Weird noise ... HAC??

bluewolf

bluewolf

New member
Occasionally on steep inclines our 4Runner will make a very strange "grinding" noise, kind of like a plastic gear which is slipping (not exactly, but for lack of a better description), and the vehicle will chirp several times. Also a dash light flashes, but I'm not sure which one since it's only happened a couple of times with me driving. This is the HAC, I assume, which engages if it senses loss of traction. I'm not fond of strange noises, so initially I was not pleased at all with the HAC, but if it's functioning properly so-be-it.

Can anyone give me a bit more detail as to how this system works, what exactly makes that "grinding" noise, and how often I would have to listen to the thing engage during more aggressive off-roading use?
 
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Occasionally on steep inclines our 4Runner will make a very strange "grinding" noise, kind of like a plastic gear which is slipping (not exactly, but for lack of a better description), and the vehicle will chirp several times. Also a dash light flashes, but I'm not sure which one since it's only happened a couple of times with me driving.
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Well, I don't know about the Hill Assist but I have had what I presume to be TRAC engage while going up a steep, uneven hillside and heard "strange" sounds from the vicinity of the wheel that was presumably slipping; sounded more like a "thunking" noise than a grinding noise. I don't remeber any chirps though and I was busy watching the trail not the dash so I wouldn't have seen any dash lights.
 
If you think the HAC noise is spooky, wait 'till you try out the Downhill Assist!

The noises are perfectly normal, and are a result of the ABS system rapidly pumping the brakes...

—Dan
 
dancojgl said:
If you think the HAC noise is spooky, wait 'till you try out the Downhill Assist!

The noises are perfectly normal, and are a result of the ABS system rapidly pumping the brakes...

—Dan
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Wonderful. I haven't had the opportunity to really test the 4Runner out on the trails, maybe this weekend. I've still got some jitters about doing anything extreme with a $30,000 rig... much more comfortable in a CJ or my 93 Toyota PU. I'm not sure I can get used to all of the "automated" BS the new 4Runner does.

Why would the ABS system want to rapidly pump the brakes if you're progressing upward on a moderate incline?
 
Why would the ABS system want to rapidly pump the brakes if you're progressing upward on a moderate incline?
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If a wheel begins to spin the Traction Control (TRAC) system will apply the brake to that wheel to prevent the other wheel on that axle from loosing torque.

If you didn't have either TRAC or a locking differential then if either wheel on an axle looses traction (slips) then the other wheel on that axle stops receiving power from the engine.
 
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Just read through Thai's excellent thread on ATRAC, VSC, etc. Very informative, I should've looked for that before posting... oops!

Thanks Thai
 
A Nissan Xterra was ahead of me going up the same hill where my TRAC system activated and the Xterra slipped back down the hill at the same place where my TRAC system activated. Later I asked the driver what had happened, hadn't he been in 4WD. He said that he had been in 4WD but not in 4low and he thought that the center differential didn't lock unless the Xterra was in 4Lo. Does anyone know about the Xterra 4WD system?
 
dcampen said:
Does anyone know about the Xterra 4WD system?
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Prior to 2003, Xterra had a basic part-time 4wd system. That is, on dry land, it MUST be in 2wd. Only on off-roading terrain/heavy rain, you can switch to 4-HI or 4-LO. There isn't a real center differential. Once you're in 4HI, the front and rear axles are tied together (similar to when you lock the center diff on your 4runner). So, that Xterra driver is wrong.

It is likely that the Xterra in the post above just lost traction due to driver error, shitty tires, lack of suspension travel (leaf springs don't travel much in stock form), lack of technology (ATRAC), or everything combined. Xterras do NOT come with ATRAC, so they don't have any system to transfer torque side-to-side.
 
Re: Hey bluewolf...

Coolhand said:
Check out my thread re: nasty grinding and chirping noises. I simply don't believe that this is a normal mode of operation. It definately does not feel like the ABS engaging.

http://www.toyota-4runner.org/showthread.php?threadid=3999

My rig is in the shop now and they're looking into this problem.

Justin
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Sounds like you may of damaged something? Does anyone have specs on how much weight the down hill assist can handle? It might not of been designed to use it for towing down hill with heavy loads.
 
Well I sure hope I didn't but man that noice and vibration gave me the creeps. BTW - it would be the hill start assist, not the downhill assist, at least in my case. I was always trying to go uphill or hold on an uphill when my cases occured.

I was also wondering about how much weight the hill start assist can handle. Since the vehicle is rated at something like 7,000 lb, you'd sure think they wouldn't put a system on there that's going to fail on a boat ramp at less than 50% of that...

Otherwise, I absolutely love my 4Runner. I just hope I haven't screwed it up.

J
 
Coolhand said:
Well I sure hope I didn't but man that noice and vibration gave me the creeps. BTW - it would be the hill start assist, not the downhill assist, at least in my case. I was always trying to go uphill or hold on an uphill when my cases occured.

I was also wondering about how much weight the hill start assist can handle. Since the vehicle is rated at something like 7,000 lb, you'd sure think they wouldn't put a system on there that's going to fail on a boat ramp at less than 50% of that...

Otherwise, I absolutely love my 4Runner. I just hope I haven't screwed it up.

J
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Remember that hill start assist may not stop the truck from rolling back, especially with the boat and trailer behind. I've found that it slows the speed, but may not stop the truck depending on the grade.
 
jharris2 said:
Remember that hill start assist may not stop the truck from rolling back, especially with the boat and trailer behind. I've found that it slows the speed, but may not stop the truck depending on the grade.
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Exactly.
 
Hey guys, that's good to know (that the hill start assist may not completely stop the vehicle). But am I to believe that if it rolls slowly backwards you hear (and feel) gears grinding and warning tones chirping? That sounds like something failing, not operating as designed.
 
But am I to believe that if it rolls slowly backwards you hear (and feel) gears grinding and warning tones chirping?
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Yes. The "gear grinding" is the ABS pulsing, not the transmission. The warning tone is to let you know that the system is active and that you now have two choices: step on the gas and drive or step on the brake and stay put.

Remember, HAC is only active for about five seconds...

That sounds like something failing, not operating as designed.
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It's normal. The tone is warning you to take action (step on the gas or step on the brake)...

Think about warning systems in aircraft, like TCAS. It beeps and tells you "Terrain! Pull Up!" That's normal—you want the warning *before* you hit the mountain!

—Dan
 
I am not sure about "gear grinding" but chirping, yes. The chirping tells you that ABS is pulsing your brakes to keep you (or attempt to) from rolling backwards.
 
Alright I'm feeling better about this. When I heard the 'grinding' or ABS pulsing or whatever it is, I also felt a vibration almost directly under my feet. Would this also be ABS? Is there some Toyota technical resource that talks about the warning chirp, including when it does or doesn't chirp?

Thanks guys, time for bed I think....

Justin
 
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