TCM on the carb - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Perchjerker · 03-21-2005, 06:03 PM

I have a factory stock truck, 307 2bbl with the selenoid on the side of the carb. I have seen it here referred to as the TCM switch. My question is, why does the switch on my truck only engage when I shift into 2nd gear (3 on the tree)? When is the switch supposed to engage??? Also, If it is supposed to engage in neutral or another gear, how would I adjust it?
Thanks

**john** · 03-21-2005, 06:51 PM

Could it be a TCS (transmission controlled spark) . That only allows vacuum advance in 3rd gear. If it is you will have vacuum line going to vacuum advance on dist. If not I have no idea. John

Perchjerker · 03-22-2005, 10:27 AM

Actually, this selonoid on the carb has a place where the vacuum line from the carb goes into it and then a line from it into the vacuumm advance on the dizzy. When this is hooked up correctly, I find that I get a little better gas mileage.

Perchjerker · 03-23-2005, 11:20 AM

Any more help???

68speedalert · 03-23-2005, 03:14 PM

John is correct, and so are you for that matter. That is an emmissions control that was common on a lot of vehicles of that vintage.

You're is working correctly, which is a surprise in itself! You might get better performace and milage if you bypass the solenoid.

**CPNE** · 03-23-2005, 03:40 PM

It is a Fast Idle Solenoid

A fast idle solenoid, also known as an antidieseling solenoid, opens the carburetor throttle plates during engine operation but allows the throttle plates to close as soon as the engine is turned off. In this way, a faster idle speed can be used while still avoiding dieseling (engine keeps running even though the ignition key is turned off). This is a particular problem with newer emission controlled vehicles due to higher operating temperatures, higher idle speeds, leaner fuel mixtures, and lower octane fuel.

When the engine is running, current flows to the fast idle solenoid, causing the plunger to move outward. The throttle plates are held open to increase engine speed. The plunger is adjustable, so the idle speed can be adjusted. When the engine is turned off, current flow to the solenoid stops. The solenoid plunger retracts and the throttle plates are free to swing almost closed.

It is working correctly

68speedalert · 03-23-2005, 04:10 PM

CPNE,

Your example doesn't cover the vacuum hoses he asked about.

I don't remember if trucks with v-8s had them, but cars DID have a compo unit of the vacuum controlled spark advance system and a idle control solenoid in one.

Perchjerker, is the solenoid mounted on the drivers side of the carb or on the pass side on the manifold?

**CPNE** · 03-23-2005, 04:24 PM

Quote:

Originally Posted by 68speedalert

CPNE,

Your example doesn't cover the vacuum hoses he asked about.

I don't remember if trucks with v-8s had them, but cars DID have a compo unit of the vacuum controlled spark advance system and a idle control solenoid in one.

Perchjerker, is the solenoid mounted on the drivers side of the carb or on the pass side on the manifold?

True, my genericized example was a simple explanation of why the switch engaged/disengaged. I believe the trucks had the same TCS/Fast Idle Solenoid combo unit too.

**john** · 03-23-2005, 06:09 PM

The TCS solenoid is controlled by a switch on the trans and a relay on the firewall to control the vacuum. I dont have a diagram. It also has a temp switch so it only works after engine is warm John

tom hand · 03-23-2005, 08:19 PM

I had the TCS on my truck...no part of it was on the carb

JimKshortstep4x4 · 03-23-2005, 09:02 PM

The solenoid valve on your engine is controlled by temperature of the engine, (over 82 degrees F and whether or not the truck is in direct drive, (high gear). It should not move when the truck is in any other gears. It is a dual function switch. It serves as a "deceleration hydrocarbon control" as it keeps the idle speed up when in high gear. It also opens up a valve that allows vacuum to go to the distributor.

Anti-dieseling on AC equipped trucks is done by a temporary turning on of the compressor when the key is turned off.

The service manual has an excellent write up on the system which covers more details thanI have listed. I can fax the pages to you next week if you don't have the manual.

Jim

68speedalert · 03-24-2005, 01:04 AM

I don't think the idle up unit was used on the 4 bbl. At least numberoneson's '72 350 q-jet has the solenoid mounted on the intake.

Perchjerker · 03-24-2005, 03:45 PM

68sppedalter, the seloniod is on the driver's side of the carb. It has an electical connection at the bottom end and one vacuum line going into it from the carb and one vacuum line going out of it into the vacuum advance on the dizzy.

I dont' think that my selonoid is controlled by temperature, but I could be wrong. It seems to work only if I put the truck into second gear (on the three on the tree shifter) and will disengage if I take it out of second gear.

My carb is a 2 bbl. The truck is an original non-AC truck. I have played with disconnecting it for a while, and the truck seems to run better and get a little better gas mileage when it is hooked up correctly---I don't know why.

It sounds like the thing should be engaging in 3 rd gear, not second. ANy more info will be appreciated.

Jimkshortstep4x4, I don't have a service manual, but it would be nice to have one. I'll pm you my fax number.
Thanks, Perch

68speedalert · 03-24-2005, 07:59 PM

"68sppedalter"

Who the heck???

JimKshortstep is right, it should work in 3rd for sure. With truck idling, feel for vacuum at the hose coming from the carb. If you don't feel any, rev up the engine and you should feel some. If it operates this way, just hook a hose from the carb to the dizzy and don't worry about the solenoid

If you feel vacuum at an idle, it will still work to run the hoses like I describe, you may need to readjust your idle speed afterwards.

Perchjerker · 03-25-2005, 06:02 PM

I don't have to feel for the vacuum to know when it is engaging. I can just put it into second gear and it wil engage and the rpms will rise slighty. But, like I said, if I take it out of second it will disengage. I'm just wondering why and if it needs to be adjuted, how do I do it. Is it controlled by the transmission or a part in the shift colum???
Thanks, Perch

68speedalert · 03-26-2005, 12:44 AM

It has a switch on the transmission that grounds the wire that goes to the relay that then sends voltage to the solenoid on the carb that extends the solenoid that opens the vacuum passage that allows the vacuum to pass from the intake to the vacuum advance and advance the timing and lower emmissions and increase gas mileage. Wew!

It should send vacuum in 3rd gear.

There is an adjustment for the idle speed as referred to by CPNE. It has no bearing on the amount or when vacuum is sent to the dizzy.

My suggestion for feeling for vacuum was for BYPASSING the solenoid . Since you're idling when you are placed it in 2nd and the idle increased, you have manifold vacuum to the solenoid. Unless you are restoring the truck, there really isn't much need for the stuff. You will get better performance and fuel mileage by BYPASSING the vacuum portion of the solenoid.

03-21-2005, 06:03 PM	#1
Perchjerker Beer Power!!!!!!!!!! Join Date: Nov 2003 Location: Nacogdoches, TX Posts: 280	TCM on the carb I have a factory stock truck, 307 2bbl with the selenoid on the side of the carb. I have seen it here referred to as the TCM switch. My question is, why does the switch on my truck only engage when I shift into 2nd gear (3 on the tree)? When is the switch supposed to engage??? Also, If it is supposed to engage in neutral or another gear, how would I adjust it? Thanks __________________ Rather be fishin'

03-21-2005, 06:51 PM	#2
john member #16 Join Date: Dec 1999 Location: Yuba City Ca. Posts: 3,618	Could it be a TCS (transmission controlled spark) . That only allows vacuum advance in 3rd gear. If it is you will have vacuum line going to vacuum advance on dist. If not I have no idea. John __________________ john http://67-72chevytrucks.com/vboard/showgroups.php board vendors http://www.flickr.com/photos/1745471...7602865984234/

03-22-2005, 10:27 AM	#3
Perchjerker Beer Power!!!!!!!!!! Join Date: Nov 2003 Location: Nacogdoches, TX Posts: 280	Actually, this selonoid on the carb has a place where the vacuum line from the carb goes into it and then a line from it into the vacuumm advance on the dizzy. When this is hooked up correctly, I find that I get a little better gas mileage. __________________ Rather be fishin'

03-23-2005, 11:20 AM	#4
Perchjerker Beer Power!!!!!!!!!! Join Date: Nov 2003 Location: Nacogdoches, TX Posts: 280	Any more help??? __________________ Rather be fishin'

03-23-2005, 03:14 PM	#5
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	John is correct, and so are you for that matter. That is an emmissions control that was common on a lot of vehicles of that vintage. You're is working correctly, which is a surprise in itself! You might get better performace and milage if you bypass the solenoid. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

03-23-2005, 03:40 PM	#6
CPNE Resident Curmudgeon Join Date: Dec 1999 Location: NH Posts: 6,662	It is a Fast Idle Solenoid A fast idle solenoid, also known as an antidieseling solenoid, opens the carburetor throttle plates during engine operation but allows the throttle plates to close as soon as the engine is turned off. In this way, a faster idle speed can be used while still avoiding dieseling (engine keeps running even though the ignition key is turned off). This is a particular problem with newer emission controlled vehicles due to higher operating temperatures, higher idle speeds, leaner fuel mixtures, and lower octane fuel. When the engine is running, current flows to the fast idle solenoid, causing the plunger to move outward. The throttle plates are held open to increase engine speed. The plunger is adjustable, so the idle speed can be adjusted. When the engine is turned off, current flow to the solenoid stops. The solenoid plunger retracts and the throttle plates are free to swing almost closed. It is working correctly Attached Images __________________ Currently on or near the homestead: 67 Chevy SWB 2WD stepside 350/3 on tree (Pat's) 67 GMC SWB 2WD Fleet 402/auto (Brian's under construction) 67 Chevy 3/4 ton 2WD 402/auto (Business Hauler) 67 Chevy 1 ton dually 2WD 396/4 speed (Former business hauler, Needs TLC) 68 Chevy 1/2 ton Suburban 2WD 250 six/3 on tree (Brian's Needs TLC) 70 Chevy 3/4 ton 4WD 350/4 speed (Pat's - Disguised as a 68 GMC) 71 Chevy SWB stepside (Crushed by tree - parts donor) 72 Chevy 3/4 ton 4WD (Parts donor) 72 Chevy 3/4 ton 4WD Suburban (Parts Donor) 72 GMC 3/4 ton 4WD 292 six/4 speed (Mine - Disguised as a 67 GMC) 81 GMC 4WD Dually Dump Body 350/4 speed (Business Hauler) 82 Camaro Z/28 355/Super T-10 (Pat's toy) 93 Caprice 9C1 (Brian's Cop Car) 02 Toyota Camry (Reliable but a souless steel and plastic hulk) 2011 2SS RS Camaro M6 Factory Hurst Shifter Maybe I need to sell some of this crap Yet another Bozo with a sawz-all Last edited by CPNE; 03-23-2005 at 03:45 PM.

03-23-2005, 04:10 PM	#7
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	CPNE, Your example doesn't cover the vacuum hoses he asked about. I don't remember if trucks with v-8s had them, but cars DID have a compo unit of the vacuum controlled spark advance system and a idle control solenoid in one. Perchjerker, is the solenoid mounted on the drivers side of the carb or on the pass side on the manifold? __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

03-23-2005, 06:09 PM	#9
john member #16 Join Date: Dec 1999 Location: Yuba City Ca. Posts: 3,618	The TCS solenoid is controlled by a switch on the trans and a relay on the firewall to control the vacuum. I dont have a diagram. It also has a temp switch so it only works after engine is warm John __________________ john http://67-72chevytrucks.com/vboard/showgroups.php board vendors http://www.flickr.com/photos/1745471...7602865984234/ Last edited by john; 03-23-2005 at 06:40 PM.

03-23-2005, 08:19 PM	#10
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	I had the TCS on my truck...no part of it was on the carb __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

03-23-2005, 09:02 PM	#11
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	The solenoid valve on your engine is controlled by temperature of the engine, (over 82 degrees F and whether or not the truck is in direct drive, (high gear). It should not move when the truck is in any other gears. It is a dual function switch. It serves as a "deceleration hydrocarbon control" as it keeps the idle speed up when in high gear. It also opens up a valve that allows vacuum to go to the distributor. Anti-dieseling on AC equipped trucks is done by a temporary turning on of the compressor when the key is turned off. The service manual has an excellent write up on the system which covers more details thanI have listed. I can fax the pages to you next week if you don't have the manual. Jim

03-24-2005, 01:04 AM	#12
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	I don't think the idle up unit was used on the 4 bbl. At least numberoneson's '72 350 q-jet has the solenoid mounted on the intake. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

03-24-2005, 03:45 PM	#13
Perchjerker Beer Power!!!!!!!!!! Join Date: Nov 2003 Location: Nacogdoches, TX Posts: 280	68sppedalter, the seloniod is on the driver's side of the carb. It has an electical connection at the bottom end and one vacuum line going into it from the carb and one vacuum line going out of it into the vacuum advance on the dizzy. I dont' think that my selonoid is controlled by temperature, but I could be wrong. It seems to work only if I put the truck into second gear (on the three on the tree shifter) and will disengage if I take it out of second gear. My carb is a 2 bbl. The truck is an original non-AC truck. I have played with disconnecting it for a while, and the truck seems to run better and get a little better gas mileage when it is hooked up correctly---I don't know why. It sounds like the thing should be engaging in 3 rd gear, not second. ANy more info will be appreciated. Jimkshortstep4x4, I don't have a service manual, but it would be nice to have one. I'll pm you my fax number. Thanks, Perch __________________ Rather be fishin'

03-24-2005, 07:59 PM	#14
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	"68sppedalter" Who the heck??? JimKshortstep is right, it should work in 3rd for sure. With truck idling, feel for vacuum at the hose coming from the carb. If you don't feel any, rev up the engine and you should feel some. If it operates this way, just hook a hose from the carb to the dizzy and don't worry about the solenoid If you feel vacuum at an idle, it will still work to run the hoses like I describe, you may need to readjust your idle speed afterwards. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

03-25-2005, 06:02 PM	#15
Perchjerker Beer Power!!!!!!!!!! Join Date: Nov 2003 Location: Nacogdoches, TX Posts: 280	I don't have to feel for the vacuum to know when it is engaging. I can just put it into second gear and it wil engage and the rpms will rise slighty. But, like I said, if I take it out of second it will disengage. I'm just wondering why and if it needs to be adjuted, how do I do it. Is it controlled by the transmission or a part in the shift colum??? Thanks, Perch __________________ Rather be fishin'

03-26-2005, 12:44 AM	#16
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	It has a switch on the transmission that grounds the wire that goes to the relay that then sends voltage to the solenoid on the carb that extends the solenoid that opens the vacuum passage that allows the vacuum to pass from the intake to the vacuum advance and advance the timing and lower emmissions and increase gas mileage. Wew! It should send vacuum in 3rd gear. There is an adjustment for the idle speed as referred to by CPNE. It has no bearing on the amount or when vacuum is sent to the dizzy. My suggestion for feeling for vacuum was for BYPASSING the solenoid . Since you're idling when you are placed it in 2nd and the idle increased, you have manifold vacuum to the solenoid. Unless you are restoring the truck, there really isn't much need for the stuff. You will get better performance and fuel mileage by BYPASSING the vacuum portion of the solenoid. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab



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