So, circuit description: Please excuse the state of my breadboard - it's been many years since I touched any electronics. Just for testing I have used fixed resistor values on the 556 timers, which gives a frequency of about 20hz on the astable side and around 1.6 second timer on the monostable side. 12.5k Ohm on the astable should give a frequency of 21.7Hz. 14k Ohm on the monostable will make it time for 1.54 seconds, so that needs to be closer to 18k Ohm for 2 seconds. From my own car, I think 1.5 seconds should be sufficient however. I couldn't work out what the delay jumper was supposed to do in the circuit. On my breadboard it is just a loop of wire across the resistor so I can take it on and off easily.
Circuit connections and testing: I have a 12V power supply running and I've got the outputs hooked up to a Fluke digital scope so I can see what's coming out. The short yellow looped jumper cable is the astable to the relay and the short white looped jumper cable is the monostable output to NPN transistor. These are in place so I can easily pull them out and look at them on the scope. To simulate the ENGRDY signal going from +12V to <1V, I'm manually moving ENGRDY from +12V to 0V to simulate the +12V during engine running and <1V when in AS.
Starting with ENGRDY at +12V, I switch ENGRDY to 0V. The relay momentarily clicks, for about 0.1s. On switching back from 0V to +12V, there is a delay of around 1.2s before the relay clicks open for 1.6 seconds. That delay before the relay is triggered, I think, is what the delay jumper was intended for, to switch it in and out. If that's the case, the jumper is in the wrong place. It needs to switch the uppermost 100uF capacitor in and out of the circuit (to the left of the diode on the right hand PNP), since it's this capacitor charging up which causes the delay. I think I have the charge/discharge thing the right way round, but either way, if you take the capacitor out, there is no delay before the relay clicks.
I wanted to know the reason for the delay being there. I can kind of see that it allows some time where the engine will start and the car will start to creep and then the circuit modifies the VSS signal. So the ECM sees the car move off and then thinks it has gone above 9mph, rather than it immediately seeing the car do 9mph when the engine starts. Is that correct?
I don't think the relay is supposed to click as I touch ENGRDY on 0V. This would mean as the engine goes into AS it would send a 9mph signal to the ECM for a fraction of a second and I'm not sure if it would like that. It's either doing it because:
I'm not switching seamlessly from +12V to 0V
I made an error on the breadboard
Something is charging/discharging and triggering the monostable
I think 1) isn't helping. If I were to hold ENGRDY high at the resistor and flick it to 0V directly at the transistor base, it might help. 2) everything looks ok but obviously it's just me looking at it. 3) can't decide what that would be but maybe reducing the value of the capacitor on control pin 11 on the 556 may help.
I had another look at this yesterday. For point 1) above, I modified the circuit slightly so that ENGRDY was held high with a pull up resistor and I could flash it to 0V to simulate AS kicking in. That didn't cure the issue. Moving onto the capacitors to investigate point 3), I changed the capacitor on the Control line to 10nF instead of 1uF in case it was that capacitor discharging but it made no difference. That left the big 100uF capacitor on the Trigger line, which I've circled red. Take that capacitor out completely and the spurious click goes away. I'm not sure what it's there for and I think that now makes the diode redundant, but I'll leave the diode in place just in case.
So I am going to transfer this to Veroboard for use in the car with the following provisional modifications:
-Remove the delay jumper and replace the 1.5K resistor with a wire link (in case it needs to be put back in later).
-Put a jumper in series with the 100uF capacitor circled blue so a delay can be switch in and out using the jumper.
-Remove the 100uF capacitor circled red.
-On/off switch connected to Ignition +12V to switch the circuit on and off. With it switched off, the relay is closed and the car simply operates as normal as it just receives its normal VSS signal.
I might need some help with what wires to cut and tap into. I'm hoping to put this in front of the shifter box like joecvt did, but he had MIMA_L installed so I don't know if I'm going to find the same wiring. Assuming those blue connectors are there, will I find VSS (blue with white stripe?), ENGRDY (black with yellow stripe?) and an IGN +12V (?) in there? Grounds look like they're all black.
Sorry for the some dumb questions, I'm more of a points and carburettors kind of guy when it comes to cars so this electronics trickery is new to me.
Finally got this built up and tested on the bench. I did try to do it with neat links and clever layout, but gave up after a while in favour of actually getting it finished and working. Hopefully won’t take another 6 months to get it fitted...