• Marshall

How to tow without a transmission lockup kit

We often get asked for advice on how to tow if you don't have a lockup kit installed. Should I use 4th or 5th gear? How do I know if the transmission is getting hot?

How do I get the best economy? How do I know if the torque converter is locked? What can I do to protect the transmission from over-heating damage? These are the most common questions.

1. Know your transmission oil temperature

Regardless of whether or not you have a lockup kit the first thing we advise is to monitor your transmission oil temperature.

Once the transmission starts to get hot you can be proactive to reduce the temperature and avoid expensive damage. Early intervention is the key, but you need to know when.

Once the oil gets to 100 degC you should start going easy on the vehicle. Unfortunately, vehicles don't have a gauge that displays the transmission temperature. Instead, vehicles have a transmission over-temp warning lamp. These are usually set to a very high (extreme) temperature after which the oil has already begun degrading badly. The oil starts degrading and laminating/depositing on the internal surfaces once above 125 degC, so this is the maximum temperature we recommend before stopping for a cool-down.

125 degC is the maximum temperature after which the oil starts to degrade

The warning lamp in the Pajero is 147degC, Challenger/Triton 125degC and Ford Ranger 140degC. If the warning lamp comes on you should definitely stop and let the transmission cool down, and you should later also have the oil checked for discolouration (burning). Note there is no relationship between engine coolant temperature and transmission oil temperature. Just because your engine temperature is ok it doesn't mean the transmission is.

How to display the transmission oil temperature

The most popular way to monitoring your transmission temperature is to install an OBD2 reader. Dedicated devices such as an UltraGaugeMX or ScanGaugeII are excellent. The cheapest option is an ELM327 bluetooth OBD2 dongle from eBay with an old Android phone running an app (like Torque Pro). Note: iPhones requires a WiFi dongle and an app like DashCommand.

OBD2 readers are also great for displaying and reseting ECU error codes.

Cheap eBay dongles can be hit and miss though. Be prepared to toss it if it doesn't work well!

But, not just any OBD2 reader will work. You must program the device to read the transmission temperature sensors, and not all OBD2 devices allow vehicle specific PIDs to be programmed. Also, different transmissions need different PID setups. The example photo is for a JATCO transmission.

For more information on setting up the UltraGaugeMX or ScanGaugeII see our article here.

2. Tow in 4th gear and try to keep the torque converter locked

The primary source of heat that causes a transmission to over-heat is a slipping torque converter under load. To prevent over-heating you should try to keep the torque converter locked whenever possible. Under load, a slipping torque converter can heat the oil by +20-30 degC. At this rate the transmission cooler cannot keep up, and the oil pan temperature slowly rises and rises.

The factory transmission ECU will normally lock the torque converter when in 4th gear and when above ~90 kph. So, when towing at highways speed it is best to use SPORT mode in 4th gear. Towing in 5th (or DRIVE) will leave the torque converter unlocked and cause slipping (and heat).

Despite the RPM in 5th being slightly lower than towing in 4th, you'll find in 5th gear the engine labours more, and coupled with the loss caused by the slipping torque converter, it actually uses more fuel.

You're better off towing in 4th and revving the engine (at ~2800) rather than labouring in 5th. It's better for the engine, transmission, and your pocket.

Highway driving in 4th is the only situation when the factory ECU will keep the torque converter locked and keep the transmission cooler. As soon as you get off the motorway and fall below 90 kph or are in 3rd gear or lower, it will unlock. Then, the only way to manage transmission temperature is to monitor it and go easy on the vehicle or stop if it's getting too hot. This is why lockup kits are so popular, as they keep the transmission cool even when off the highway. And, to save fuel, they'll allow you to tow in 5th gear when the conditions are favourable.

But without one, it's better to chose a lower gear and rev more rather than labour in a higher gear, as there will be less torque converter slippage and thus it will generate less heat.

auto-mate from MM4X4 is the most sophisticated transmission lockup kit available

auto-mate ensures maximum use of the torque converter lockup clutch in a wider variety of driving situations (including low range 4WD) to ensure the transmission stays cool. It's like a lockup-kit and transmission remap all in one unit which is optimised for towing, and can be adjusted to suit your individual vehicle. With in-built protections such as SafeLock, it ensures maximum longevity and reliability of the clutch. Learn more about auto-mate here.

4. Replace your oil every 40,000km if you work your transmission hard

You hear transmission specialists say this, and they're right. Don't believe the 'sealed for life' statements. Under ideal driving conditions the AISIN transmission service interval is specified at 190,000km, and under severe conditions 95,000km. The image below shows a comparison of new oil and the oil from our Pajero NX after just 50,000km. In the first 50k of life the vehicle spent 35,000km either towing a 2.4t van or in severe off-road conditions (towing across mountain ranges, beach work, desert crossings, Victoria high country steep climbs etc).

This oil sample above was taken (before a lockup kit) after we were experiencing issues with our transmission, and this then ultimately led us to the develop our lockup kits. Necessity is the mother of invention, as they say.

Even though we monitored the transmission oil temperature using the UltraGauge, and didn't let it go above 130degC, we still burnt the oil, and damaged the transmission. The lesson learnt was we shouldn't have let it get so hot before stopping to cool down (but this can be a hard call when you're in a convoy!)

There were too many real-world driving situations when it's next to impossible to effectively manage the oil temperature without a lockup kit, such as steep climbs, beach driving, winding mountain roads, strong head-winds etc. Even without towing, the long, steep, low range climbs of the Victorian High Country over-heated the oil.

When you have a lockup kit, the oil temperature is typically 60-70 degC, and rarely goes above 80 degC. This protects the transmission, extends the oil life Oil changes become more of a monitor and replace if needed activity.

5. How to know if the torque converter is locked

There are two ways to know if your torque converter is locked or not. Firstly, if your vehicle has an AISIN transmission (eg. PajeroNT+, Triton MQ, Prado150, D-MAX,LC200) you can use an OBD2 reader to monitor the two temperature sensors (pan temperature and torque converter output temperature, shown on the photos as Pan and Atf respectively).

If they are the same value (+/- 1 degC), the torque converter is LOCKED. It takes about 20 seconds after lockup activates for the temperatures to stabilise at the same value.

Note the JATCO transmission (Challenger, Triton MN, Pajero NS) has just one sensor so this method can't be used. These just measure the Pan temperature.

The other way is to gently push the accelerator pedal. If the RPM jumps up a bit, the torque converter is UNLOCKED. If the RPM doesn't jump (or stays about the same) as car slowly accelerates the TC is LOCKED. The TC will not be locked in 3rd gear or below. Over time, you'll also learn the RPM when the TC is locked at a certain speed and gear. eg in 5th at 100kph, 2000 RPM means it's locked, 2300 RPM is unlocked.

Happy travels, and ensure you learn how to look after your expensive transmission.


Marshall and the MM4X4 team

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Tea Tree Gully, Adelaide, South Australia

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