Retrofit G33 screenwash low level sensor (Polo Mk4 9N3)

Post Instructions on DIY modifications here - use all information with caution!
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Sam78
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Joined: Fri Jul 22, 2022 1:34 am
Drives: 9n3 2006 1.2 12V
Location: UK

Retrofit G33 screenwash low level sensor (Polo Mk4 9N3)

Post by Sam78 »

I've recently done a retrofit install of the windscreen washer fluid low level sensor (G33) in a 2006 9N3 1.2 12v Polo righthand drive (S trim). I've seen guides posted elsewhere but several required removal of the bottle itself (via removal of bumper, undertray sound plate, wheel arch trim etc) or they installed the sensor in an inappropriate position. The way I did it only requires the bottle to be drained below where you want the sensor (if you're thinking 'what about clogging the pump with debris drilling the hole in the bottle?', read on). Only the front foglight and fog grille need to be removed - nothing else - and so I thought I'd share it with you. As far as retrofit jobs go it was quick.

What you need:
- Standard portable power drill (ie not a large heavy drill as there's not a lot of space to play with)
- A 25mm diameter flat hole cutter (see photos below) - the reason for using this tool and not a hole saw is the flat hole cutters are on a long shaft which you'll need when poking it through the space in the bumper front; hole saws tend to be shorter stubbier things unless you can extend one.
- Other tools: Headtorch, soldering iron, sharp hobby knife, also a wire snake (straightened coat hanger and tape), wet-dry sand papers rough (180-400 grit) down to finer (1000-1500 grit)
- Windscreen wash fluid low level sensor (G33) 7M0919376 and rubber grommet 7M0919382
- 2-pin VAG plug for fluid level sensor: 7M0973202 with 2x female terminal contacts and 2x wire seals (get it all together as a bundle to avoid ordering the wrong sized contacts, and don't order the standard type of 2 pin VAG plug as the terminals won’t be the correct distance apart).
- Two lengths of different coloured wire (the VW wire spec is 0.5mm2 / approx. AWG-20). The wire to the dash unit obviously has to be longer - I started with 4 or 5 metres and then cut when done. The ground wire length depends on which earth/ground bolt you use - mine was about 1.5m. Just measure out your chosen routing path length first.
- 6mm inner diameter ring terminal grounding solder lug (measure your chosen earth point first as it may need a larger e.g. 8mm ring lug).
- Licensed version of VCDS and a laptop which you'll need to code your instrument cluster to read the sensor and display the caution lamp.


1. Gain access to the front of the washer bottle (this is for a RHD Polo 9N3 with front fogs):

Optional: Open the bonnet for more light, chock wheels and jack up the front nearside just to bring the bumper a bit higher, but not so far that the front wheel lifts off the ground.
Pull out the nearside fog light grille (there is no fixing screw - that's only the offside grille). See Photo 1. Remove the 3 Torx screws (they have a washer on them) from the fog light surround). Don't stick your screwdriver into the funnel shaped hole as that one adjusts the aim of the lights. Reach around the rear of the fog light and unplug the connector, note there is very little spare play in this cabling. Take a moment now to thank The Good Lord that VW did not use the standard VAG wiring connector for the fog light and instead used a very easy-to-remove pinch plug. Phew! Remove fog light (photo 2) and place where it's not going to get smashed. That's it. No need to remove any undertray, engine sound plate or the wheel arch liner.
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2. Decide where to locate the sensor on the washer fluid bottle:

Decide where you want the sensor. In one guide I saw online, the bloke had drilled the hole so low down the bottle that it was barely higher than the pump intake - this is far from ideal since the whole point of the sensor is to tell you in good time when you start to run low, not when you are down to 2 or 3 squirts left..! Now, as shown in photo 3, although those nice people at VAG couldn't be arsed to spend $15 putting a washer fluid sensor in your car, they did kindly go to the trouble of imprinting a 25mm circular panel guide about 10-12cm up the front of the bottle which is what you will be using. But, if you want it lower or higher than this then that is entirely up to you – if you do, then stay clear of the existing imprint to avoid grommet fitting issues later. Wipe clean the front of the bottle to locate this guide clearly, and also to minimise ingress of dirt during the drilling. If your bottle didn't have this guide, then it's about 10-12 cm up from the bottom. Don't go too far to the right (offside) as the bumper will be in the way when you try to drill straight.
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3. Drain the washer fluid reservoir to about 3-4 cm below the bottom of the hole:

Ideally, you will already have used up much of your fluid in advance of embarking on this job. If, like me, you didn't (it was summer), you can either 1) remove the filler trap in the bottle opening and use a pipe to siphon it out, 2) remove the wheel arch liner, remove the washer pump and collect it (messy), or 3) if you don't have a siphon tube, you can raise the bonnet, support two raised cups (or bottoms cut from plastic bottles) under each of the nozzles, remove the 20A windscreen wiper fuse, and then just pump it out into the cups in the normal way. Don't forget to replace the wiper fuse before driving...

4. Create the hole for the sensor and install rubber grommet:

Using a screw, hole starter or other sharp point, make a starter indent in the dead centre of that circle guide. This can be done by eye if you're feeling lucky, or using a card template prepared earlier with compasses or other such method. Place a small bit of masking tape on the centre of the hole to stop the drill bit dancing, but don't obscure the hole marker indent as you want to monitor drilling progress. Place masking tape around the fog light edge surround to protect paintwork from the drill chuck as it spins, as there won't be much clearance.

Then just carefully and slowly drill as straight as you can (photo 4). Don't worry about any plastic swarf debris from drilling entering the hole, even small pieces (see final step).
Once you have drilled your 25mm hole (photo 5), you can fish out larger plastic debris through the hole with your finger as it will be floating on the surface of the fluid (if you filled it 3-4 cm from the bottom of the hole as in step 1 but is not essential right now. Sand evenly around the hole to get the edge as smooth as possible so the rubber grommet gets a tight seal. The grommet fits into a 26mm hole so you can afford to sand a tiny bit away – use the guide imprinted on the bottle to keep to a good circle. Keep trying the grommet frequently between sanding until it just fits. Cut or sand any swarf attached to the hole edge inside the bottle (it can fall into the fluid -see final step). See photo 6.
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3. Install the low-level sensor:

The sensor will (should) be a very tight fit inside the grommet (photos 7-8). Use a smear of dish washing liquid to help ease it in if needed. If it really won’t go, remove the grommet and sand a tiny bit more. Repeat until you get a very tight fit. You want to get the sensor rotated so that the pins lie in the horizontal plane (lower the car down flat first if you had it jacked up for drilling). Also consider whether you want the push clip on the VAG connector facing up or down, for easier future removal. I prefer to have it facing up, which means the curved side of D-connector needed to point left (towards the engine).
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4. Prepare the wiring harness and install:

There are several ways of skinning this cat. These instructions are if you want to connect the sensor directly to the dash instrument cluster via the (vacant) bonnet release cable grommet on the nearside (on a RHD car). I chose this as I already had 7 or 8 wires running from engine bay to cabin along this route. If you wanted to make it OEM-style, you could connect it via the bulkhead coupling station, but be warned: if you don’t already have a wire connecting the instrument cluster to the coupling station on the inside it is a complete dash out job (see end of guide for info on this method).
First, prepare the plug. Connect a female VAG contact terminal to one end of each wire using a fancy crimping tool or just solder it on (I did the latter), remembering to slide the soft plastic waterproof seals onto the wire ends first. Then insert the contacts into the plug from the rear until they click home. The shorter earth/ground wire goes to contact terminal 1 (it's numbered on the plug), and the longer wire to the dash cluster goes to terminal 2. You may wish to slide some form of protective sleeve on to the wires or later tape up – it’s up to you.
Solder the other end of your ground wire onto a ground lug with a 6mm inside diameter.
Push the wired plug onto the sensor until it clicks home (photo 9). Use cable ties to keep the hanging loop of sensor wiring clear from interfering with fog light wiring to prevent snagging when removing them. Make sure there’s enough free play to remove the sensor plug but not so much that the cable rests on the undertray where grime and water accumulate.
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5. Routing the wires and grounding the sensor:

Push the wires from the sensor up through the space under the headlight and tie to existing harness or chassis. I then ran the wires up and inside the plastic harness protector which runs up parallel to the fluid bottle neck.
The ground wire from pin 1 of the sensor plug can be fixed to any chassis earthing point, it does not matter which one, just make sure the contact is a good one. (The OEM connection is via an earth connection #329 in the engine wiring harness). I used the one behind the battery on the nearside suspension tower not far from the ECU (Earth point #29 – see photo 10), simply because of its easy access and also I did not want to interfere with the very tight (and harder to access) bolts for some of the larger grounding cables used for critical engine and steering components nearer the washer bottle.
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The wire from pin 2 of the sensor plug runs to the dash through the vacant bonnet release (photo 11). It’s easiest to remove this grommet, make a small hole in the tip and thread the grommet onto the wire. Now for the hardest part – wiring into the cabin. Remove the glovebox (search other guides online) and side panel facing the door, and open the passenger door fully. You are permitted (and encouraged) to swear and curse aloud during this process: Reach around the HVAC fan assembly and fumble around behind the sound insulation mat until you find the grommet used for the LHD bonnet release cable and pull it out. From the cabin side, push a straightened length of wire (eg coathanger) through the hole and then another 10cm or so. Go around to the front and from the space under the windscreen pull it through with pliers (you can pull up the plastic and rubber trim here for access), pulling the metal wire through the grommet hole. Securely tape the end of the sensor wire to the coathanger wire and carefully pull it into the cabin from inside the car. You can thread it through the grommet and reattach both grommets. Ensure you have the wiring routing sorted in the engine bay then you can take up the slack and route the sensor wire behind the dash through to the instrument cluster. I use the route along the existing harness below the passenger airbag, then behind the radio unit, then under the dash cluster - take care to route and cable tie away from the airbag, steering and adjustment lever.
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6. Connection to the instrument cluster module:

Pull out the dash instrument cluster (see photo 12). Behind it there are 2 connectors, an 8 pin and 32 pin. Release the locking clip of the green 32-pin connector and disconnect it. Remove the outer casing of the plug and slide away to reveal the terminals. You need to connect the sensor wire to pin 30 - tiny numbers are printed on the plug (photo 12b). The plug is easy enough to open up, but just be careful not to pull out any of the existing contact terminals. Several allocated pin slots won’t have wired contacts, so if pin 30 is vacant, obtain a suitable terminal and crimp or solder the sensor wire onto it. Despite not having the G33 sensor, my car already had a wire connected to pin 30 (it only went to the bulkhead coupling station), so I just lightly soldered my sensor wire to that, keeping the original wire in place.
Reassemble the plug, reattach to the rear of the instrument cluster, secure the locking clip, reinstall the dash cluster with the 2 screws, and replace the black trim piece.
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6. VCDS coding the instrument cluster module:

This step cannot be done using an unregistered (shareware) version of VCDS or VCDS-lite. I am assuming you know the very basics of VCDS. Anyway, turn the ignition on without starting the engine. Navigate to gateway 17-Instruments, then click on Coding. Make a note of the code shown in case you balls it up. You need to change the first two digits of the code which corresponds to Equipment installed, adding the numbers shown in the ‘helper balloon’ (see photo 13). The first 2 digits in your code should tell you what you already have. So if you already have brake pad wear sensor (+01) and we want windshield wash level sensor (+04), then you just need to change the first 2 digits to 05, then click ‘Do it’. Close the controller and VCDS and turn off ignition.
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7: Test, refill washer fluid and remove drilling debris:

Turn on ignition. If you kept the washer fluid level below the sensor, the amber warning light should come on after about 30 seconds (this allows for ‘swishing’ over and under the sensor pins while driving). Now refill the washer bottle above the sensor and check that the light goes out after 30s. Check the washer bottle to ensure fluid is not leaking past the grommet and it is a good idea to do this after first few drives.
Now, remove the filter cup from the top of the washer bottle, fill the washer bottle to the brim and you should be able to fish out any drilling debris and swarf with a spoon or tweezers (photo 14).
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Replace the filter cup and close filler cap.
All done! All that remains is to reattach the fog light connector, the 3 Torx fixing screws, and replace the fog grille trim.

Final Note: If you decide you want to connect the sensor signal wire (from pin 2 of the sensor) via the bulkhead coupling station to replicate the OEM installation (photo 15), the 6-pin connector (code T6q) is red and you want pin 4 (pin 1 is front fog lights, 2 is for heated washer jets, 3 is the horn and 5 is brake pad wear sensor). But be warned – this can be tricky to access in the engine bay, and even trickier in the cabin as it is a dash out job if you don’t already have a wire installed to the dash cluster.
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