Darwin's Theory - March - 2018

Rise of the MAF

From the beginning of the electronic era, we’ve been chasing inputs and outputs as root causes for failures. In this case, we’ll talk about how no line rise can cause the 3-4 clutches to burn in a 4L60E.

Here’s the customer complaint: The driver gets in his truck, gets ready to take off, and puts his foot to the floor in his heavily loaded, half-ton pickup.

As the vehicle pulls away, the 1-2 shift is very long and drawn out. Then the 2-3 shift occurs, and finally the 3-4 shift. But it just doesn’t feel right. We’ve all done enough of these 4L60Es and if you haven’t, just know that one of the most common issues is burnt 3-4 clutches. This is almost always caused by a line rise problem.

This is where you need to be cautious and do your due diligence by making sure you have all the outside-the-transmission sensors checked and verified. Out of several root causes, the mass airflow (MAF) sensor is the most common to fail.

Here’s what happens: The MAF reads how much air is flowing from the air filter container to the intake. It sends its reading to the computer, which uses the signal to create the perfect air/ fuel ratio for driving conditions. For example, if the driver presses the throttle to the floor, then the MAF should see a massive amount of air traveling past the filament.

Of course, there are backups and failsafes allowing the vehicle to continue driving, but not as efficiently. And, if the computer sees a discrepancy while checking and rechecking the inputs, it’ll usually set a code.

But there’s something about the hotwire style MAF that can affect line rise and cause other problems: The MAF sensor usually reads 0.70V at idle and 3.50V at WOT. Its voltage moves up and down, varying with the amount of air passing through.

Where it can go wrong is when the filament gets damaged or dirty and the reading faults or sticks at a set voltage, say 0.90 volts. Still within specs, yet, the command input is too low.

The most common cause for this on a hotwire MAF sensor is the filament gets dirty (figure 1). The MAF shown is used in many GM cars and trucks. If you look closely at the sensor, you’ll see three tiny, platinum filament wires positioned in the air stream (figure 2). The computer heats these filaments by supplying a constant signal to them.

When air flows past the wire, it cools the wires, decreasing their resistance. This allows more current to flow through the circuit. The computer then converts the current into a mass airflow signal.

If the wires are dirty, the dirt acts as an insulator, hindering the cooling action of the air flow. The computer uses the faulty command and adjusts the line pressure too low.

If there’s a MAF code, identifying and repairing the problem is easy. But if there aren’t any codes, take a snapshot, movie, or monitor the inputs, especially the MAF sensor. If the MAF sensor signal is out of specs, you have a couple options:

The first is to remove the MAF and inspect the filaments for damage or contamination. If the filaments are contaminated, you can attempt to clean them with compressed air or with a lint-free cleaning wand.

There are sprays sold for cleaning the MAF, but the technique recommended isn’t adequate: simply spraying at or into the MAF won’t fix the problem. While this may clean off some contamination, it isn’t a complete cleaning.

A thorough cleaning requires nothing more than some lint free wiping swabs and some compressed air. Cotton swabs can work; just make sure you get all the material off the filaments.

Remove the MAF sensor from the housing to gain access to the filaments (figure 3). You want a clear view of the filaments, so you can get in and clean the side of the hot wires that faces the incoming air.

Gently wipe the area of the hot wires, one at a time, using a new swab for each wire (figure 4). Blow compressed air across the filaments to complete the cleaning process.

Your only other option is to replace the MAF, which is a good call if the sensor is damaged or you aren’t willing to go through the cleaning process.

All transmissions operate under guidelines set by the computer; some different than others. But when a sensor is reading within its parameters but reading wrong for the conditions, sometimes a little cleaning is all that’s necessary.