaRelay' that slips inside the tailgate (there's got to be room, surely? There's already a wiper motor in there after all), and all it requires is a single low-resistance switched positive, and maybe one or two other tiny wires for sending a data signal - I'm confident I could handle that wirelessly, but I don't necessarily trust the cyber-security of wireless over a wired equivalent.

This whole system would consist of two small circuit boards (which could probably be packaged down the line as relays with a ton o' pins if desired):

- One would probably end up under a kick-panel somewhere, near all the existing relays, and would have all the existing connections 'terminating into it'. This would turn the existing power connections into simple signals (i.e. the current-carrying rear demister would now be a binary on/off signal).
- The other would be in the tailgate and would route the proposed single switched live, as appropriate, to the relevant accessories. This would be achieved based on the aforementioned data wire(s).

Essentially, this would keep all the existing functionality, it would just condense the 8 or so wires that are currently mounted in the hinges, into only a couple of wires. Given these would have to be added in anyway, they could then be brought inside the car, and any section exposed to flexing be made pluggable at both ends allowing for massively increased serviceability if it were to go wrong.

I think I'll leave the conceptual rambling there for now, please let me know your thoughts and also please play Devil's Advocate for things I'm likely to overlook. Obviously, the first victim of such an experiment would be my own vehicle.

P.S: A rough block diagram is attached and linked, wiring colours should be fairly accurate, but this is only for demo purposes.

Link: https://drive.google.com/file/d/1XdQ...usp=share_link

People seem to be quite concerned about this, but I'm not sure why, really. I've had the car for nearly thirteen years and some 110,000 miles, and had wiring changed twice. As I have dogs the tailgate gets a lot of use, so failure of wiring wasn't a surprise. The first time it was replaced by a friend with me as 'labourer', it took maybe an hour or so; the second time the garage replaced it, took them half an hour! The only drawback is that the earth wire inside the hinge had broken so demister efficiency is variable but it didn't seem worth all the hassle of taking the hinge off again. The looms cost peanuts, labour about £30, so next time it goes it will simply be replaced as original. The first replacement was done not long after I bought the car so lasted well, at least ten years.

It's not a good design, I agree, but I don't suppose it was expected to last for twenty or thirty years!

[Bob Meadows]

Can you get the new loom through the original channels within the hinge?
It all gets a bit tight in there even with shrink down sleeving.

From memory the hinges also have an earth tag link between them but can be replaced/altered when new wires are introduced.

Quite common for tailgate looms to fray or break.
Regards Bob.