Resident autotragic expert strikes again - Bulletin Board

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Resident autotragic expert strikes again
Posted by S Carlson [Email] (more from S Carlson) on Wed, 29 Nov 2017 21:27:08

In Reply to: Automatic - Throttle Pressure Cable Adjustment, tweetiedancer, Wed, 29 Nov 2017 18:11:56
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Having been a victim of autotragedy, I know well the throttle cable and its effects. Basically, the throttle cable affects the system pressure, which is basically a pressure signal that varies depending on two inputs: RPM and throttle angle. The higher the RPM, the higher the system pressure - linearly, and with a fixed ratio because the pump moves faster as the engine moves faster. The more open the throttle, the higher the system pressure, depending on the cable position. With higher system pressure comes more abrupt, but also firmer (less slippy) engagement (think about those hydraulic pistons with lots of pressure behind them). Thus firmness is good because your transmission won't be as likely to slip. But also with higher system pressure, the shift points will be higher, since the automatic shifts using shift valves, which compare governor pressure (varies with speed, independent of system pressure) to system pressure. There are other fudge factors going on, but this is the basic explanation. At full throttle a valve is also opened that causes the transmission to kick down a gear.

Higher RPM shifting could be caused by several things. If it's dependent on engine temp (more ok when warm), it's a stuck governor. You will have to clean the governor, backflush the transmission cooler where dust likes to collect, change the fluid, and then probably clean the governor again several times. Then when you're at your wits' end and covered head to toe in red type F (blood of the beast 666), dump in a bottle of Lucas and pray. Otherwise, it could also be general wear and tear. Worn bands and clutches will take longer to engage, and engage more abruptly possibly due to slight flares between gears (engine momentarily in neutral. BANGshift). You can adjust the second gear band, but you can't adjust third gear engagement because that's a clutch. The dust from worn bands and clutches can adversely affect all the valving, making the thing more sluggish to make decisions since these are mainly piston-style valves (piston slides in cylinder, opening or closing passageway ports in cylinder walls). Governor is most affected, but other valves that you can't clean out are also.

A shop service manual is worth its weight in gold, but you can also go to the link I have linked and see what is what in the PDF (more than the Bentley Bible can offer I assure you). I never trust the position of the crimpy thing. I adjust by the seat of my pants, generally based on the explanation on page 440-32. You can fill out the worksheet a few pages later if you're feeling ambitious. I err on the side of firm and high shifts (tighter throttle cable), figuring that even though this sacrifices a bit of fuel consumption, at least I will be firmly in one gear or another and won't give up any band or clutch material to the dreaded slip. I think originally the transmission was intended to shift lots and at low RPM (looser throttle cable), but the thing gets really slippy (a satisfied customer might say "smooth" but not realize how the thing is grinding itself up). Based on my experience, I think it's pretty hard to damage things with hard shifting.

posted by 75.69.61...

Autotragic manual

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