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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Ok, been working on an 84 trans am I just picked up. Previous owner had stripped the electronic distributor and left the electronic carb. I purchased a new distributor, ran better but then started getting mixture control solenoid code. Being the car had set for some time and the carb was in pretty rough shape I then replaced the carb. Beautiful, MCS finally clicking, easy starting, no codes, everything was looking good.
Them the other day no clicking from the MCS, hard idling and poor performance. Checking the voltage and resistance at the MCS I noticed that it is getting constant ground and engaging once when the key is cycled on. Resistance across the MCS is good at 24 ohms. Giving momentary contact of the connector clucked the MCS with every contact. From what I can tell there are no shorts to ground. I know the easy way to diagnose would probably be to disconnect the ECM to verify it is indeed sending ground to the solenoid but haven’t had the chance to check. Just wondering if there would be any other reason the solenoid would suddenly stop clicking and fully engage with key on. I am leaning towards ECM although everyone says they don’t go bad. Solely because who knows when it last actually controlled anything and was suddenly put back in service. Thanks for any insight.
I would suspect a short to ground in the ECM lead to the MCS.
Try disconnecting both ends of the lead at the ECM connector and at the MCS and check continuity to ground. Maybe bypass that lead with a temporary jumper see if you get the clicking.
Check your dwell diagnostic lead, it may be inadvertently shorted to ground. I do not recall that they are electrically connected but shorting it to ground could do 'something' to the rest of the system. I'll have to look at that one day.
I’m at a loss of explanation. Disconnected ECM and solenoid still clicked and engaged.Recheck resistance across MCS, 24 ohms. Disconnected ECM wire at dwell connector, still engaging. No ground present, simply an open wire. Leading me to believe an internal short to ground within the MCS. Contacted the carb rebuilder and explained, they will be sending another out. So, put everything back together and...working. WTH? I’ll just cough it up as an intermittent issue and install the new carb when it arrives. While digging around I did find this connector which looks really familiar from past vehicles but can’t place it. Any idea what this goes to?
looks to be the O2 sensor lead. should reach across the manifold to the sensor in the driver's side manifold. should be a little bracket it goes through to keep it above the manifold where it crosses over.
Yup; single purple wire would be O2. Somebody pulled it to the wrong side of the motor.
Not having that connected will cause all manner of things related to the MCS to not work properly. Check Engine light, not the least.
I see you have the new-style coolant temp sensor & connector on that car... that's a good thing. The pigtail needs to be soldered and heat-shrinked rather than those butt splices though.
Was definitely the O2 sensor wire. Thanks! Slowly getting this thing put back together. 02 sensors in my pile of parts to replace. Funny how I didn’t throw a code though. One thing I like about this car, It’s all here just been hacked over the years. Plenty of questions coming soon and vacuum routing and smog. Thanks again for the help!
Was definitely the O2 sensor wire. Thanks! Slowly getting this thing put back together. 02 sensors in my pile of parts to replace. Funny how I didn’t throw a code though. One thing I like about this car, It’s all here just been hacked over the years. Plenty of questions coming soon and vacuum routing and smog. Thanks again for the help!
Bulb works and does come on not running. Been chasing codes for the past few days. But never a o2 related code. Actually was running with no code triggered after MCS issue was resolved.
