Paul Brand: Debris may be causing Tahoe's ABS to activate

Q I have a 2005 Chevy Tahoe with about 40,000 miles on it. A couple of weeks ago when it was 20 below zero, every time I touched the brakes, the pedal pushed back as if it were going into antilock mode.

Off and on since then, when I first start the engine in the morning or at the end of the workday, and then step on the brakes to put it in gear, the pedal pushes back at me. The other day I had to brake hard as traffic suddenly slowed down, and the pedal pushed back. Is this a safety issue? Do you have any idea what is causing this and how to fix it?

A My Alldata automotive database pulled up a GM service bulletin that addresses a similar symptom. It's technical service bulletin 03-05-025-007C dated March 2006, and it identifies the possibility of rust and debris causing an increase in the air gap between an ABS sensor and the reluctor ring on the hub and potentially triggering low-speed ABS activation. The bulletin suggests first testing the sensor, then removing it to clean its mount. Retesting after reinstalling the sensor will determine whether the sensor needs replacement.

A scan-tool check for ABS fault codes would identify any other issues with the system.

Q I'm trying to get a sense of whether I need to replace the power steering pump on my 2002 Chrysler Town & Country minivan. It whines pretty much constantly, but it doesn't squeal. I will try all the things you suggest, but if they don't work, at what point will I have to replace it? Does it pose a hazard?

A Try adding a couple of ounces of SeaFoam Trans-Tune to the power steering fluid first. This will demoisturize and clean the pump, spool valve and steering gear. If this doesn't reduce the noise, a complete flush and refill with the proper Mopar power steering fluid is worth a try to remove any contaminants and debris. Chrysler has a very specific bleeding process that uses a vacuum pump with 25 inches of vacuum applied to the reservoir to help purge any trapped air in the system. This might also be worth a try.

But until there's a functional symptom such as a lack of power assist or an intermittent increase or decrease in steering effort, I don't think replacement of the pump or steering rack is warranted.

I can shift between all the gears with ease, except for shifting from third to fourth gear. I have to force and push it hard into fourth gear in order for it to engage. If I try to shift normally, it will not engage at all. There isn't any crunching noise in either my successful or failed attempts to shift to fourth.

Please help me diagnose this. How do I tell if it is a bad clutch, a shift linkage problem or a failing transmission? I am hoping to repair it myself if possible.

A I own a very similar BMW -- a '95 325is -- that exhibits a little bit of the same characteristic. Upshifts from third to fourth gear are balky, meaning I feel more resistance when moving the shift lever into this gear than during any other upshifts. And like your scenario, there's no gear "clash" or crunching.

I know it's not a clutch issue, and the downshift from fifth gear back into fourth gear is smooth and silent. One interesting characteristic I've noticed is that the upshift from third to fourth is cleaner and smoother when the transmission is still cold. I attribute this to the more viscous lubricant in the transmission doing part of the syncro's work by more rapidly slowing down the input shaft and gears to better match the gear speed to the output shaft speed for smoother engagement.

So the problem appears to be a worn syncro hub on fourth gear. In this case, there's no easy fix. A complete repair would involve removing the transmission to replace worn components -- a rather expensive proposition at best.