Gu patrol auto 4500

Submitted: Saturday, Feb 23, 2019 at 12:48
ThreadID: 137855 Views:997 Replies:4 FollowUps:5
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Do we use od on or off all the time when towing
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Reply By: Mudripper - Saturday, Feb 23, 2019 at 13:09

Saturday, Feb 23, 2019 at 13:09
Without knowing what you're intending to tow (weight wise) I'd suggest that the general consensus when heavy towing is to switch Over Drive off.
AnswerID: 624050

Follow Up By: Member - Keith P (NSW) - Saturday, Feb 23, 2019 at 13:54

Saturday, Feb 23, 2019 at 13:54
Yep....what he said

Cheers Keith
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Reply By: Ron N - Saturday, Feb 23, 2019 at 14:48

Saturday, Feb 23, 2019 at 14:48
There are several things to be aware of when towing in overdrive.

In virtually all of todays automatics, there is a lock-up clutch arrangement in the torque converter that effectively bypasses the normal fluid drive mechanism of the torque converter, and which effectively turns the drivetrain into a manual-style, direct-mechanical-drive. This is the "overdrive" in todays automatics.

Thus, there is no power loss or slippage when in this state, as the torque converter is not in operation.

If your lock-up clutch stayed engaged full time, your engine would stall when you braked to a stop.

However, most torque converters don't lock up until a road speed of about 80kmh or 90kmh is reached.
Once a torque converter locks up at this speed, the torque converter slippage is eliminated, the engine speed is reduced - and accordingly, transmission heat build up from torque converter slippage is also reduced.

But against that scenario - because the engine is turning at reduced RPM in torque convertor lockup mode (in comparison to an equal road speed with it unlocked) - the engine is effectively "lugging" - i.e. running at a speed which makes RPM gain slow and difficult, if the accelerator is pushed down.

Towing a heavy load, when the engine is lugging, sharply increases exhaust gas temperature in the engine. This leads to burnt valves.

Under light loads, such as when not towing, low engine RPM in overdrive at highway speeds is quite acceptable, because the engine is capable of accelerating at an acceptable rate when the accelerator is floored.

However, if towing a heavy load, the engine needs to run at an engine speed whereby it's not "lugging", and it can increase its RPM relatively easily when the accelerator is pushed down.

As a result, it's important to run the engine under the heavy load of towing, at an engine speed whereby it's not "lugging", and the engine is capable of accelerating at a reasonable rate, when the accelerator is pushed down.

As a general rule, you can use overdrive when towing, if the road is flat, the load being towed is a moderate weight, and there's no headwind.

However, if the road is hilly or quite undulating, and the load being towed is heavy - or if there's a strong headwind - you will find it's much easier on the engine, to switch off the overdrive.

Cheers, Ron.
AnswerID: 624054

Follow Up By: RMD - Saturday, Feb 23, 2019 at 15:12

Saturday, Feb 23, 2019 at 15:12
If overdrive, ie, converter lockup option is used/selected, the auto will use lockup until the load, which is caused by terrain or accelerator position, will simply "unlock" it and then operate in the torque converter slip of the gear it is in. It isn't going to hold that for very long because the ECU programme is set to, not have it hang in lockup under load.

That feature ensures the engine does not/is not lugging. The auto selects ;ower gears as appropriate.
If the load is suitable for a lower gear ratio of the auto then that gear may allow lockup to occur in that LOWER gear and if load increases it will change down and possibly select Lockup in that gear. Most autos of any repute use a lockup on most higher gear ratios.
Depending on the van/trailer load it may be best to manually select the next lower gear and allow it to lockup in that gear load position instead of simply switching off the lockup feature and causing the auto to run in slip mode of the highest gear ratio.
An exhaust temp probe and the instantaneous fuel usage should/can be used to determine the most economical gear selection for the conditions.ie, not too hot and lowest rate of drinking.

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Follow Up By: Athol W1 - Sunday, Feb 24, 2019 at 09:52

Sunday, Feb 24, 2019 at 09:52
Ron
Whilst you have mentioned a few very good and valid points in your reply there are also a few very obvious and glaring inaccuracies which are as follows IN UPPER CASE.

I have copied and pasted your reply.

In virtually all of todays automatics, there is a lock-up clutch arrangement in the torque converter that effectively bypasses the normal fluid drive mechanism of the torque converter, and which effectively turns the drivetrain into a manual-style, direct-mechanical-drive. This is the "overdrive" in todays automatics. COMMENTS RE THE TORQUE CONVERTOR ARE CORRECT, HOWEVER THE TORQUE CONVERTOR HAS ABSOLUTLEY NOTHING TO DO WITH THE QUESTION OF OVERDRIVE, AS THE OVERDRIVE IS A STEP UP GEAR RATION WITHIN THE GEAR BOX.

Thus, there is no power loss or slippage when in this state, as the torque converter is not in operation. CORRECT

If your lock-up clutch stayed engaged full time, your engine would stall when you braked to a stop. CORRECT

However, most torque converters don't lock up until a road speed of about 80kmh or 90kmh is reached. BASICALLY CORRECT, BUT ROAD SPEED CAN BE AS LOW AS 60KPH IN SOME VEHICLES.

Once a torque converter locks up at this speed, the torque converter slippage is eliminated, the engine speed is reduced - and accordingly, transmission heat build up from torque converter slippage is also reduced. CORRECT

But against that scenario - because the engine is turning at reduced RPM in torque convertor lockup mode (in comparison to an equal road speed with it unlocked) - the engine is effectively "lugging" - i.e. running at a speed which makes RPM gain slow and difficult, if the accelerator is pushed down. THE PURPOSE OF THE PCM (POWER TRAIN CONTROL MODULE) IS TO PREVENT THIS HAPPENING, AND THAT SYSTEM IS NOT AFFECTED BY THE SELECTION OF OD ON OR OFF.

Towing a heavy load, when the engine is lugging, sharply increases exhaust gas temperature in the engine. This leads to burnt valves. MORE IMPORTANTLY THE EXCESSIVE LUGGING OF AN ENGINE LEADS TO CRACKED PISTON SKIRTS.

Under light loads, such as when not towing, low engine RPM in overdrive at highway speeds is quite acceptable, because the engine is capable of accelerating at an acceptable rate when the accelerator is floored. THE FIRST PART IS CORRECT, HOWEVER THE PCM WILL DECIDE WHICH GEAR IS SELECTED FOR THE PREVAILING CONDITION OF ACCCELERATION.

However, if towing a heavy load, the engine needs to run at an engine speed whereby it's not "lugging", and it can increase its RPM relatively easily when the accelerator is pushed down. AGAIN CONTROLLED BY THE PCM

As a result, it's important to run the engine under the heavy load of towing, at an engine speed whereby it's not "lugging", and the engine is capable of accelerating at a reasonable rate, when the accelerator is pushed down. 1st PART CORRECT, HOWEVER THE PCM WILL SELECT THE APPROPRIATE GEAR FOR THE ACCELERATOR POSITION.

As a general rule, you can use overdrive when towing, if the road is flat, the load being towed is a moderate weight, and there's no headwind.

However, if the road is hilly or quite undulating, and the load being towed is heavy - or if there's a strong headwind - you will find it's much easier on the engine, to switch off the overdrive. AGREED.

MY COMMENTS
There are 2 things to consider when deciding to, or not to, use OD in any vehicle. The first is if there is a known weakness in the OD gear set, as any OD gear set by their very nature are the less strong (mechanically) of any of the gear sets available for use, and as such there are several manufacturers who recommend that for heavy load use you do NOT use OD.

The second consideration is as to the conditions being right to allow the torque convertor clutch to 'lock up' when using OD, or any other gear, and if not then it is beneficial to use the next lower gear ration to allow the convertor to "lock up" to reduce the massive heat build up that happens when the torque convertor is in 'slip' mode.

You may have noticed that I have not referred to a Direct Drive gear ratio, as not all vehicles have such a gear ratio (Ford Ranger & Mazda BT50 come to mind and they have both 5th and 6th gears are overdrive ratios, all other gears in these vehicles are underdrive ratios).

Hope this helps.
Athol (retired motor mechanic)
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FollowupID: 897458

Follow Up By: Ron N - Sunday, Feb 24, 2019 at 12:27

Sunday, Feb 24, 2019 at 12:27
All good points, RMD and AtholW1, I have no problem with being corrected.
Unfortunately, modern electronics, as related to transmission control, have only increased the potential for problems, not reduced them.

The transmission control module is engineered to take input from many sensors - but it can be fooled, because there are areas such as EGT which is not normally measured by any transmission sensor, so the EGT temperature is not part of the transmission gear selection electronic process.

So, on a long steady pull on a long grade in undulating terrain (as in a lot of rural and remote Australia), EGT can sneak up steadily , whilst all the electronic sensors are still telling the transmission to hold the particular (high) gear it is in.

The very fact that we see and hear about regular transmission failures and engine damage, even with late model vehicles, shows that the engineers and manufacturers can't foresee every situation when designing electronics.

Cheers, Ron.
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Follow Up By: RMD - Sunday, Feb 24, 2019 at 13:50

Sunday, Feb 24, 2019 at 13:50
To drive the rig along a road takes energy, if the torque converter is stopped from slipping by using a lockup clutch in other than it's overdrive gear, then that torque loading is being taken through a little clutch in the TC which is now receiving more torque than it is designed to do. It's life will be shorter.
If the auto is running in slip mode, then yes, there is more heat developed in the auto fluid an dhas to be cooled by the radiator water/airflow.
If in lockup, less heat from the auto, BUT, that heat being derived from the fuel being burnt is now at a raised level in the engine cooling system. So either way, trying to run in TC lockup or unlocked, both cause more heat energy needing to be dissipated by the engine coolant.
Each time a lockup is used or unlocked under load, the thermostat opening increases to cater for the additional head being developed in eother the auto or the engine. No free lunch here. When the thermostat reaches max opening it is because the cooling rate of the radiator is at a max, mostly because there is not much difference in the cooled recirulated water/coolant temp from exit to return to engine. Either way you are now at the sudden overheat situation.
Instead of taking it easy, ie a lower gear to ease load, many drive up to near the max point without realizing.
If you had a thermosat opening position sensor would alarm many people. That is why some newer vehicles have trouble as Ron mentioned above.

The temp of fluid exiting the engine and the temp of fluid entering the engine cooling system is a great thing to monitor. The closer they are to each other the less reserve of cooling engine AND auto. When one of those overheats/fails or stops, so does the whole rig. No one does monitor this concept though.
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FollowupID: 897467

Reply By: nickb - Saturday, Feb 23, 2019 at 21:20

Saturday, Feb 23, 2019 at 21:20
I always turned OD off when towing in my old patrol.
AnswerID: 624062

Reply By: Gbc.. - Sunday, Feb 24, 2019 at 10:53

Sunday, Feb 24, 2019 at 10:53
It’s an old school 3 speed box with an overdrive. You should never use the overdrive while towing anything of substance - your user manual will also point this out. The problem with that particular box is that it only has a torque converter lockup on the overdriven gear, so serious towing is rewarded by and aftermarket electronic lockup kit for third gear which is common enough. On that model it will pay for itself in fuel savings alone while towing.
AnswerID: 624066

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