Yes, they were doing a 'proper' diagnostic on the engine and tranny mounts to see if they were affecting the alignment.
You found a 'good' shop there in my opinion!
Lots of shops will just put it on the machine and line it up. They don't check for a toasted mount that 'will' affect the alignment or the CV or u-joint life that will cause you vibrations down the road.
From my 30+ years in automotive repairs, I think you have found a 'winner' for a good shop.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Can someone tell me if the incident below is normal procedure for identifying a bad motor mount? Or were these mechanics purposely damaging the motor mounts in my van? I think they were but I'm not an auto mechanic.
My 1991 minivan needed a front end alignment so I took it to a nationally known auto repair center for new tires, balancing and front end alignment. After a couple of hours in the waiting room, I looked out into the garage and saw two mechanics with my van in the middle of the garage with the hood up. One man was in the driver's seat and the other was looking under the hood.
They were gunning the engine over and over. What they were doing did not appear to be related to the alignment so I walked into the garage right up to them and asked the man under the hood what they were doing. The man only said that I had a cracked motor mount. The man in the car did not see me and continued to gun the engine.
I could see that what he was actually doing was keeping the brake on and taking steps shifting the automatic transmission through each gear and flooring the the gas pedal. The car was not in park! He was shifting into drive and reverse with the brake on!
I watched this action cause the the engine to lurch violently inside the car each time he floored the pedal.
The man inside my van did not initially see me and when he did, he immediatly stopped what he was doing. The man I was speaking to yelled at him to continue so he he continued shifting the gears and gunning the engine with the brake on. This continued for maybe one more minute. The mechanic went on and explained that a bad mount could affect the cv joints and eventually cause the van to pull to the right. I went back inside and waited another hour for the work to get done. I estimate the engine gunning incident lasted a total of 5 minutes.
My main issue is that I would never attempt that with any car. Logically I would think that at the least it could damage the transmission.
Has anybody heard of mechanics doing this type of thing? What is your opinion about this?
Yep, sounds to me like they thought they might have spotted a cracked/busted/weak/otherwise defective engine mount, and were trying to verify it. Without seeing it myself, I wouldn't want to say it was a definite diagnosis, but I'd have done similar in an attempt to check my initial thought. Based on what you're describing, (the violent lurch) it even sounds like they were in the process of verifying a correct diagnosis. I'm hoping that when you say he was flooring the gas, that he wa doing it *AFTER* he was in gear... Floor it, THEN go into gear is the only way I can see him "hurting" anything. In gear, then floor it is pretty much "business as usual" for the system. (Think about what happens at a stoplight, and how often)
What they were doing was looking for engine motion that would indicate a mount wasn't doing its job properly. And if your description is accurate, it sounds to me like they were seeing it. Such motion would show up quite well under these conditions if the mount was broken/bad/dying, since the torque of the engine is always trying to "spin" the whole unit. A textbook example of Newton's third law of motion "in the flesh" - for every action, there is an equal and opposite reaction - Anytime the engine is running, the engine block is trying to spin in the direction opposite to that of the crankshaft's rotation, with a force equal to that exerted by the crank, with actual motion achieved being modified by the difference in mass between the spinning crank and the block, any factoring for leverage due to offsets from the rotational center of the crank, and how the whole thing is held together and tied down into the car.
Winding it up some, then shifting between drive and reverse while braking (unless done at ridiculously high RPMs) isn't going to do any significant damage to an automatic tranny in this short a time/few repeats. Heck, except for the "stand on the brake" part, that's how one "rocks" a stuck car loose. (If you're in snow country, I *KNOW* you've at least seen it being done... :) ) It just gives the engine something more to push against, making the motion from a bad mount more apparent. Changing gears while the engine is PARTIALLY wound up makes any motion there is to be seen even more obvious than it would be it one were to shift into gear at low revs, then wind it up.
Mine's detailed above. Summarized: They were using a legit method to diagnose a suspected problem. Depending on exactly how high the guy in the seat was revving it, and whether he was shifting BEFORE, or AFTER punching the pedal down, they *MIGHT* have been pushing the limit of reasonable a bit, but not knowing what the tach reading was when he ran it through the gears leaves me (and you) needing to take a guess and hope we get it right. What their exact findings were is a topic you'd be best off discussing with them, since they were the ones "live at the scene" to see it for themselves.
Revving the engine aginst the brake in drive and reverse ONCE OR TWICE is a normal way to check for a bad motor mount. Doing it over and over for several minutes is questionable, at the very least.
I wouldn't worry about it. Unless there was already something wrong with your tranny before they did that, I doubt they'd damage it. The amount of time they did it according to your story seems a little much, but unless you see the tires turning, they're not doing anything wrong. You say they were 'flooring' the engine, but there's a big-big difference between accellerating an engine and flooring an engine.
I'd worry if they were doing a 'brake stand' and telling you they were looking for broken stuff. For those who don't know, a brake stand is when you floor the engine while holding the brake. The driver then slowly lets off pressure on the brake pedal. The engine torque overcomes the brakes and the tires break loose on the drive axle while the other axle remains locked by the brakes. The vehicle barely moves at all but the tires on the drive axle are turning at around 80 mph+ and results in a lot of smoke and noise. Pouring bleach on the tires before doing so results in a LOT more smoke, but I wouldn't know anything about that personally - nope - not me. =8D
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