Model S/X: LDU problems (the large rear motor) — seal leak and failure

Summary

The large rear drive unit (LDU) on early Model S/X (2012-2020) is prone to coolant leaks through the seal on the rotor shaft. Coolant that reaches the stator/inverter degrades the insulation, triggers BMS_w172 (SW_Drive_Iso_Warning) and eventually kills the unit. Tesla itself dropped rotor cooling in the late revision-U motor (t.me/teslacar_chat/497725), which amounts to an engineering admission of the defect.

📌 The seal leak affects every LDU from 2012–2020, Raven included. Tesla removed rotor cooling only in the late revision U (~2022) (t.me/teslacar_chat/152376).

How to recognise it

• An insulation fault (BMS_w172, SW_Drive_Iso_Warning; often paired with BMS_u008) is almost always the first and main symptom (t.me/teslacar_chat/497784, t.me/teslacar_chat/651460).
• Fogging or coolant traces around the speed sensor (where the rotor exits). This is the key diagnostic trick: unscrew the sensor and look inside (t.me/teslacar_chat/152369).
• Coolant slowly disappearing: 0.5 litre out of a ~14-litre system goes unnoticed, so the low-level warning usually never fires in time (t.me/teslacar_chat/651460).
• A buzzing or grinding noise that keeps getting louder (at later stages).

What to do

Diagnostics: remove the speed sensor from the back of the motor and shine a light in. It should be dry. If you see drops or moisture, the car needs a shop.

Repair options, from cheapest to most complete:

1. DIY caps plus an angle grinder (~$50). Enthusiasts in the US plug the rotor cooling channels. The risks: you need good hands, a grinder and a clear idea of what you are unscrewing. This is for people who have already resoldered an MCU and are not afraid. (t.me/teslacar_chat/602688)

2. A Revolt-style bypass kit (~$300; $299 on the Revolt site). A ready-made kit that reroutes the cooling loop around the rotor. It is installed at a shop.

3. Seal and bearing replacement plus stator cleaning ($800–2500). The traditional repair, and only a temporary one: "every seal replacement ends up leaking again after a while" (t.me/teslacar_chat/602688). If you go this way, insist on new bearings, a Teflon seal, cleaning and drying of the stator, and polishing or metal spraying of the shaft where the seal sits (t.me/teslacar_chat/294584, t.me/teslacar_chat/317934).

4. A full LDU reman (revision-U, $7000). A new Tesla motor without rotor cooling, the way Tesla built the late units. In the US it costs $7000 (t.me/teslacar_chat/602688).

💡 Strategy for a buyer of a 2014-2019 S/X: check through the speed sensor. If it is dry, fit a bypass or caps as a precaution before the first problems. If it is already wet, weigh it up: a seal repair is temporary, a reman costs $7k.

Prevention

• Check for leaks through the speed sensor at least once a year.
• Watch the coolant level and how clean it is.
• At the first signs (a spot of moisture, an insulation fault), act quickly. The longer you drive, the more coolant ends up in the stator and the more the repair will cost.

What it costs in Belarus

• DIY caps (if you do it yourself): $50 plus time and risk.
• Bypass kit plus installation: ≈ $800–1 200 (the $299 kit plus shipping/customs and labour).
• Early stage, seal plus bearings plus cleaning: ≈ $1 000–2 500. Motor R&R alone: 60 000 RUB ($700) just to remove and refit it (t.me/teslacar_chat/722086).
• Deep repair with disassembly, stator rewinding and inverter restoration: ≈ $2 500–5 500.
• LDU replacement/reman (donor unit): ≈ $3 000–6 000 (t.me/teslacar_chat/564854). A new revision-U from the US: $7 000.

Year-by-year clarifications and repair in Belarus (tsk.by)

• The short LDU service life is characteristic above all of motors built in 2013–2018: with a water-cooled rotor, antifreeze gets into the stator through the rotor seal over time. Tesla only refined the manufacturing process that eliminated the problem by 2019–2020 (tsk.by breakdown).
• A full LDU restoration cycle includes drying and re-varnishing the stator, plus grinding and balancing the rotor (tsk.by scope of work).
• Minsk reference prices, BYN (one shop's figures, not the market): bearing replacement from 400 BYN (1–2 days), stator restoration from 800 BYN, full motor overhaul from 1200 BYN (3–7 days); repair warranty 6–12 months.
• The small motors (SDU) on Model S/X are considered reliable, provided the gearbox oil is changed every 50,000 km.

Links

• TMC overview of LDU/RDU coolant leak issues: https://teslamotorsclub.com/tmc/threads/early-ms-ldu-rdu-coolant-leakage-current-state-of-affairs.327284/
• Revolt Systems on the "coolant bypass": https://revoltsystems.com/tesla-large-drive-unit-ldu-coolant-bypass-kit-the-achilles-seal-fix/
• Gruber Motors on the consequences of a leak: https://www.youtube.com/watch?v=zBEtkGHdKwU
• Repair pricing example: https://elmob.us/tesla-ldu-coolant-leak-repair
• RTC: the best single-message overview of every option (Tesla revision-U, Revolt, $50 DIY): https://t.me/teslacar_chat/602688
• RTC: Boris on the 12-year LDU saga and Tesla's decision to drop rotor cooling: https://t.me/teslacar_chat/497725
• RTC: pre-purchase tip, unscrew the speed sensor before buying: https://t.me/teslacar_chat/152369