4WD indicator lights flashing

Hey watch it! There are women and children present!

Yes.... let us know what we are working on and we might be able to help. Without knowing the year, I can't even tell if the GEM or the 4X4 module is what needs to be polled for codes. Incidentally, without knowing the codes, you will get opinions and not facts.

Jim Warman snipped-for-privacy@telusplanet.net

OK.... so far we know that the 4X4 function in the '96 is controlled by the GEM. We also know that the GEM has seen a problem in the 4X4 system since the lights are flashing. This narrows the problem down to include (but not limited to) the GEM, the shift motor, shift motor solenoids and circuits, front speed sensor, rear speed sensor.......

Without DTCs, we are still pretty much flying blind. Some climatic areas may suffer from pattern failures, but I haven't seen any pattern with these systems in my area. The easy way to get to the root of your trouble is to have the GEM polled for both Continuous and On Demand DTCs. I am unsure whether the code readers some parts stores use are capable of accessing the GEM. Perhaps another person can step up to the plate for that one.

Jim Warman snipped-for-privacy@telusplanet.net

On our '96 XLT we had the same problem. Sporadically, the 4x4 lights would flash 6 times every few minutes with the switch in the 4wd Auto position. The effect seemed to be that the whole thing was "locked" as if it were in

After a lot of investigation, I removed the front and rear speed sensors on the transfer case and cleaned them off. Both had a small "lump" of metallic gunk on the Hall Effect sensors (they have magnets in them that are part of the speed sensing physics). The goo attracted to the magnets was really very very fine metal shavings. Nothing to be concerned about with the shavings, they are apparently part of normal wear.

Anyway, after wiping off the sensors the 4wd system has performed normally.

See, here's what happens. When the front sensor either goes bad or cannot read properly (due to a buildup of metallic gunk on the sensor), the GEM computer thinks the back tires are going faster than the fronts. It thinks the rear tires are thus spinning. When this happens, the GEM tells the transfer case to put more power to the front axle. Since the front speed sensor is not giving a good signal, it seemed to put as much power as it could to the front, i.e, putting the transfer case into the same lock-up as you'd see in the 4wd Low lockup mode (which is normal for THAT mode).

Then when you start driving at normal speeds, the lights begin to flash since the GEM finally figures out that it's impossible at 35 MPH for the front axle to not be turning at all (due to gunked up or bad front sensor) yet the rear tires are turning normally. Therefore, the error condition.

I had seen several postings over the years that describe this condition as either a bad/dirty front speed sensor or that the connector near the transfer case needs cleaning.

So, that solved it. They are both held in by one nut, fairly easy to get to except for the rear one where you have to remove the transfer case shift motor assembly (that's not too bad). It's a medium difficulty DIY job, so if you're not comfortable with doing it yourself, you could ask a mechanic to do it. It's worth having it done to eliminate it as a cause anyway.

Mike

Congrats on the easy fix, Mike.... however, Davids problem still needs some more info before we can make any accurate long distance diagnosis. What you found may be a pattern failure though I haven't seen it as a problem in our area.

This particular system is far from simple.... the front and rear output shaft speeds are compared and the GEM will adjust the duty cycle of the electromagnetic clutch depending on the difference in shaft speeds and throttle position.( wide open throttle being the only time that the duty cycle will achieve 100%). Turning the Mode Select Switch to 4WD commands the shift motor to lock the mode shift collar.

I'm not going to discourage David from removing the sensors, other than the usual warning about plastic parts, since it is an easy check...... I do try to discourage shot gun or random diagnostics, though. All too often, I see the unwary create a nightmare when they wade in over their heads, creating needless expense they can ill afford on the way. Knowing the DTCs we are dealing with and seeing if they are readily reproduced or not can colour our diagnostic efforts, shortening the time required and money spent. Additionally, we must remember that there are those people that are all thumbs (my BIL comes to mind ...... nothing to be ashamed of as long as they don't pick up any tools) and others that resemble a bull in a china shop in their repair efforts..... what comes easy to one person is an arduous task for another....

Jim Warman snipped-for-privacy@telusplanet.net