Help! Low Voltage to MAF sensors. Fuse? Relay?

[Timmah]

I am having problems with my car not starting, sprayed fuel in the intake and it starts, got it to the mechanic, who scanned it.

both MAF sensors showing 0.0

Mechanic said to check MAF voltages. There should be a 12v and a 5v on two of the wires. I got the 5 volts on one wire, but only 1.75v on the wire that should have 12v.


has anyone had this happen? Is there a fuse or relay for the MAF sensors? Which one is it?

[nubcake]

Fuse 34, also powers a bunch of other engine sensors/solenoids.

EDIT: possibly fuel pump relay J17, located "In the micro-central electronic behind storage compartment on driver's side, relay position 4". Above "terminal 87F".

[Timmah]

Fuse #29, #32 and #34 are not getting voltage to them with the key in any position. is there a main engine management fuse/relay anywhere further back in the system that could be causing this lack of voltage?

Fuel pump relay is good, all the fuses under the dash by the relays have power with a test light.

[nubcake]

It's hard to believe, that fuel pump relay is good with the symptoms you describe.
It has a direct connection to the +12V, and directly powers fuses 28, 32 and 34. Think you meant those 3, and not 29? Cuz 29 is powered by Motronic relay, and if that was dead - you wouldn't be getting any power to the ECU.

Make sure the fuel pump relay is getting +12 at terminal 30, and +12 at terminal 86 at ignition on.
If it does, replace with a known good spare (or test this one on the bench).
If that doesn't help, trace relay terminal 85 to the ECU pin 65.

[Timmah]

Fuel pump relay is getting 12v at terminal 30, and 12v at terminal 86 at ignition on.
Fuel pump relay terminal 85 has continuity to the ECU pin 65.
I have 3 fuel pump relays that all bench test good. Unless there is a different way to test VAG relays

I am only getting 2.5v in the 87F and 1.7V at pin 2 of the MAF sensors

[nubcake]

Well, next step is to verify ECU is actually giving out the "turn fuel pump on" signal. It'll ground the pin65 (term85) briefly when you turn ignition on (and then when cranking). Basically, energizing the relay coil.

If this checks out as well, then I'm at a loss. :vhmmm:
Unfortunately, I'm not much more of an electrician, than "trace this wires, check connections & voltages".

EDIT: can ofc try the obvious - bypass fuel pump relay from t30 to t87f (and t87a!). If that makes the car run, it's effectively the same test as above. ECU is not giving out the signal for some reason.