Short on Mass airflow sensor on 2007 Cee'd CRDI 1.6

northlodger · 28 Jan 2019 at 18:19

I had a breaking down on M25 last Friday night. The car ran as normal at 70 MPH till the speedometer suddenly dropped to zero and I lost power completely. At that time, the engine was still running, but no power was delivered to the wheels and I had to pull over and switched off. After that, I could not switch on the engine and had to call rescue.

The following day, I found a fuse, that is next to the engine is blown. It is #11, 'SNSR2'. I put on new fuses, and they got blown straight away. I then disconnected a few sensors and put on new fuses to determine which sensor is at fault. It turns out to be the MAF sensor. I leave it disconnected, the engine can be turn on, but runs without power, which is expected.

I clean the MAF with some Halfords electric contact cleaner and replace the air filter, but it doesn't make any difference. As soon as I connect the MAF sensor, the #11 fuse gets blown.

I tried to find information on web, but nothing seems to match the symptoms. Has anyone got similar experience or insight about the problem?

Lashout_UK · 28 Jan 2019 at 20:21

You've probably got a short in the wiring harness somewhere. Time to start inspecting the wiring...

northlodger · 28 Jan 2019 at 20:27

Lashout_UK said:
You've probably got a short in the wiring harness somewhere. Time to start inspecting the wiring...

Thanks for the advice. I am new to car electrics. I'm wondering if this is due to harness short, then why the fuse stays OK when the MAF is disconnected?

Lashout_UK · 29 Jan 2019 at 10:20

northlodger said:
Thanks for the advice. I am new to car electrics. I'm wondering if this is due to harness short, then why the fuse stays OK when the MAF is disconnected?

I'm not an expert in electronics but I believe it'd be a result of there not being a complete circuit – it's only when you plug that MAF in that current tries to flow around the circuit, finds an alternate route (worn insulation on a wire touching the chassis, for example) and blows the circuit. There's nothing in the short-circuited loop to limit the current draw so the fuse promptly blows to protect the wiring and electronics.

Otherwise, it'd just be an open circuit (not plugged in) and harmless.

The MAF could have an internal short, alternatively, but I'd start with the potentially easier approach of visually inspecting as much of the wiring beforehand.

northlodger · 29 Jan 2019 at 17:12

Lashout_UK said:
I'm not an expert in electronics but I believe it'd be a result of there not being a complete circuit – it's only when you plug that MAF in that current tries to flow around the circuit, finds an alternate route (worn insulation on a wire touching the chassis, for example) and blows the circuit. There's nothing in the short-circuited loop to limit the current draw so the fuse promptly blows to protect the wiring and electronics.

Otherwise, it'd just be an open circuit (not plugged in) and harmless.

The MAF could have an internal short, alternatively, but I'd start with the potentially easier approach of visually inspecting as much of the wiring beforehand.

I did remove the MAF and measure all combinations from the 5 pins with an multimeter and found no shortage.
What is strange is that when I measure the voltage on the harness side, I coundn't find 12v pin. I think pin #1 should be 12, but it shown 0v. Now I am wondering if there is a short circuit on the wire to pin#1 to the chassis, but if so, the fuse should be blown without connecting the MAF sensor, but it wasn't the case.