First of all, you need to appreciate precisely what the TM-2 will tell you. By securing the sender unit to any surface, the TM-2 will accurately relay the information to the readout unit.
The choice of sensor position is a matter of personal choice, ability, and a matter of what you want to look at.
Below are pics of actual units fixed in different places around the engine, with an explanation of what you will be seeing at the readout unit. The TM-2 is a very useful and informative piece of equipment and ideally should be used to its full capacity.
An upper alarm can be set such that you will not only get a visual display, but can set the alarm to warn you of impending problems. Personally, I have the audible alarm set to point where the fans are about to kick in. ie: thermostat has lost control.
There following diagrams explain the various points of fixing.
exhaust side (original recommended position)
Initially, I fitted mine to the cylinder head on the passenger side – at the time, this was the recommended fixing point.
Because this point is directly above the exhaust manifold, it registered much hotter than the coolant temperature. I was quite happy with this position because at the time, I was not particularly interested in the coolant temperature. Haydn subsequently changed the recommended sensor position to the rear of the head. (see next pic)
Rear of head (current recommended position)
The current recommended position is as shown in the pic.
This position will ‘simulate’ the coolant temperature.
The temperature at this point on the head is very similar to the hottest coolant temperature. However, from the pic, it can be seen that the sensor is mounted approximately 50mm away from the nearest coolant chamber.
The sensor is actually on the same level as the rocker shaft. Whilst producing a close approximation of the hottest coolant temperature, it still remains a simulation. In fact, in this position, the readout will give a reasonably accurate temperature of the rear camshaft pedestal bearing.
The original reason I didn’t fit mine here is that I could not get to the lower of the bolts to remove the cover and gain access. This is due to the flooring installed by 321 Away covering the bolt.
Front of head (Helens current position)
Helen has her sensor fixed to the 2nd rocker cover bolt, and it was placed here because it is very close to the hottest coolant flow.
Again, whilst giving a very close approximation, this is also a ‘simulation’ – because again, it is fixed to the rocker cover and is in the region of 60mm (measured) away from the nearest coolant chamber.
Because I was (and still am) interested in the actual coolant temperature at the thermostat, none of the previous positions was ideal.
First of all, I looked at the absolute ideal which is to use the existing Mazda sensor which feeds the Mazda gauge. I know it would have meant disconnecting the Mazda gauge so it was basically redundant. No great loss there because it is useless anyway as all will agree. However, it would have meant connecting the TM-2 to the original sensor and introducing something similar to the Mason modification. In other words, ‘fudge it’. Those that know me will testify that ‘fudging’ things is not my way. I’ll leave that to the politicians.
The next option was to use one of Haydns ‘in-line’ adaptors. I was sorely tempted, but the thought of introducing another joint/clamp into the system with the associated potential leak-point, did not appeal at all. Shame.
My way
So what I finished up doing was getting as near to this solution as I could by clamping the sensor to the metal part of the top hose connection at the point where it leaves the cylinder head. see pic above (In fact it was Kirsty who did the actual job for me).
The coolant flow at this point in the head divides into two. One goes to the top hose and the other goes directly to the thermostat. Both identical temperatures, so BINGO – job done.
The results of this is that I now have an accurate digital readout of the temperature at the thermostat. I have monitored this since fitted last year and the figures I’m getting are very informative indeed. It shows the thermostat actually working. I fitted (well Ady fitted) a new Mazda thermostat for me.
After bleeding. we raised the temperature of the engine to see when the fans kicked in. I don’t know what others Bongos do but mine kicked in at precisely 96°C which coincides exactly with when the thermostat loses control. They switched off again at 91°C, when the stat had regained control.
I have the TM-2 alarm set to 96°C so that I get a warning if things are erring towards the hot side.
On my way home, I was a VERY happy bunny. Meaningful readings.
Personally, I would recommend this fixing point to ALL TM-2 users, but that’s only my opinion, being the control freak that I am.
