Ian:
If you are still suffering the TCS blues, I thought I might add some suggestions and comments. You may have checked these already. Also, my car is OBDII so some of my suggestions may be off for your 91 car.
The 3-1 trouble code for the traction control system indicates problems on the serial data line between the ECM and TCS. From the shop manual, it appears that this is a two way communication port and the TCS and ECM do a handshake on start-up. With the ignition on - TCS starts signaling the ECM, once engine is started the ECM receives the TCS signal and signals the TCS back. If you have a serial data line fault, I would not expect the MIL light to come on until the engine is started.
On the OBDII cars, absence of a signal on the serial data line will cause the ECM to light the MIL and generate a DTC (P1676 or P1677 on OBDII). As such, my guess is that you cannot simply disconnect the TCS from the ECM without having the ECM generate a DTC.
According to the shop manual, the failure options for the 3-1 trouble code (as you have noted) are:
- The serial data line has an open circuit or is shorted to ground.
- The ECM is faulty.
- The TCS is faulty.
Your description of the problem makes me think that you have an intermittent open or ground on the serial data line (which would be a best case scenario). This could be a failure in the actual wire (between terminal 24 on the ECM connector F and terminal 14 on the TCS 20 pin connector) or a problem with the terminals themselves. I don't know about your '91 car; but, on later cars the serial data line is shielded. This shield is grounded at the TCS end only through terminal 5 of the TCS 20 pin connector. As such, it is possible (probably a low probability) that you could have a ground located in the middle of the serial data line without having damage to the external jacket on the cable between the ECM and the TCS. From this thread and your previous thread on this problem, it wasn't clear to me whether you had checked for problems on the serial data line. I suspect that you have a shop manual; but in case you don't and for others that may not have access to a manual, the shop manual test procedure for the serial data line is as follows:
Ground or open check
Check for ground- pull ECM F connector (26 pin) and TCS 20 pin connector. Using an ohm meter, check for continuity between terminal 24 on ECM connector F and the body ground. Continuity indicates an insulation failure on the serial data line. The shop manual doesn't include this but I would also check for continuity between terminal 5 and terminal 14 on the TCS 20 pin connector. Continuity here would indicate a failure between the serial data line and its external shield (you could have this failure without a body ground on the serial data line).
Check for open - with both connectors removed, you could use your ohm meter to check for continuity between terminal 24 on the ECM F connector and terminal 14 on the TCS 20 pin connector. This test is not in the shop manual.
When doing the ground and open checks, having somebody wiggle the cable which may help to show an intermittent failure in the cable.
The manual also has checks for the ECM and TCS ports on the serial data line (to determine a TCS or ECM failure).
ECM check - reconnect the 26 pin ECM F connector. Turn ignition swich on (II) and check for voltage at the back of the ECM F connector on terminal 24. It should be approximately 5 volts to ground. Absence of voltage indicates a faulty ECM. You can also check for voltage at terminal 14 of the still disconnected TCS 20 pin connector. This gives you a double check on the continuity of the serial data line.
TCS unit check - turn ignition off, reconnect the TCS 20 pin connector. start the engine and measure the voltage between terminal 14 at the back of the TCS 20 pin connector and body ground. The best way to do this would be with an oscilloscope as there should be a square wave toggeling between 0 and 5 volts on the data line (the shop manual does not specify the frequency). In the absence of a scope, an analog voltmeter should read between 2.5 - 3.5 volts. The shop manual states that absence of voltage indicates a TCS fault.
Both the TCS and ECM use a resistor connected to 5 volts to pull the serial data line up to 5 volts and a transistor to pull the data line to ground. This is how they create the data bits, although I have no clue as to what communication protocol Honda is using. If the ECM and TCS check out in an initial test, I don't expect that there would be an intermittent internal fault in either unit associated with the serial data line.
Given the intermittent nature of your fault, if it continues, it would be nice if you had access to a hand held portable scope such as made by Fluke. You could connect it to the serial data line and watch the signal to see if it goes away on you as you drove the car around.
I have a scope. Its relatively small but requires AC power. It could be used to check the operation of the TCS with the car parked (in desparation it could be powered off a small inverter to do a mobile test if you had access to one). Since we both live in Regina, if you are still having problems with your TCS and want to pursue checking the serial data line operation with a scope, PM me and maybe we can make some arrangements.