Heater Blower Motor and Resistor on Smart Fortwo
Service interval: Replace on failure — not a scheduled item
Tools you'll need
- Socket set (10mm and 7mm common on Smart trim)
- Plastic panel removal tool
- Multimeter for diagnostics
- Phillips and flat screwdriver
What this is and why it matters
The cabin fan on a Smart has two failure modes that look similar from the driver's seat but are different parts. The fan only working on the highest speed is almost always the resistor pack. The fan not working at all, or making a grinding noise, is almost always the motor itself. Knowing which is which before you order parts saves a wrong-parts return and an extra weekend.
The resistor sits between the fan switch and the motor and drops voltage for the lower speed settings. When it fails, the lower speeds get no power but the high-speed circuit (which bypasses the resistor) still works. So you turn the fan to 1, 2, or 3 and get nothing; turn it to 4 (or whatever the top setting is) and it blows. That's the resistor, not the motor.
When the motor itself fails, you get nothing on any setting, or you get noise — bearing whine, scraping, or a thunk on startup. That's the motor.
What you'll need
A basic socket set covers most of the trim fasteners; 10mm and 7mm are the common Smart sizes for dash and HVAC area work. A plastic panel removal tool keeps you from snapping clips on glovebox and footwell trim — useful enough to be worth the $5.
A multimeter helps with diagnosis. You can also do a brute-force test by jumpering 12V directly to the motor terminals (with the connector unplugged) — if the motor spins, it's the resistor. If it doesn't, it's the motor.
While you're in there, replace the cabin filter. It lives in the same area on most Smart Fortwos, the labor is half-shared with this job, and a clogged filter is what kills blower motors prematurely in the first place.
Step by step
The exact teardown varies by Smart generation — the 450, 451, and 453 all locate the blower assembly slightly differently behind the dash, and access is from either the passenger footwell or behind the glovebox depending on the model. Confirm the location for your specific car in the workshop manual on Manualslib before committing to a teardown path.
- Confirm the diagnosis first. Turn the fan through every speed. Only-high-speed-works points at the resistor; nothing-on-any-speed points at the motor or a fuse. Check the fuse for the blower circuit before pulling any trim. A blown fuse is a 2-minute fix that looks like a dead motor.
- Pull the relevant trim panel. Glovebox usually unclips and tilts down out of the way after a couple of fasteners. The blower assembly sits behind it on most variants. Take photos as you go so you can put it back the same way.
- Locate the resistor pack. It's a small plastic-and-metal block with a multi-pin connector, mounted on the side of the blower housing where it can be cooled by airflow over its fins. Two screws or a single bolt typically hold it.
- Diagnose with the connector unplugged. Use a multimeter to check resistance across the resistor pack terminals — an open circuit on one of the speed taps confirms a failed resistor. Or jumper 12V directly to the motor (carefully — motor only, not into the resistor connector) and confirm the motor spins. Motor spins = resistor problem. Motor silent = motor problem.
- Replace the failed part. Resistor: unscrew, swap, reconnect. Motor: usually three or four fasteners holding it to the housing, plus a connector. The motor lifts out as an assembly with the squirrel-cage fan attached on most variants.
- Reassemble in reverse and test on every speed. All speeds should work, the fan should ramp smoothly, no grinding or whine.
Common gotchas
Buying a motor when it was the resistor is the most common mistake on this job. Five extra minutes with a multimeter or a 12V jumper saves an unnecessary part purchase. The two parts are not interchangeable and the resistor is two to four times cheaper.
A motor that grinds or whines but still moves air is on borrowed time. Replace it before it seizes — a seized motor with a working circuit feeding it can blow the fuse or burn the new resistor you just installed.
Cabin filter neglect is the leading cause of premature blower failure. A clogged filter forces the motor to work against extra resistance, the bearings wear, and you're back in here in a few years. Replace the cabin filter every service, not just when the airflow is obviously bad.
Some aftermarket resistor packs are noticeably lower quality than OE. If a cheap aftermarket resistor fails within a year, that's not bad luck — that's the part. Buy the better-known brand the second time.
The blower assembly on some 451 builds requires more dash teardown than other variants to access the motor. Check the workshop manual for your specific year before assuming this is a one-hour job — it can be more.
When to skip DIY
If the symptom is the fan running by itself with the key off, the AC system kicking on with the fan switch off, or the fan running at a single fixed speed regardless of switch position — that's usually a control module or relay problem, not a blower motor or resistor. Different repair, different diagnosis path. A shop with the right scanner is the call.
If pulling the trim reveals previous repair attempts — broken clips, hacked wiring, missing screws, a mismatched aftermarket resistor wired in funny — it's worth pausing and getting a shop to take it from there. Compounding someone else's mistake is more expensive than starting clean.
If the blower works fine on every speed but the cabin temperature is wrong (no heat with engine warm, or no cold air with the AC running), this isn't your problem — that's a heater core, blend door, or AC system issue, not a fan issue. Different page.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Blower motor resistor pack | $25-60 | Search Google |
| Blower motor assembly | $80-180 | Search Google |
| Cabin filter (replace while you're in there) | $10-25 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread.
Manual references
- Browse Smart workshop manuals on smartcarmanuals.com — model-specific reference manuals on the home page; pick your chassis code section for torque specs and detailed procedures.