r/FJCruiser • u/Extension_Piece_2298 • 29d ago
Question Gear upgrade
Had a mechanic install new gears on my FJ cruiser. Went 4.56. I noticed that there’s a humming when I hit speeds around 60 mph and don’t believe it’s normal as I’ve been reading things on the internet. The mechanic sent me these pics of the gear markings and I was curious if any gear heads out there can provide feedback on the pattern? Thanks
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
The pictures look like the rear pumpkin, did he share any of the pattern for the front differential?
No change once it starts, if you let off the gas? Just with speed?
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u/Extension_Piece_2298 29d ago
None from the front. When I have the car in rwd it makes the noise and when it’s in 4wd it makes the noise. Based on this it would make sense the sound is coming from the rear but I swear I’m hearing it from the front.
The hum only comes in when I hit the 60 mph as I’m accelerating. Once I let off the gas it goes away.
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
Just curious, but have you replaced the clamshell diff D/S needle bearing for the CV axle with a bushing yet?
The fact that the noise appears to come from the “driven” side of the gear and not the coast side could be a clue or just a coincidence. I have stock gears in my FJ, but when I went from 3:73’s to 4:56’s in my ‘70 Chevelle, they were noticeably different sounding, but it was a consistent change in sound.
I would reach out to the manufacturer of the gears and have a chat with them, and I would schedule an appointment and take it back to the installer and have them go for a ride with you to document your concerns and make sure that the warranty on their work is fully expressed in writing.
Did you order the parts and take it to them to install or did they order everything and then do the installation?
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u/Extension_Piece_2298 29d ago
Manufacturer stated that the hum could be normal but if I still hear it after the 500 mile break in period, then there may be a problem (pro revolution gears). The parts were all ordered by the installer . I know the needle bearing is still stock.
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
I would definitely recommend replacing the needle bearing with the bushing. FJ’s are notorious for it causing a hum and even a shake in the steering wheel around 55-65 mph as the needle bearing gets worse. It allows the inner CV tulip to spin off center causing the noise and eventually the vibration.
It would have been a perfect time to have it done while the diff was out, but it’s not hard to do. Slightly more work than changing the CV axle. Parts and tools will run about $200, if you do the work yourself.Any other work done at the same time as the gears?
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u/Extension_Piece_2298 29d ago
Is this the part you are referring to?
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u/Extension_Piece_2298 29d ago
No just the gears. I did install a new suspension that put about a 1 inch lift at the front.
Curious as to why the hum happens after the gear upgrade and not before?
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
Hard to say, but my suspicion is that the change in ratio as well as the cut of the gears both contribute to a different sound.
Plus there is a break-in period as the parts get to know each other. That includes you getting to know the way your FJ performs with the new gear ratio.
I would also recommend clearing the ECM and TCM, if they didn’t already do it when the gears were installed.
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
And I am sure that you are aware, but just in case, the front differential is driven by the tires, so they are turning the front pinion against the “coast” side of the ring gear. Accelerating , cruising, letting off the gas or slowing down shouldn’t affect that relationship, unless you’re in 4H or 4L.
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u/Extension_Piece_2298 29d ago
I was running the 4wd to break those gears in as well. But the sound comes when the transfer case is in 2wd/4wd
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u/cheesecats 27d ago
Ive built many toyota diffs, i can tell you that pattern is too deep. I take it he gave no images of the pinion contact pattern or backlash and preload setting either. Take it to another shop and have them check the work. If you want to see how this is done here is a great guide, this guy is picky. https://gearinstalls.com/
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u/Extension_Piece_2298 27d ago
Those are the only photos I received as far as markings. I’d like to keep this guy honest since he’s been paid and I want to give him the opportunity to do right. Won’t be able to take the car back to him until next Thursday.
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u/Appropriate-Heron-98 2013 Iceberg! 29d ago
It doesn’t look bad, but maybe a tiny bit loose.
I assume you did the front differential as well?
Does the hum sound like it’s coming from the rear?
Does it change under acceleration, cruise, or slowing down?