mikec, on 20 May 2020 - 12:09 PM, said:
The ECU sends two signals to the ignitor: one tells the ignitor when to monitor for engine knock and the other tells the ignitor "cold engine or high altitude, either way advance the timing an additional 5 degrees." The ignitor does not send anything back to the ECU other than ignition pulses - the same signal that drives the tachometer. This comes from the ignition coil actually. The ECU uses these pulses to know if the engine is actually running (if the engine stalls/stops, the ECU cuts power to the fuel pump in case the engine died from the car being in an accident - don't want the fuel pump running, pushing fuel out busted fuel lines) and, if the throttle body nose switch says the throttle is at idle, the ECU looks at the ignition pulse rate to determine engine RPMs and adjusts the Idle Speed Control (ISC) motor as appropriate to get the proper idle speed.
So, if your engine will run and the tach works, you can be pretty sure the ignitor is functioning. It's a simple "electronic ignition" box, not a highly sophisticated computer.
Do you live at "high altitude" - above 3900 feet or 1200 meters barometric/pressure altitude? If so, the ECU should be requesting the timing advance. If not, you'll still have the timing advance on a stone-cold engine; it'll suddenly go away once there is some temperature in the coolant. Look for the wiring harness bundle between the air filter canister and passenger fender... there should be a pair of connectors hanging from it (or taped to it) that are not plugged into anything but should have protective caps. One will have a single wire, one will have multiple wires. On the 2-wire one, jumper the two wires together. That forces the system into "warm engine, low altitude" mode so you can verify the ignition timing is a proper 10 deg BTDC once the engine is fully warmed up. Let the engine cool completely and restart... timing may be a little different as idle RPMs will be higher, affecting the distributor timing advancer components. Remove the jumper. Did the timing increase (advance) 5 degrees? If so, the ignitor is 100% functional.
Folks have long complained that StarQuests have a weird/unsteady idle. I've only driven 87-later models that don't have the solenoid and pressure sensor; the 87-later cars have a dedicated barometric sensor in the airflow sensor assembly and don't measure boost pressure at all. So they don't have this 2-minute cycling thing; I've never experienced it myself to know if your car's symptoms are typical or abnormal. Mitsu did a lot of experimenting in the 86 ECU programming too; some 86s for example run just fine with the primary airflow sensor removed - relying on the MAP sensor only - while others flat-out suck without the airflow sensor. That tells us the software in 86 ECUs varied over the production year. One thing about the ECU error codes: the ECU only looks for significant "out-of-bounds" errors from most of the signals; it really isn't that good at detecting most failures. So, just because the ECU isn't throwing error codes, don't assume that means the sensors are 100% okay.
With the engine warmed up and idling, the output voltage of the pressure sensor should be between 0.2 to 1.2 volts, momentarily jumping to 1.5 to 2.6 volts every two minutes. This is on pin 8 of the pressure sensor which is one of the two pins closest to the locking/indexing tab of the connector. (the other pin is +12volts if the engine is running so it'll be obvious which pin to test/use) Use a voltmeter to measure the voltages.
Mike I was hoping you would chime in, as I have learnt a lot from your replies in the past.
I thought the plugs on on the passenger side hardness were for the ECU/02 codes and fuel pump diagnosis? Every time I do a TPS/ISC reset I use my timing light to verify the timing and RPM's its right around 10 degrease when warm, I haven't checked what it is when cold though.
when I bought the car is had been hacked up, was only running on MAP and no ISC etc and I have been slowly piecing it back together and the last hurdle was the solenoid switch, I have no idea what the car was like before it was messed with.
When I said I probed the ECU I mean back pinning with a multi meter against the FSM spec's and everything seemed to be functioning as it should the Map was working correctly and you could see it momentarily jump to 1.5-2.6 volts as it checked for atmospheric pressure but when it does that the idle drops and goes lumpy then within 5-10 seconds it flips back to normal. However sometimes on a cold ish engine it will get stuck on the atmospheric switch and the only way I can get the car to come out of it is to rev it.
This behavior has me running round in circles, I have a laundry list of parts that I have replaced as maintenance and the CTS is one of them before some mentions it.
I am starting to wonder if the ECU is just on the fritz.