Thread: Correct auto towing procedure

It will be locked at 100 kph

You will see/feel it lock at around 80 kph

If towing a decent load, you may find it is lugging a bit at those revs, dropping back to 3rd will get the motor spinning much happier

I'm a bit of an auto novice here, but my TD5 cruises quite comfortably at
about 2250rpm at 100km/h. I assumed it has locked at this speed

Yes when you are cruising at over 80Kmh the trans is locked and it locks in 3rd or 4th.
But look at the tacho when you are doing say 70Kmh up a steep hill it will sit on up to 2800rpm, just before the transmission changes down or if you are gentle at say 60 Kmh it will run up to 2800RPM in 4th. That is the Torque converter stall speed ie the torque converter is varying the ratio by "slipping" and it is this slipping which generates heat.
Regards Philip A

Just as an interesting side point to heat creation, was very interested to see that my car ran hotter towing in 3rd lockup than it did in 4th lockup at the same speed approx 100kmh,

I'm guessing this would have something to do with the speed the air is going through the small intercooler ,ie not enough time to cool down,as both inlet air temp and engine temp went up, maybe one causing the other.

If you are towing anything heavy it is also a good idea when moving from stationary to have the auto manually locked in first. This will lock the rear band and take the strain off the one way sprag, which is a known failure point.

Do you mean simply moving the gear selector to first to do this or some other means?

Move gear selector to first. When ready to change to second throw it into drive and then drive it in auto.

If the sprag fails the vehicle will not move unless it is manually locked in first. Theoretically you could manually drive it forever, provided you gave it a very thorough oil change as the failed sprag produces fine metal filling that can jam valves.

I have repaired the odd three speed auto in XF Falcons that have had sprags fail.

A sprag clutch is used in some auto transmissions as a method of allowing the transmission to smoothly change gears under load. Various models of GM gearboxs have used this system.
A sprag clutch is used in most older automatic transmissions and some newer ones for shifts which require the synchronized engagement of one clutch with the disengagement of another. Using an overrunning clutch instead of a hydraulically actuated one makes this synchronization automatic. The sprag clutch simply "lets go" once the reaction force it provides is no longer needed.
In a Simpson gearset, common to many automatics, this happens on the 1-2 shift, and reverses on the 2-1 downshift. The 2-3 shift is done by engaging a single clutch, and nothing is released. Some newer electronically controlled transmissions make "clutch to clutch" shifts, without any sprags.
This can also be used exclusively in first gear (transmission shift lever in D, but auto valve body or management selecting 1st) on some autos. That way it will automatically provide forward drive, but will not provide any engine braking. This is done not so much to avoid engine braking per se, but rather to allow a low throttle opening 2-1 downshift as a car decelerates, to avoid a loud (and potentially abrupt) and unnerving jolt as a result of the downshift. On transmissions so equipped, manual selection of 1st gear typically engages an additional band that grips the same section as the one way clutch would engage, and thus allows for engine braking

By locking the selector in first, this band takes the strain off the sprag

Another way to tell whether the trans is locked up is if you take your foot off the throttle the revs will stay up in relation to the speed when locked up.

cheers james

Originally Posted by shack

Another way to tell whether the trans is locked up is if you take your foot off the throttle the revs will stay up in relation to the speed when locked up.

cheers james

This is the method I use. If not locked revs will drop substantially and instantly if you back off the throttle.