Hondata

This kind of happened out of nowhere, but I’m having serious issues sourcing this problem down.
1994 EG Hatch
B18b stock internals
3582 Turbo
4 Bar Hondata map sensor
Edelbrock Victor X Manifold
MSD External Coil

For a month or longer now I’ve noticed my ECT no longer communication with my ecu. I’ve gone through 4 sensors, all bench test fine. Rewired sensor straight to ecu and grounded to chassis, gone through Haynes specs and tests, wire tests, new tune files, the whole nine yards. It seems as if my coolant temp is constantly reading 158-160F and will not change. The only instance I’ve seen a change is if I unplug my map sensor, which the reading drops to ~135F. Has anyone seen this problem before? Is it a corrupt s300 unit? Any advice would be GREATLY appreciated since it’s casing my car to run like dog****, and cold starts next to impossible. I’ve had to tune around it by disabling it but the car is not liking that. Thank you everyone who takes a minute.

-Edit-
I know which sensor is the one for the ecu, so it’s not a simple matter of having the wrong sensor or wrong plug. It’s not the single wire SENDING unit, nor the 2 wire fan switch on the thermostat housing. I’m talking about the 2 wire on the head next to the sender.

You should try your ECU in a different vehicle and or try someone else's ECU/s300 in your vehicle and see if reads the same way. This will help you determine if it an ECU issue or an issue with the vehicle. This is most likely not an issue with the s300 as the ECU is what reads the sensors and is more than 25 years old and could be damaged.

Could the polarity of the wires be backwards? Would that blow a component in the ecu or just cause false readings? I’ve looked at diagrams and looking at the connector, +5v seems be the bottom.
I remember my coolant temps working before though which confuses me.

I don't think that would be the issue. You really need to determine where the problem lies before troubleshooting further.

I’ll try to find another person with one. Any ideas why unplugging the map sensor would cause the reading to change though? I know the circuits are intertwined through the sensor grounds but, it’s baffling me that it’s still doing it even with it being directly wired to the pin and body ground, not through a harness.

Spunkster wrote: Fri Jan 31, 2020 3:57 pm You should try your ECU in a different vehicle and or try someone else's ECU/s300 in your vehicle and see if reads the same way. This will help you determine if it an ECU issue or an issue with the vehicle. This is most likely not an issue with the s300 as the ECU is what reads the sensors and is more than 25 years old and could be damaged.

Little update, tried it in my friends car to no avail. I’m having a big issue trying to find another s300 to try. Also re-downloaded the most recent version, deleted all my folders associated with the software and started fresh. Still no change. I feel as though the most recent update was the last thing to happen before this headache started.

Doubt it's the problem because it would still read it would just be off but you should never ground PGMFI sensors to the chassis or body, they should always be grounded at the sensor ground input to the PCM. You introduce voltage offsets this way and if the wire isn't isolated from the rest of the wiring harness you'll cause a ground loop or worse, massive voltage spikes back feeding during cranking.

I was having very similar issues and swapped the Hondata into a different ECU. Problem solved.