IPSC

While cleaning up the mess of a wire bundle the PO made doing the MAF install I found what looks like a variable resistor in the middle of the wire bundle coming off of the MAF.

What does this adjust?

I did not attempt to adjust it but, I now want to know what it does.

IPSC

Mike Murcia

It is a resistor that Autothority put in place of the temp sensor in the stock AFM. I didn't like the design, so I installed the original AFM sensor into the Autothority MAF.

The original AFM temp sensor (thermisistor) supplied a variable resistance to the DME with changing air intake temperatures. The DME was then able to adjust fuel properly for hot and cold air intake. Autothority put the variable resistor in place, so you can adjust the resistance, but it remains constant no matter what your intake temps are. This leads to a car that may idle well once warm, but on cold days it may run quite poorly, or vice versa depending on what resistance the variable resistor is set to put out.

IPSC

Hummmm mine idles fine in varying temps so I won't mess with it. How did you put the temp sensor into the MAF?

IPSC

streckfu's

What he said.

testarossa_td

What they said ^^^
I want part numbers and pictures

Mike Murcia

My car ran fine also, but the idle changed a couple hundred RPMs depending on whether it was a really cold or hot day. Now the idle speed is much less temperature dependent. I just don't like the idea that an "upgrade" took away some functionality that came from the factory. Most of the other MAFs offered for our cars come with a temp sensor, and it is not hard to update the "original" MAF offered for our cars. I'll post details on what I did when I get to my home computer tonight. All of my pictures are there.

Mike Murcia

I used the temp sensor located in the stock AFM. It is located under the black cap, and you have to be careful to not destroy it while pulling it out.





Once pulled, I could measure and drill the holes in the MAF housing.



The sensor was then mounted in the MAF housing using loctite on the bolts and silicone to make an airtight seal.



After pulling the resistor out of the Autothority wiring harness, I spliced connectors onto the ends of the leads and connected the temp sensor wires.





Viola! The APE MAF now has the proper intake temp compensation.

Dal Heger

But. . . the MAF uses a hot wire to measure not only air flow (a simple function of the old style flapper air box) but also the density of the air and most importantly the temperature of the air passing by it - and cooling it off. The old flapper style air flow meter needed the temperature input in order to figure out how rich to run the car when the air was cold (and therefore more dense), but the MAF has that built into the measurement of the hot wire. If it's cold outside, then the MAF sensor reads more current needed to keep the platinum wire at the proper temperature, which then outputs a higher (richer - more load being measured) voltage to the DME.

What you are basically doing now is doubling the fuel enrichment correction being shown to the DME, or conversely making it run leaner as the air temps increase. The variable resistor was being used as a simple, static fine tuning device for the DME. If you were having a poor running problem, why didn't you just try a different setting on the resistor?

Just my 2 cents.

Dal.

95ONE

the chip he is using takes care of this issue. (from what I have just read on the website.)

Ski

Dal, the hot wire in the MAF doesn't measure the air temp passing over it. All it does is try to stay a certian temperature(my understanding and I've been wrong before). The resistance level of keeping it to that temp, as air is passed over it, is driving the fuel delivery that is calculated by the DME, in conjunction with the TPS and speed sensor, 02 if not open loop at full throttle and a given set of fuel maps. All LR and VR MAFs ( set to certian calibrations) have a seperate temp sensor.

Mike Murcia

Hi Dal. I should have clarified. Let me first give the disclaimer that I am not a Motronic expert, so all points are open to debate, but I believe the intake temp sensor is only used as a reference for fuel scaling by the DME when the engine temp sensors are reading a "cold" condition. This resistance value that the temp sensor puts out allows the DME to calculate the proper fuel enrichment on cold startups. By having a fixed resistance value being sent to the DME, the startup fuel enrichment will always be the same no matter if the outside temperature is 90 degrees or -20 degrees. It's my understanding that the MAF's hot wire cannot provide that much compensation on a cold startup for a cold day.

I would think (rather hope) that the resistance would not be used under all operating conditions. Otherwise the constant resistance from the adjustable resistor that APE put in place would give a constant fuel enrichment signal and it makes no sense why they would do that.

The airflow signal to the DME is used to establish a stoich mixture only. Since on a cold start, the engine needs additional fuel (more than stoich), the air flow signal alone can't do this. The temp sensor is then used as a reference point so that the signal from the AFM or MAF can be scaled to the proper enrichment (I think I read up to 15% somewhere). The MAF's hot wire alone can't achieve this kind of temperature compensation. Even APE decided this and put the variable resistor in. The problem is that the variable resistor is usually set to one resistance, implying only one cold start temperature. The temp sensor fixes that. Depending on what your climate is, you may not notice much of a difference without the temp sensor, but the temperature can fluctuate wildly where I live, so it made sense.

Dal Heger

The cold start is handled by the coolant temperature sensor, not the air temp sensor.

I'm still learning about this myself, so I might be mistaken, but from what I've read/figured out the air temp compensation is on the order of 4% enrichment, which isn't much. The MAF can easily handle this by the way it normally operates.

As to the temperature of the air not affecting the amount of current needed to keep the wire at the desired temperature that's obviously incorrect. I agree with Ski that the output is passed to the DME and the air flow meter basically measures the load on the engine, but the density and temperature of the air blowing by the wire WILL effect the temperature drop of the wire and therefore the voltage output (read as load) sent to the DME. Humidity also effects the amount of temperature drop across the hot wire (humid air is more dense than dry air - and cold humid air will take more heat away from any source).

As to the chips compensating for the active air intake temperature changes, assuming that you're running the chips that came with the MAF, why do you think that they put the resistor in there in the first place? I have no idea what Lindsey Racing/Vitesse are doing with the temp sensor or the maps in their chips.

The airflow meter doesn't set the engine to a stoichiometric mixture, it just tells the DME what the load on the engine is via the amount of air passing it. The DME then uses this, the output from the reference sensor(s) and the TPS to find what cell it should be looking at in the chip to see how long to hold the injector open for and when to fire the spark plug. The O2 sensor is the trim device that tries to keep the engine running at stoiciometric ratios at part load. The O2 sensor (and the AFM output) are ignored when the full throttle switch is activated. The DME then goes into full open loop mode and just reads the reference sensors to determine what rpm the engine is turning.

If I'm wrong, tell me!

Dal.

Currently hunting a pinging (massive advance) on a newly built 2.5l engine. Thinking it's the new fidanza flywheel - reference sensor might be off a few degrees.

ZPmadA

What does the blue wire connect to? Just the potentiometer, or is it attached to the ground (black) aswell? My Ground wire is ripped out of the oem socket. Do you think I could re-route the maf ground wire to the headlight ground until I get a new maf harness?

TurboTommy

I used to have an Autothority MAF.
I had a discussion with one of the techs, there, in regards to that resistor.
The bottom line is that it only has a very small effect on the air/fuel ratios, and at idle only. It is possible that it is there to fine tune the idle because of very low air flow going over the hotwire. Any throttle opening and that thing has no effect whatsoever.
I don't think it was designed to replace the factory intake air temp sensor which, I believe, is a much more significant input signal to the DME.
Even the temp sensors that some of our friendly tuners use with their MAfs are only there to aid in cold start/running conditions, which is over and above the self temperature compensating hotwire.

By the way, Dal;
humidity, by definition, is evaporated water.
Once evaporated, it will have no cooling effect on a hotwire. It will also take up space once occupied by air molecules, and it will have less mass per volume. So, higher humidity air is actually less dense.

H Dog

Hi,
I'm having drivability issues with my K26/6 Authothority MAF setup. Like mentioned above, it won't hold idle when cold. After the car gets hot, it won't return to a normal idle; instead, it hangs at high revs making 3000-4000 rpm upshifts (requiring the engine to return to 2500 rpm or so) impossible. And at stop lights, the engine idles at 2000/min.

On the track, it runs great.

Will the splicing of the temperature sensor in lieu of the potentiometer fix this?