Air mass meter innards

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Thanks Mike, I had a reply from them that they cannot help with this one.

AJH

What nissan is it, and is it turbocharged or not?

They are generally similar across the model range but can cause lean conditions or really rich conditions and kill your cat. You could do with finding out what the error is caused by - its obviously going to be visible electronically so you should be able to see the fault with enough patience.

Yes the one from an 1800 turbo subaru I fitted to see what the problem was works fine but it is excessively rich, so I think the cat would get swamped if I drove with it.

And that's where I need a little help. The fault took a long time to develop. At first it was just the occasional misfire as the ecu cut the spark momentarily as soon as it senses a fault condition, this caused a hot exhaust as a unburnt mixture was converted in the cat. It was only when the OBD1 code showed up as a MAF fault that I started looking at it. It checks out OK with a dmm with volts rising to 3+ when sitting with the torque converter stalled at 2000ish revs, but the ecu senses a fault. Running with the subaru MAF triggers no faults.

It could be something simple in the circuitry but I'm not sure what to look for.

As the hot wire effectively changes its resistance in response to air velocity over it with the reference resistor calculating ambient temperature and the circuitry converts this to a voltage in the 0-5V range in both MAFs I surmised that the only significant difference would be the size of the port. So wondered if the subaru MAF hot wire and circuitry could be fitted into the original tube?

AJH

Sounds more akin to a bad wire or contact somewhere than an outright failure, maybe the scooby one fits into the contacts tighter so doenst cause the fault?

Really all you can do is check soldered joints internally and hope you spot a dry one or a breaking wire. Any electronics that are failing are either going to be obvious (burnt) or totally un-obvious!

The two wires and bias/reference resistors could be totally differnent. Coupled with the difference in body diameter and hence cooling at the same flowrate Id suspect its not that simple. They all use 0-5v purely as its easier for a microprocessor to read directly, it doesnt mean all the circuits are the same unfortunately. I'm not overly familiar with hot wire methods as my cars are either MAP or flapper-style AFM. Maybe someone on the nissan usenet groups would be better suited to directing you to an easy fix. IF not, and if you have no other use for the scooby one - pull them apart and give it a shot, you only have a misfiring car to lose!