Murphy Posted August 24, 2012 Share Posted August 24, 2012 After reading Cliff's book on Q-jet rebuilding, it has me interested in my on Q-jet. I then discover this really odd solenoid on mine...... This Monte is a 71, SBC350, TH350 with AC. Going thru Cliff's book & website, my Haynes manual and internet searches, I finally found mention of an idle-stop solenoid for AC cars only! Anyone have any more insite?? One comment I found stated "New for 1971" this was on a corvette site.... Quote Link to comment Share on other sites More sharing options...
Leghome Posted August 24, 2012 Share Posted August 24, 2012 That solenoid is to speed up the idle when the AC is running. I just got one from my son in-law for my Eldebrock Carb. When I turn on the AC the idle drops about 150RPM.What color is the wire running to the solenoid. I am getting ready to instal mine and would like to know what color of wire to look for in the harness. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 24, 2012 Author Share Posted August 24, 2012 I'll check it out tonight & let U know...I beleive there were 3 wires????? Quote Link to comment Share on other sites More sharing options...
Toppless72 Posted August 25, 2012 Share Posted August 25, 2012 When your engine is idling, the throttle plates are slightly open. This is the normal idle setting. This is accomplished by energizing the idle stop solenoid. It becomes energized as soon as the key is turned on and remains that way until the key is shutoff. As soon as the key is turned off, the solenoid de-energizes and the throttle plates, no longer held open by the solenoid, close completely. This prevents any air from entering the engine and stops the engine from dieseling. It, in affect, chokes the engine to a stop. This is explained in detail on pages 6T-8~9 of the Chevrolet Chassis Service Manual for 1972 “Emission Control Systems”. It appears that it is required for all engines regardless of the A/C option therefore, the wiring should be present in all the engine harnesses. The wires consist of two tan 20g wires and one 20g dark blue wire. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 25, 2012 Author Share Posted August 25, 2012 Thanks Joe, great break down. If a carb were replaced, I then assume that the solenoid should be moved over to new carb. Quote Link to comment Share on other sites More sharing options...
stangeba Posted August 25, 2012 Share Posted August 25, 2012 I have a 71 350 4BBL. NO A/C! When I removed the TCS solenoid from the carb, it's function (as I understand it) is two fold, 1- was to energize (in high gear) allowing vacuum to advance the timing and 2- keep the throttle plates open slightly more than idle screw speed for leaner foot off the gas or coasting emissions. It also energized in reverse so the speed could be adjusted and function verified. Bruce Quote Link to comment Share on other sites More sharing options...
Toppless72 Posted August 25, 2012 Share Posted August 25, 2012 Some of these solenoids do have dual purposes however; Murphy's question addressed the idle stop solenoid so that is what I commented on. He has identified an idle stop solenoid in his picture. It has no vacuum lines running from it so it is not controlling the spark advance in his case. The Transmission Controlled Spark system (TCS) has a completely different purpose. It controls the spark advance based on the conditions of: 1. Engine coolant temperature is above 82 degrees 2. Transmission in high gear. 3. The 20 second warm up delay relay has been met. If all that is true the TCS will allow ported vacuum to draw on the distributor advance diaphragm. If not vacuum is denied and there is no advance via vacuum source. I would like to correct one comment I made earlier. The idle stop solenoid by itself should only have one wire running to it... 20g tan. The idle stop solenoid is energized at all times while the key is in the run position. Murphy: If you change your carb to an Edelbrock for example, you could tune it to eliminate the idle stop solenoid if you wanted to. It was put there forty years ago to help the tremendous dieseling problem many car makers had at the time. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 25, 2012 Author Share Posted August 25, 2012 Thanks for the additional info 'Topless', I've been researching this as well then your post popped up and you are correct in that the TCS is a different animal as there are NO vacuum lines, only a 2 prong connector which consists of 2 Tan wires on one side and a green and a blue connected to the other spade connector. As I previously mentioned, earlier research I stumbled upon this corvette site discussing the same issue and it was mentioned that this idle stop solenoid was new for 71 and is not mentioned in the manuals until 72. My 71 chassis manual, Haynes manual show nothing! My car build sheet is dated June 28th. I just found the part on ebay "of Course" the only difference with mine and this photo is the plastic connector is shaped differently. Quote Link to comment Share on other sites More sharing options...
Toppless72 Posted August 25, 2012 Share Posted August 25, 2012 Murph, I’m not sure where to go from here. I only have a manual showing the 1972 systems and they may be a bit different than 1971. The picture you have in your last post is a TCS Solenoid. The carb. Vacuum is connected to the outer port while a hose to the distributor advance diaphragm would go on the inboard port. I’d be afraid to comment further if I don’t have supporting documents defining the true system layout. My wiring diagram doesn’t show a green wire at all that I can see connected to this system. I will say this... If you turn your key to the “run” position, the plunger on your solenoid should pop out and push the throttle lever open a small amount. If it doesn’t, it isn’t working properly. Check for 12 volts at the tan wires with a test light to confirm if it’s the solenoid or the power. If you have no power, trace it back from there and see where it’s failing. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 25, 2012 Author Share Posted August 25, 2012 That's probably correct (TCS solenoid) that explains the different plastic housing but it is about the same size as mine even tho it's label on ebay as "idle Stop" I just found mention of another device called an "Idle Speed" solenoid and I have cut a portion of that doc and pasted it below...... Some air conditioned vehicles with the Quadrajet carburetor use an A/C idle speed solenoid mounted on the carburetor float bowl in place of the idle stop solenoid. The A/C idle speed solenoid is electrically energized only when the air conditioning compressor is engaged (air conditioning “On”) to maintain proper idle speed when the air conditioning is in operation. Normal curb idle speed (air conditioning “Off”) is made using the idle speed screw on the carburetor bowl. This is where I got the notion that it was only on AC vehicles. This document was found on a corvette forum. Quote Link to comment Share on other sites More sharing options...
Toppless72 Posted August 25, 2012 Share Posted August 25, 2012 Murph, That has to be the difference between 1971 and 1972. The system was used differently in 1971 vs. 1972. The 1972 manual clearly states the ISS is energized at all times with key on. I learned something today. During that time GM's engine management controls were evolving and this shows there are some significant differences in these systems. Well you could try turing your key on with the A/C in the on position and see if the solenoid pops out. That would be another place to start. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 25, 2012 Author Share Posted August 25, 2012 Thanks, Joe..........Half the fun of restoring old cars is the detective work IMO. I'll check out its functions tomorrow once I've finished installing the exhaust system. Later, Murphy. Quote Link to comment Share on other sites More sharing options...
wallaby Posted August 25, 2012 Share Posted August 25, 2012 I'm trying to remember, but I think there are some special mounting points on the carbs that used that solenoid. Look closely; you may find that the solenoid won't mount to your replacement carb. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted August 25, 2012 Share Posted August 25, 2012 My 2 barrel had it on there when I switched over to the q-jet it wouldnt fit on it so I left it off and havent had any problems at all. When I switched over to the pertronx system I used the power from the live selonied wire for that it works fine. Of corse my a/c dosent work so im not sure about the idle drop when its turned on. Quote Link to comment Share on other sites More sharing options...
Murphy Posted August 25, 2012 Author Share Posted August 25, 2012 Further update...after closer inspection, the plastic connector on this solenoid DOES have 2 vacuum ports! It was turned downward so the ports weren't visible. My apologies! I turned the key to test voltage and the solenoid pops out immediately, there is 11.4 volts on tan, when AC switch is turned on I then see 3.7 volts on green & blue side. Nothing is connected to vacuum (obviously or I would have noticed). I'm going to finish with exhaust install so I can fire the old girl up without waking up the entire neighborhood, I then will report further operation/voltage at that time...... Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted August 26, 2012 Share Posted August 26, 2012 Wake up the neighborhood they really like ya know they just wont tell you that they do. Quote Link to comment Share on other sites More sharing options...
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