Thread: Super MPGuino?
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Old 02-14-2010, 01:35 PM   #28 (permalink)
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Join Date: Jan 2010
Location: Newark, DE
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'91 CRX - '91 Honda CRX DX
90 day: 34.91 mpg (US)
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Originally Posted by gtkid2002 View Post
On Honda's pre-1990, I'm pretty sure most of them don't have electric speedometers, which would make things difficult.
I thought I would clarify a few things regarding Honda instrumentation.

First, their tachs have been electronic for a long time... Probably since they switched from points/condenser systems to magnetic pickups. That means the '89 Prelude should have a pulse-driven tach. The ignition control module in the distributor generates a signal pulse every time it fires the ignition coil -- twice per crankshaft rotation.

As far as I know, the cable-driven speedos found in fuel injected cars still have a pulse-type pickup in them so the ECU (and other devices, such as cruise control) can read vehicle speed. The transition period for cable to electric driven speedos was '92-93, depending on the model line. I'm not sure about the '88-91 Prelude since it's sort of a... umm... ******* child of the B-series engine family, but many times the electronic VSS (vehicle speed sensor) units found installed on the transmissions of newer vehicles are drop-in replacements for the cable drive gear found in older cars. The biggest trouble there is getting the old drive gear assembly out, as they tend to bond to the hole in the transmission... 5 mins with a propane blow torch and a good set of channel lock pliers with sharp teeth will often do it. Try to find a decent Honda/Prelude enthusiast forum to find more info.
Once you have the pickup in the car, supply it with power (either 5V from the ECU or 12V from the main relay, both should be available in the engine harness, check your ECU wiring diagram/VSS troubleshooting procedure for which to use), ground (the connection at the engine thermostat housing should do) and run the signal wire to the gauge cluster, splicing into the existing speed sensor wire so the ECU and other devices get the speed signal, as well as whatever you plan to use for the speedometer/odometer.
FYI, I've done this conversion on my CRX and am now running a '94 Integra gauge cluster... It's fully functional except for the warning lights for equipment I don't have, such as ABS, SRS and the low fuel warning light.

For coolant temperature, there should actually be two pickups. A single-wire sender for the gauge, and a two-wire sensor for the ECU.
The gauge sender doesn't respond linearly to changes in temperature. At some point Honda came to the conclusion that the normal variations in engine temperature gave drivers the impression that the cooling system wasn't doing what it's supposed to. So, they replaced the gauge sender with one that responds normally up to the bottom of the normal range of operating temperatures, plateaus for a bit and then continues behaving normally just above that normal range... There is very little change in sender reading within that normal range.
On the other hand, the ECU's two-wire (sensor ground and signal) Engine Coolant Temperature sensor responds in a far more normal manner. I would guess the ECU uses a simple resistor network to read the value of the ECT sensor, so you should be able to just tap into that red/yel wire and read the 0-5V signal.

You should be able to read the Intake Air Temperature sensor the same way as the coolant temp sensor.

MAP (manifold pressure relative to full vacuum) should be another simple to read 0-5V signal, as should the Throttle Position Sensor.
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