Why the heck would you care what your Automatic Transmission Fluid temperature was in your DB9?   Most  DB9’s, DBS and Rapides between 2004 and 2014 were fitted with a 6-speed  ZF HP626 automatic transmission.  I’ve written an extensive article about the transmission here, and it also concludes that the transmission fluid should be changed at least every 8 years.   If you are going to change you transmission fluid (check out my video on how to do that here), or, merely want to check the level is correct (check this article), you need to do this when the fluid is between 30°C and 50°C with the engine running and idling.    Too cold or too hot, you’ll have an incorrect level.   Let me show you how you can check it properly.

The big challenge is that the fluid is inside the transmission and actively circulating because the engine is idling.  How can you measure it?   We have two options really.

Infrared Thermometer

The less precise way to measure the fluid temperature will be to use a non-contact infrared thermometer.   Essentially this is very cool device that will read the surface temperature of most anything you point it at.  It even has a cool red laser dot to see where its pointed.  You can get one like I did for about $23 USD from Amazon.com.

The idea with this is that you open up the fill plug and rely on a small stream of fluid to come dribbling out of the transmission, and measure the temperature of the fluid stream with this thermometer.  The problem with this approach is that there needs to be a big enough target (stream) for it to get a reading from.  If your fluid level is just perfect, there should be no more that a drip/drizzle, so there won’t be a sufficient stream to measure.

Alternatively, you can point it at the black plastic oil pan that makes up the sump of the transmission.  It will be filled with the fluid, but there will be a large delay between the actual fluid temperature inside it and the exterior surface temperature of the sump.  The actual fluid could be many degrees hotter than the sump, so this would lead to a less than accurate result.

You might make do with the Infrared thermometer (better than merely guessing), but the next approach is the best way to do it.

Using an OBDII Scan Tool

The transmission has a temperature sensor built right into it.  All you need is the right tool to talk to the transmission and ask it what the current temperature is.   I’ve written extensively about how an OBDII scan tool is an essential part of your Aston toolbox (check out the collection of articles here), and hopefully you have one already.   Not just any OBDII tool will work.   We need one that can talk to the Automatic Transmission Control module.   Most OBDII Scan tools can manage to talk to the engine control modules via the “OBDII” port, but a rare few are smart enough to talk to the other 11 control modules through the “Body” port.

One that can is the Foxwell NT510 (or later models NT520 and NT530).  The magic of the Foxwell is that it can speak the actual native Aston Martin Codes.   I’ve written about the Foxwell scanners here.   Fortunately the Foxwell tool is also very affordably priced.   There may certainly be other code readers that can do this, and as long as they can talk to the Transmission control module via the Body port, you are all set.

Reading the Transmission Fluid Temperature using a Foxwell Scan Tool

Let me show you how to use the tool to read the transmission fluid temperature.  In the article below I will be using my Foxwell NT510, but the process will be similar in the other models.

The process will only take 2 minutes to get it going.

• Connect your Foxwell code reader to the BODY port. In my car (LHD) this is what the ports look like in the drivers footwell.  The Body port is the right hand one.
• Insert your ignition key and turn it to the On position (all the idiot lights on dash lit).  The car can be running if you need it to be, but at a minimum it needs to be in this key ON position.
• With the main menu showing, use the arrow pads to  highlight the “Aston Martin” codes and then press the Enter button.
• On the next screen that appears, use the up/down arrows to highlight your generation of Aston Martin and then press Enter.  For my 2005 DB9 I selected this one.
• On the next screen it will be prompting you to be sure to have connected to the correct OBDII port.  As I already noted above, for this procedure you need to be connected to the BODY port.   Press the F1 button under the OK onscreen.
• Next select your model of Aston Martin.  My 2005 DB9 would be the second option on the list.  Use the arrow pad, highlight it, and press Enter to proceed.
• On the next screen we need to choose “Control Unit”.  We do NOT want to Auto Scan (that’s a different use of the OBDII reader).   Use the arrow pads, highlight “Control Units” and then press Enter to proceed.
• On the next screen use the arrow buttons and scroll down to highlight the menu item “TCMZF”.  This stands for Transmission Control Module for ZF Transmissions (the Automatic Transmission in the DB9 is a ZF Model 6HP26).   Once highlighted, press Enter to continue.
  • Note – if you didn’t have the key in the On position, or you weren’t connected to the Body OBDII port, this is the step where you will get an error failing to communicate with the module (since its Off or your on the wrong communication bus).
    
• On the next screen arrow down to the “Live Data” option and press enter to continue.
• Now you’ll see a list with dozens of live data items you can monitor, but we are only interested in one right now.  Arrow down to highlight the 2nd option “Transmission Oil Temperature”.   You’ll notice there are small check boxes to the left of the item, and they are all unchecked initially.
• Now press the enter key to ‘check’ it.  You should see a check box appear to the left of it.
• Press the F3 soft key under the display to select “View Data”.   This tells the Foxwell to start asking the TCM for the live data values for any variable that you checked off.   In our case, the Transmission Oil Temperature.
• It should think for a moment, and then begin displaying the live real-time transmission oil temperature.   In my example here, the fluid temp was 113°F (45°C) since I had recently been driving the car.

The live data will stay on screen now.   If the car wasn’t running yet you could start it when you are ready to check the fluid level and the real-time information will keep updating on the screen.

How Fast Does the Fluid Heat Up?

With the ability to read the temperature in real-time I thought it would be helpful to measure how long the fluid takes to warm up from 30°C to 40°C.   This is the critical window where you need to be working under the car, with it idling, and pull out the fill plug and monitor the drip/drizzle flow and either drain or add fluid.  Many people talk about ‘you need to work fast’.  But is this 1 minute or 10 minutes?

To find out I left my car in the garage overnight and came out in the morning to start it up and measure the temperature rise (without driving).  Merely idling from cold start, just like you’d do in the shop when changing/checking your fluid.

Unfortunately its a hot August day here in California, and by 9am it was already 87°F in the garage (31°C).  So, I started already a few degrees into the 30°C to 40°C range (86°F to 104°F).

The transmission reported its starting temperature as 90°F (32°C).

The result – it took almost exactly 10 minutes for the temperature to rise (at idle) from 90°F (32°C) to 104°F (40°C).   A little more than a minute per °C.

This is good news.  It will likely be a bit cooler for you when you do the work, so you’ll have longer than 10 minutes to start the car, crawl  under and get the work done.

I’d suggest that you enlist the help of your Sweetie to sit in the car and call out the fluid temperatures to you as they rise.  The green-light to pull the plug out at 30°C and then the status reports on the climb as you dial in the level before it gets too hot.

You can use this knowledge in my series on how to Change the Automatic Transmission Fluid (check it out here).

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Video

Here is a short video of how to use the Foxwell, and a neat time lapse I shot of the temperature timing test.