your-real-dad

Here's a puzzler for any Gen3 4runner gurus out there. Bought a used 2002. At 30mph and above, a distinct cyclic "thrumming" sound appears. "Thrummm, Thrummm, Trummm". Sounds forward, like front diff or maybe trans case. The noise frequency correlates with wheel rpms, but not with engine rpms. The weird part is that this noise is much slower than the wheels. At 30mph, the thrum is 2.2cycles/second (2.2hz), but the (calculated) wheel rpms are 5.4hz (315rpm). The thrum is 2.5x SLOWER than the wheels. This relationship holds true at any speed: 20, 30, 40 mph all tested.

Here are details if you are intrigued. Purchased from a young guy who made some mods. I think it has one of the "Andy mods". He said he replaced the transfer case, and that made the noise "go away, for a while, but it came back". It looks like he also replaced the TC actuator and the ADD actuator with Dorman models. He said "it needs a new front differential, but I didn't want to take that on." The vehicle 4WD will NOT engage; just the orange LED and four green ones all flashing.

The thrumming noise has a sound "profile", for lack of a better term, that reminds me of a beat frequency. (If anyone remembers what a beat frequency is, from HS physics...).

As a side question: many posts refer to the ADD actuator being vacuum driven, but the after-market ADD unit here is definitely motor driven. Is that normal for a 2002?

aowRS

Yes, the ADD on both of my 3rd gens ('01 & '02) is electric.

Your description of the noise reminds me of when I lost a tooth or two on a Subaru ring gear, many years ago. Maybe pull the front driveshaft and see if the noise changes or stops?



Andreas

your-real-dad

Thanks Andreas. I've since stumbled on a TC diagram, and as soon as I noticed the planetary Hi/Lo gear, I had a suspect. The Hi/Lo gear reduction is 2.58, which means the six little planet gears are rotating 2.58 times slower than the main input output shaft regardless of whether the vehicle is in Hi or Lo. My mystery noise measurements are around 2.6Hz, probably not a coincidence. If any of the six planet gears is going south, it would make this kind of noise. Or, as you noted, if there are a couple of teeth missing from the ring gear it's going to make weird noises also.

I'll report out when I get a look. Currently with my arm in a sling and it's about 8F outside...