More testing of the bluetooth module.

I have been out for two test drives this weekend with the aim of proving the bluetooth module a bit more. I installed it on the right hand side of the car, which as many will know is just behind the rear wheel, fairly low down. I have now got it to the point where I'm confident it will run acceptably in the car but this was not without some teething troubles.

During the first drive I found that about 20 minutes into the drive the bluetooth module shut down. This was evident because logging on the phone stopped and the terminal application I'm using for logging disconnected from the device. A quick look at the module showed that the usual bluetooth LEDs were not lit up, meaning it had stopped running.

The engine bay is a pretty hostile environment and my immediate suspicion was that the module had gone into thermal shutdown. Sure enough when the car was brought home and it cooled down a bit (engine kept running throughout) the bluetooth module came back to life.

The fundamental problem is that on the right hand side of the car the standard position of the Cat ECU is 12-18 inches away from the catalytic converter. As a result it has to withstand a lot of radiant heat yet it has no heat shield; only the substance it is potted with. The potting I've used initially is standard epoxy resin and this is not particularly insulative.

I have had no luck with finding the thermal properties of various potting compounds. Some talk about the low exothermic properties (this is the heat generated when the compound 'goes off' when first poured in around the electronics) and others talk about insulating properties but only from an electrical perspective. If anyone knows of any potting compound that is has good thermal insulation properties I'd be intersted to hear.

For the time being (and possibly permanently) I've moved the module further away from the exhaust. It is now located higher up, just behind the rear lights on the right hand side. It is simply fastened to an existing vacuum pipe with a tie-wrap. The advantage of putting it here, other than it being further away from the heat, is that the existing cables are all within the required length so it's a simple job to relocate it.

Here it is, located in its new spot. The connector in the forefront of the image is for one of the rear lights.

I took the car out for a second test this weekend and hey-presto - no thermal shutdown! So I'll call this 'solved' for the time being but will keep in the back of my mind that other solutions might be of value. One other thought I had was putting it inside some thermal sleeving normally used to insulate inlet pipes in the engine bay. These come in a fairly broad range of diameters and are easy to get hold of. However most are metalized in some way so may have an impact on the bluetooth signal.

I took a data log whilst I was out. This log was captured from the right hand Cat ECU using bluetooth on a mobile phone. The two lines are as follows:

  • Blue - Temperature in celsius (left hand scale)

  • Orange - Voltage output (right hand scale)

The two obviously track each other almost exactly except for the startup sequence where you have to manage the fact that there is no valid output value for less than 300 celsius. Thats why there is a flat line at the beginning of the orange chart, until the blue line goes over 300.

I have continued to develop the Android app but it's not in a state where I feel it should be presented yet.

So what's next?

  • Build a left hand bluetooth ECU so I can monitor both sides wirelessly

  • Build out the android application

  • I'm probably going to have to build another right hand ECU as well, because I managed to lock the firmware in on the current one as I posted in my last update.

  • I suppose I still ought to think about how I might offer these to other people. Still not sure on that yet.