The blower in my 2007 G35 sedan recently stopped working. I've tried the following steps to troubleshoot the issue to try to find the root cause:
(1) With the fan enabled and the car turned on, I used a test light to check the two blower motor fuses. What is weird is that I get a light on one side but not other side of each blower fuse. I swapped both fuses with spares and I get the same result. I think the fuses are not tripped.
- Given this, I thought maybe the blower relay may be bad (not sure if that's a safe conclusion).
(2) I took out the blower relay (the bottom relay at the panel) and swapped it with another relay. That made no difference. The blower still doesn't work.
(3) With a test light and car turned on/fan enabled, I tried to check if I'm getting a power signal from the connector to the blower motor itself. No power.

I've searched within some threads and some have had to change out the relay or the motor itself. I'd like to try to narrow down the problem a bit further before I just start swapping parts.

Does anyone have any perspective on how I should better troubleshoot this?

 

Here's the schematic for the AC blower relay located on the BACK of your cabin Fuse Block - J/B.

Swap the AC relay with the ignition relay, if the engine won't start you know it's the relay.

Did you visually inspect the fuse to see if it was popped? You should definitely have the same voltage present on both sides of the fuse.

 

Thanks for your reply. I tried swapping the blower relay earlier and the engine started fine.

I visually checked both fuses and they don't look tripped.

Based on this, do you think my blower motor itself is bad?

 

No because you haven't successfully tested for voltage at the motor harness. Start by getting a digital multimeter, even the cheapo Cen-tech one from Harbor Freight is a very functional little DMM.

 

Thanks. I'll be getting a multimeter and I'm pending delivery.

In the mean time, I purchased a replacement relay and blower motor. Replacing both did not give me positive results.

I'll test with a multimeter once it arrives but I'm still scratching my head on what may be wrong.

If when I'm using a test light, both sides of the fuse does not light up (only one side) but the fuse is not tripped, does that make sense???

I've tested this with the blower enabled and car turned on.

 

Did some more troubleshooting.... My fuses were blown. I replaced them with two new fuses and they immediately tripped.

Clearly, there is a short somewhere... Could a bad relay cause fuses to trip?

 

Unplug the relay and see if it still trips, then unplug the motor and see if it still trips.

That will point to either the wiring between the motor and fuse, or the motor itself being bad.

 

cleric670 - thanks a ton for your help!

Quick update - I had a bad relay and bad motor. Once I replaced both, all is good in the world again.

Some steps I recently took to figure all of this out:
1. Replaced the two 15 amp fuses for the blower that were tripped.
2. Replaced the relay
3. Disconnected the connection to the (old) relay motor
4. I turned on the car and no fuses tripped.
5. I made the connection to the (old) relay motor and turned the fan on. It ran for a little bit (made some funny noises) and then I saw/smelled smoke. Turned the car off.
6. Replaced the relay motor with a new one and turned the fan on. Success.

 

Awesome, glad you got it all figured out. Probably the failing motor is what killed the relay, locked rotor current (basically as the motor quickly spins up to it's operating rpm you have the amperage spike then quickly level out, when it doesn't spin "locked rotor" it just generates a TON of amperage) going through a relay generates a lot more arc and heat and burns up the contact points for the few fractions of a second it takes for the fuse to finally blow. The motor probably got worse and worse over time and took a heavy toll on the relay finally killing it's contacts.

The opposite can happen as well though, as relay contact points wear out it creates resistance, higher resistance creates more inrush amperage when a motor initially kicks on, the higher amperage generates more heat which damages the motor windings.

This is why you should always replace a motor AND it's relay at the same time when one component fails so.

Starter + starter relay

AC motor + AC relay

Fortunately those are the only two big motor loads in a car so it's fairly simple for automotive.

Same reason you replace your home AC / heat pump relay regularly, 3 years for a heat pump, 5 years for an AC (places like Arizona where it's hot year-round shorten the maintenance interval on the AC). When the relay wears out it puts more load on the compressor motor startup.