So this came up in the general forums but I feel it's better suited for this section. For those of you running an aftermarket ECU, if you have a free programmable PWM output, I have finally found a way to use that to drive the Econometer gauge to display customized, useful information! In my case, I'm using a spare Megasquirt output to display AFR on my '88 325iS! To accomplish this involves some minor surgeries, both on the ECU and SI board in the cluster, but it's all 'reversible' and IMO it's well worth the effort!
First, some background on how this silly gauge works. The SI board is what actually drives both the tachometer and the econometer (and temperature) gauges; the signals only pass *through* the cluster, but all of the important signal processing happens on the SI board. On the SI board, one of the little IC chips receives TWO separate input signals, one a 12v square wave from the speedometer for vehicle speed, and the other a 12v square wave for fuel rate from the ECU. It uses both of these variables to calculate fuel economy, and finally drives the 4 separate output pins that actually control the gauge. Ok, so now for the fun part:
What you need to do is bend the vehicle speed signal pin from the SI board out of the way so that it NO LONGER receives the VSS pulse from the cluster, and then bridge the fuel rate input to back of the now-disconnected VSS pin on the SI board. This way, the SI board will see the SAME pulse for both inputs, and you can drive the whole gauge based on providing it with your custom 12v pulse output via the fuel rate pin. So firstly, the schematic of what we're doing, for reference:
Step 1, bend vss input pin on the SI board back out of the way. Bend rather than cut so the process is reversible. On my SI board it's pin 10 (1 is on the far right if you're looking at the cluster, left in the pic):
Step 2, bridge the fuel rate input pin (9) to the VSS pin (10) on the bottom side of the SI board:
Step 3, Make SURE that the signal you're providing is a 12v square-wave. Use a pull-up resistor (1k~10k ohms) to +12v if you need to (my Megasquirt needed this; a open collector or 5-volt square wave will NOT work; you can even pick up 12v for this right at the SI board from pin 5). Then just connect this to Pin 11 on the white connector (yellow/white wire at the cluster, white/black wire from the ECU connector), and program your duty cycle accordingly! I tested this using the Fidle output from my test MS2 (easy to test this way by adjusting CLT on the stimulator), set to a frequency of 124hz. My results of gauge position (MPG) vs duty cycle are attached, both 'normal' and 'inverted' are included depending which way your signal goes:
Final note, remember that the Econometer gauge will NOT work until there's an RPM signal; aka it won't do anything with key-on-engine-off. Engine needs to be running before it 'comes alive,' even though RPM has no influence on the reading. The final pic for proof!
First, some background on how this silly gauge works. The SI board is what actually drives both the tachometer and the econometer (and temperature) gauges; the signals only pass *through* the cluster, but all of the important signal processing happens on the SI board. On the SI board, one of the little IC chips receives TWO separate input signals, one a 12v square wave from the speedometer for vehicle speed, and the other a 12v square wave for fuel rate from the ECU. It uses both of these variables to calculate fuel economy, and finally drives the 4 separate output pins that actually control the gauge. Ok, so now for the fun part:
What you need to do is bend the vehicle speed signal pin from the SI board out of the way so that it NO LONGER receives the VSS pulse from the cluster, and then bridge the fuel rate input to back of the now-disconnected VSS pin on the SI board. This way, the SI board will see the SAME pulse for both inputs, and you can drive the whole gauge based on providing it with your custom 12v pulse output via the fuel rate pin. So firstly, the schematic of what we're doing, for reference:
Step 1, bend vss input pin on the SI board back out of the way. Bend rather than cut so the process is reversible. On my SI board it's pin 10 (1 is on the far right if you're looking at the cluster, left in the pic):
Step 2, bridge the fuel rate input pin (9) to the VSS pin (10) on the bottom side of the SI board:
Step 3, Make SURE that the signal you're providing is a 12v square-wave. Use a pull-up resistor (1k~10k ohms) to +12v if you need to (my Megasquirt needed this; a open collector or 5-volt square wave will NOT work; you can even pick up 12v for this right at the SI board from pin 5). Then just connect this to Pin 11 on the white connector (yellow/white wire at the cluster, white/black wire from the ECU connector), and program your duty cycle accordingly! I tested this using the Fidle output from my test MS2 (easy to test this way by adjusting CLT on the stimulator), set to a frequency of 124hz. My results of gauge position (MPG) vs duty cycle are attached, both 'normal' and 'inverted' are included depending which way your signal goes:
Final note, remember that the Econometer gauge will NOT work until there's an RPM signal; aka it won't do anything with key-on-engine-off. Engine needs to be running before it 'comes alive,' even though RPM has no influence on the reading. The final pic for proof!
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