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Old 05-24-2005, 10:06 PM   #1
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Default Engine compression versus braking downhill

We took my brother-in-law with us camping this weekend. I down shifted out of overdrive into 3rd in my automatic when descending any grade more than a mile long. My brother-in-law, who, in fact, knows a lot more about cars than I do, suggested that I should not do that. He said it was hard on the valves and it is cheaper to replace brakes than valves.

What, if any, damage or premature wear can be caused by using engine compression instead of the brakes?

Perhaps I should only use compression on either very steep descents (where 2nd gear would be a better choice than 3rd) or for very long descents (more than 2 miles long).
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Old 05-25-2005, 06:14 AM   #2
BobRederick
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I had a mechanic tell me it was hard on the transmission to down shift due to the "pressure buildup". Engine valve damage is a new one to me.

I would like to know also. We have group members who have transmission temperature guages and I would like to know what happens to oil temperature when going down a long grade downshifted.

Several posts have noted that 4th gear (normally overdrive) is a weak gear and won't take the long-term abuse of towing. My TV owners manual says to not tow in 4th but to go directly to 3rd which is what I do with my 3326 king.

All of my life I have used engine breaking. I find it much safer than wheel brakes as it can't fail or fade due to heat and doesn't wear out.

Bob
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Old 05-25-2005, 07:54 AM   #3
Cateye
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Both of my Grand Cherokees ('99 and '05) recommend shifting down when descending hills. In fact, my '05 has a "towing" button which commands the transmission to shift down under these conditions and to not shift into overdrive. It also has a manual shift mode to simplify shifting down to assist braking.

I would recommend that you follow the recommedations in your owners manual and not your brother-in-law.
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Old 05-25-2005, 11:50 AM   #4
Bill
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The responses you have gotten so far are good ones.

If you follow any automotive forums, or listen to Car Talk, almost every week you will hear some idiot say proudly that he downshifts instead of using the brakes whenever he comes to a stop light, for example. He is proud of himself for saving the brakes, but the expert response is always just what your brother-in-law said - "brakes are cheaper than transmissions". In other words, don't use engine braking for routine stops.

But your brother-in-law is wrong when he applies this stock answer to a very different situation - keeping your speed down on a steep downgrade with 2 tons of metal pushing on your back bumper. If you use the brakes for this, it is very possible that your brakes will overheat and fade - meaning they won't brake any more. And that means you crash. And guess what? Transmissions are cheaper than crashed vehicles and lost lives.

The secret is to downshift right from the beginning. Don't allow your speed to build up, and then drop your transmission down a gear. The shock of that downshift is indeed very hard on the tranny. Instead, just before you start down, let your speed drop, ease off on the gas, and downshift while your speed is still moderate. There is much less shock and your tranny will thank you for it. If your speed builds up a little while you are on the downgrade, use your brakes a little to bring it back down. If you find yourself having to use your brakes continuously, you have no choice but to downshift again, and sure enough, that will be hard on the tranny. But what you CANNOT do is ride your brakes - that is the invitation to disaster.

Ray had a real good post on this a while back - I'll see if I can find it.

Bill
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Old 05-25-2005, 03:42 PM   #5
RockyMtnRay
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Never heard of valve damage due to use of engine braking...but suppose it would be possible if you let the engine grossly exceed the redline RPM. And I've never heard of any damage due to "pressure buildup" either...modern electronically controlled automatic transmissions manage their internal hydraulic pressures quite well. There's a whole lot of just plain bad information floating around that seems to get passed down through the years.

As Bill explains above, using and replacing wheel brakes is less expensive than replacing engine/transmission components...if...mighty big if...you can avoid brake overheating (and possible loss of brakes due to fade) when only using the wheel brakes to control speed while descending a long, steep mountain grade with a trailer in tow.

The one thing that engine braking does do...and that's hardly ever mentioned...is put a lot of loading on the thrust washers & spacers that keep all the helically cut gearsets aligned when torque is coming from the driveshaft instead of the engine. When you put torque on helical gears (which are extensively used in automatic transmission planetary gearsets), they tend to move laterally and thrust washers prevent that. Automatic transmissions have heavier duty thrust washers for front->rear torque passage (power on) than for rear->front torque passage (power off engine braking). Therefore, to avoid undue wear on these lighter duty thrust washers, it's a good idea to limit use of engine braking to those circumstances where overheating of the wheel brakes is very likely if engine braking is not used.

On nearly every trip when towing, I have to descend multiple 6 to 8 percent mountain grades of anywhere up to 10 miles in length. I therefore always use engine braking (3rd or even 2nd gear) as the primary control of my descent speed on those grades. I sporadically use the wheel brakes as needed to supplement the engine braking for speed control and for emergency braking as needed. Because the wheel brakes are not being steadily used, they remain cool and have their full effectiveness at all times. With nearly 7,000 miles of mountain towing on my truck I've yet to see any indication that using engine braking is causing any problems.

I've watched folks towing trailers (including professional truck drivers) try to descend Colorado's mountain grades using only the wheel brakes (almost invariably the rig has license plates from a flat/lowland state). It's downright scary to watch the smoke billowing from the wheels, especially considering that brakes that are overheated that badly are on the verge of complete failure. Nearly all the long descents in Colorado have "runaway truck ramps" and these get used on average 3 times a week!

As for any deleterious effect of using engine braking on automatic transmission fluid temperature, there isn't one that I've seen. Nearly all (if not all) automatic transmissions are cooled wholly (or at least in part) by a cooler in the bottom of the radiator...even if the vehicle has an external air/oil cooler. On a long descent with the throttle closed, the coolant in the bottom of the radiator drops quite low (to maybe only 20 degrees over ambient) due to abundant airflow in combination with little energy being produced. The net result is I typically see my transmission temperatures steadily drop to nearly as low as non-towing temperatures during a long descent using engine braking...even if at the top of the descent the transmission temperature was well over normal because of heating during the climb on the other side. Granted, with the torque converter unlocked while using engine braking in 2nd gear, there has to be some heat being generated in the transmission due to fluid slippage. However, my empirical evidence is that whatever heat is being generated, it's being easily handled by the transmission's cooling device(s).

Every make (and sometimes even model) of vehicle has different transmission designs and usage guidance in the owner's manual. Therefore the owner's manual should be guide. My owner's manual states to use 3rd gear (OD off) for better "engine braking" while towing. And that's exactly what I'm doing...but only for speed control on long, steep descents. For routine level ground deceleration, I use the wheel brakes because they're designed for that and a routine stop shouldn't overheat the brakes.
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The Trailer: 2002 TM Model 2720SL ( Mods: Solar Panels (170 Watts), Dual T-105 Batteries, Electric Tongue Jack, Side AC, Programmable Thermostat, Doran TP Monitor System)

The Tow Vehicle: 2003 Toyota Tundra V8 SR5 4X4 w/Tow Package (Towing & Performance Mods: JBA Headers, Gibson Muffler, 4.30 gears, Michelin LTX M/S Tires, Prodigy Brake Controller, Transmission Temperature Gauge)


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Old 05-29-2005, 09:25 PM   #6
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Default What my mechanic says abotu down shifting

Had my TV in the shop for an oil change. Been using the same shop and mechanic since 1986. I asked him about downshifting and valve problems.

He said years ago that valve problems would not have been uncommon, but not with newer (since around 1990) vehicles. The trans is more of an issue.

His recommendation is to get out of overdrive, unless you have the tow/haul switch like I have. Control speed on long downhill grades by braking lightly and infrequently useing the trailer brakes. They are the cheapest to replace. Next cheapest is the TV brakes. After that is the trans.

So, what I think I will do is just use the brakes. I'm too lazy to reach over to tap the trailer brakes. I have 3/4-ton brakes on my 1500HD and don't expect much of a problem with heat buildup.

Now, if it was a really long really steep grade I might shift down. Priest grade (the old original) out of Yosemite comes to mind, but only a fool would go straight down that hill towing a trailer instead of taking the newer bypass.
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Old 05-30-2005, 05:54 PM   #7
BobRederick
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Thanks for the input guys. Its appreciated.
Bob
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