Exhaust Gas Temperature logging

Written and conceived by Jeremy Hendy


egt_log_power_020711.gif (30546 bytes)How to modify an Autometer EGT gauge to support datalogging with the Link ECU

The Autometer EGT gauge

I used an Autometer Ultra-lite EGT gauge (model 4365), matching my boost/AF/FP gauges and mounted in the drivers side air vent.

The thermocouple used was the Autometer 5247 "clamp-on" probe.  This comes fitted in a stainless steel hose clamp which holds it in place - just need to drill a single hole in the exhaust manifold.

I chose to mount the EGT probe just after the join of the #2 & #3 runners, opposite the O2 sensor bung (which is just after the #1 and #4 "Y" on the JR manifold), about 8" from the flange.  This was to let me monitor the same EGT that the O2 sensor sees (albeit in different cylinders)


The Autometer EGT thermocouple amplifier

The Autometer EGT gauge comes with a separate interface box, containing the thermocouple amplifier itself, and a driver chip which interfaces to the gauge.

The gauge movement is a wide sweep "Air Core" type, requiring differential (sin/cos) drive.  The CS289 driver chip (right) converts the output voltage from the thermocouple amplifier into the right signals to point the gauge in the right angle.

Note that gain and offset are trimmed by Autometer for each individual driver/gauge pair.

Luckily, those nice engineers at Autometer left a spare connector on the terminal block - thanks guys!  It's the 3rd from left, and marked as "no connection" on the plastic box.

Modifying the amplifier

Even better, the thermocouple output voltage which drives the gauge chip comes straight out of the LMC6494 op-amp, so is a nice buffered low-impedance output.

Nevertheless, the safest way to get this to the outside world is via a series resistor - I used 2.2k to minimise the current drain in the event of an external short circuit.

The resistor runs from pin 8 of the CS289 (top left as viewed from the component side) to the spare connector on the terminal block.  Be sure to insulate the resistor leads with some fetching orange insulation, y'hear?

The output voltage

Next step is to hook up a digital voltmeter to the output, and go for a drive to calibrate the output voltage against the gauge reading. 

I did this at 50 - 100 degree F intervals, and got the following graph. Great - it's linear!

Note that the graph extrapolates down to zero volts at absolute zero (-459 deg F) - that's what thermocouples do..

Dividing down the output voltage

The Link ECU Aux data input has a range from 0 to 5V, whereas the thermocouple amp can go up to 8V or so (if we ever reach 2000F!)

I therefore used a couple of resistors and a capacitor connected on a bit of stripboard outside the thermocouple amplifier to divide down from 8V to just under 5V.  Using an 18k + 33k divider divides the voltage by (33k+18k+2.2k)/33k, or 1.612121, making an output value of 8V correspond to 4.96V - perfect.

Modifying the Link to get to the Aux data input

To get to the aux input on the 1.6 Link, you need to cut a track on the backside of the ECU, which links pins 2P and 2R.  Some models allegedly have a jumper linking these two - mine had a PCB track, which had to be cut with a scalpel.

Next, take out the Red/Blue wire that was in position 2P of the ECU connector, and move it across to 2R.  You can then connect your EGT datalogger output to pin 2P of the Link. 

Of course, this assumes you have some spare ECU connector pins magically available - I didn't, so cheated and soldered a 6" flying lead to the back of the PCB, routed it out of the ECU, and terminated it in an insulated crimp spade socket.  That way, I can plug different things into the datalogger input!

Calibrating the logged output

Time for another drive, this time with the datalogger connected.

The easiest way to check the calibration is to drive at a constant EGT (in my case 1400 F), then stop the datalog while the EGT is at a known level.

Using MLL version 2.3.1 to show "Aux NS", this showed raw voltage value of 3.78v for a 1400F EGT.

This in turn translates down to a slope of 0.0020333 v/deg F, with an offset of 0.933V at 0 deg F.

Final step is to create the MLL aux conversion file, then load this into MLL using Options->File Locations, reload the file, et voilá!