Although it's rare, if it happens to your car it can cause thousands of dollars in damage! The problem is that some coolant reservoir tanks were poorly manufactured and can leak at electrical sensor plug. The coolant tanks are subcontracted out by VW - VW does not build every component on the car. The coolant moves through pressure and capillary action out the leaky plug and throughout the wiring harness. It's not caused by overfilling the coolant tank and the plug should be above the coolant level anyways. This corrodes all of the electrical connectors. Some people even found coolant in their taillights and gas tank sensors, which means coolant migrated about 15 feet through the wiring harness! Even if your car does not display these symptoms, taking the free preventative steps below should help delay any migration.
Symptoms would include coolant pooled around the electrical plug on the coolant reservoir tank and unrelated electrical problems such as unexplained instrument cluster malfunctions, starter, alarm, or malfunctions in anything else which is connected to the engine wiring harness. Sometime in 2004, a new part was released but it has the same part number (1j0 121 403 b) as the old part and started appearing on some 2004 cars. The old style tank starting appearing on mk5 cars but they removed the weatherproofing seal on the sensor plug. This will prevent coolant from following the wiring loom although it could be possible, although extremely unlikely, for coolant to get inside of the individual wires.
Note: Although the mk4 B5 Passat uses the Audi style coolant tank, there has been at least one report of coolant migration. This is a rare problem on the Passat and Audi but it's possible. Almost all cars which have seen this problem used the round style coolant tank (New Beetle, Golf, Jetta).
Your coolant should be a bright pinkish color - if it's brown or muddy then someone mixed coolants, see 1000q: mk3 coolant flush , 1000q: mk4 coolant flush , 1000q: mk4 VW Passat coolant flush, or 1000q: mk5 Jetta TDI coolant flush on how to clear the system. Other cars are similar.
Clear water inside the cabin like soggy carpets could be from a faulty windshield installation (if you recently had a new windshield put in this is highly suspect), a broken or clogged sunroof drain, or clogged water drain under the base of the windshield. This can can corrode the instrument cluster, ECU, or relay panel.
Remove the coolant reservoir sensor plug and see if there is coolant
there. Below are some pictures of a defective tank that suffered coolant
migration. It should be dry. If you see pooled coolant there, there is already coolant
inside the wiring loom so do further examination of the electrical sensors, instrument
cluster, ECU, and relay panel. If the wiring loom is wet inside, you have coolant migration.
Unravel some of the wiring harness and dry it out, then check for further
damage. Also do a scan with VCDS (new name for vag com) for seemingly unrelated electrical
error codes and problems. Any corrosion or coolant behind the instrument cluster would need
cleaning or any corrosion and repair.
Here is a picture of a fuel tank sender plug - that's engine coolant!
The coolant tank which is most susceptible to coolant migration is the style
of tank pictured below where the coolant level sensor is tilted down. It
also has a weatherproofing seal around the sensor plug.
The newer style of tank (with the same part number as the old style) and the
mk3 coolant reservoirs have the sensor near the top at a horizontal angle.
The mk3 coolant reservoir has a different style sensor connector. Because this
style connector is not sealed with the wiring harness, any coolant that could get
past the mk3 style plug should drip off the two wires and not get drawn into the wiring
Mk5 coolant tanks switched back to the tilted sensor but they got rid of the
weatherproofing seal around the plug's base to let anything drip out. Mk6
and Audi A3 TDI coolant tanks are the same.
Preventative maintenance or solutions involve first preventing the coolant from migrating into the wiring harness. You can remove the bright orange waterproofing seal at the coolant sensor plug. The seal provides a tight connection to prevent corrosion of the coolant sensor from water but it also seals in the pressure that pushes coolant into the wiring harness. Exposing the wiring harness makes it more like the mk3 coolant tank and will let coolant drip down the exposed wires and not into the wiring harness.
Another solution involves making one or two small holes in the bottom of the coolant reservoir plug plastic. This will let any coolant weep onto the ground instead of being pushed into the wiring harness. Use a heated paper clip or tiny drill to put a hole into the connector plastic. In the pictures at the top of the page, the hole would go at the bottom of the connector the coolant is pooling, not through the reservoir.
A third possible solution involves also replacing the coolant sensor wires with spade connectors to break the path that the coolant can flow across. If you look at the picture (above right) of the mk3 coolant reservoir you can see how the wiring loom is exposed after the plug. A connector would stop any coolant.
Lastly, you could just replace the coolant reservoir with a new one. This condition is relatively rare so you could take one of the preventative steps listed above. New tanks are pretty cheap and if your old tank is dirty inside I would replace it just to be able to see the coolant level.
If you do find coolant migration
First follow the wiring harness and see how far coolant has migrated. It appears to collect at the taillights, instrument cluster (see 1000q: instrument cluster removal for tips), starter motor (it's at a low point), coolant temp sensor, relay panel, fuel tank sender (under the rear seats), engine wiring harness, and transmission sensors. These are relatively easily inspected. You may see light green or red crustys. Green is corrosion and red/pink is dried coolant. If it's relatively minor clean off the corrosion with a cloth and electrical contact cleaner.
A full inspection requires pulling up the carpet and dashboard to inspect the wiring and grounds under the carpet so I just suggest cleaning everything you can reach and hoping for the best.
Summary of TSB
Download the full TSB here: coolant contamination of wiring harness
Coolant leaking from coolant reservoir bottle
wiring connector contaminates the wiring
Contamination can result in various
malfunctions of the instrument cluster.
If an instrument cluster exhibits a malfunction:
– Record Vehicle Mileage (odometer
adaptation reading) (see Repair Manual).
– Print out any and all instrument cluster
coding and adaptation values using the VAS
5051 for use after repairs are complete.
– Remove instrument cluster (see Repair
– Remove blue flat contact housing cover and
inspect wire terminals for moisture or
If moisture or corrosion is found (arrow):
– Repair wiring using repair kit 1CM 998 001.
This kit contains overlay harnesses, splice
connectors, tape and installation tube
necessary to repair the affected harnesses.
The instrument cluster, coolant bottle and fuel
pump/fuel sender must also be replaced.
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