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The diagnostic port signal on my race car is "dirty". The data signals are erratic (RPM bounces wildly on OBD II monitors). This is a problem because my dash (AiM MXL2) gets RPM, among other things, from this signal.
If I run wires directly from the CAN bus ECU pins to the diagnostic port the data is perfect. But, ABS and power steering are disabled because the ECU is not getting those signals.
I can get RPM signal from elsewhere but I'd rather have the ECU correctly interface with my dash / data acquisition.
Anyone have any ideas other than a new wiring harness? I suppose I could try running new CAN bus wires from the PS and ABS to see if that works.
AiM is tapped into the hi and low CAN bus wires. Monitor is just plugged in using an OBD II reader plug. Same result either route so it's not the connector.
That doesn’t jive with your first post. Is it the same as you just stated above or is the data perfect when you tap the CAN, but the problem occurs when you use a plug-in diagnostic connector as you stated in the first post.
That doesn’t jive with your first post. Is it the same as you just stated above or is the data perfect when you tap the CAN, but the problem occurs when you use a plug-in diagnostic connector as you stated in the first post.
Sorry I wasn't more clear. The signal is bad when the signal comes from the diagnostic port connector (plugged in) or the wires as they enter the connector (Aim signal source tapped in here). This eliminates the actual connector as the problem.
The signal is good when the CAN bus harness wires are cut from the ECU connector and I splice wires directly from the ECU CAN bus to the diagnostic connector and/or the Aim.
The above method bypasses ABS and PS. Corresponding error codes show up on my reader as expected.
Thanks for the clarification. I have the MXL Pista, but haven’t fully wired it in yet. Have had connector issues using the Cobb Tuning device. The pins in the OE connector are notorious for being loose and loosing the connection. Otherwise I never really saw the specific issue you mention. It was my intention to tap the wires directly to avoid the connector problem though.
Wondering if perhaps you either have a problem in the wiring to the connector port or if maybe you are using something causing a noise interference issue? I know other people haven’t had that issue. Did you try bypassing the connector wiring and tapping directly into the pcm where it feeds those two signals from?
That's what I suspected. There must be an issue with wiring between the PCM and the port but if you ever trace the wires they are a complex mess. Probably easier to replace the engine bay harness.
Yes (see above). If I tap directly to PCM signal is clean. If the rest of the harness is connected (to acquire PS and ABS) signal is dirty.
If you refer to the diagram at the wiring end point it lists the pcm connection point information where those two wires originate from. Then just go and tap directly there, essentially replacing/bypassing the harness wiring to the port. If it doesn’t kill those two signals at the diagnostic port then it shouldn’t kill them by eliminating the wiring between there, right?
If you refer to the diagram at the wiring end point it lists the pcm connection point information where those two wires originate from. Then just go and tap directly there, essentially replacing/bypassing the harness wiring to the port. If it doesn’t kill those two signals at the diagnostic port then it shouldn’t kill them by eliminating the wiring between there, right?
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Again, apologize for not being more clear but that is exactly what I have done. If It goes directly from PCM to monitor the RPM signal is clean but no ABS/PS. If I leave the harness connected to PCM AND run new signal wires to the monitor the signal is dirty. If the PCM/port harness is connected in any way, even if there is a bypass, signal remains dirty.
I'll start shopping for a new harness in the off season. Thanks for the help.
Well it doesn’t seem to make sense for the result to be different at the end of a wire than the beginning. Now that I’m looking at the wiring diagram it appears that the ECU signal may split up at a harness connector. I’ll try to look into it further using the electrical diagram book the next time I’m at the shop. The pdf version I have is an early copy and not always reliable.
Perhaps removing the dash wiring harness removed the end termination point for the twisted pair? You would in theory need to add a 120 Ohm resistor at the now broken end point from the instrument cluster to get the correct stability.
After a much trouble-shooting it looks like the problem is a bad ABS controller unit. If this it taken out of the circuit PS works and the signal to the AiM dash is clean. AiM dash is equipped with a 120 Ohm terminating resistor for anyone who needs this info.
10-08-2017, 05:22 PM
