Anti-dieseling solenoid questions - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**IT Cowboy** · 02-28-2026, 10:36 PM

I connected my anti-dieseling solenoid on my Rochester carb tonight as it was not getting power. I connected to the 12v coil positive (hope that is ok?)

As I understand the solenoid is supposed to actually control the idle speed and the actual idle screw is only supposed to make contact at shutdown when the solenoid no longer has power.

My solenoid doesn't seem to have enough power to push the throttle idle beyond 800 rpm. That is about what the idle is supposed to be but is my solenoid bad? I put some brake cleaner on it and a little lube so I will see if it improves. Also is how I adjusted it correct?

truckster · 03-01-2026, 12:46 AM

Yours would probably be the only one in the known universe that hasn't been tossed. Set the timing and idle correctly and you won't need it.

howzzzit · 03-01-2026, 02:28 AM

Quote:

Originally Posted by IT Cowboy

I connected my anti-dieseling solenoid on my Rochester carb tonight as it was not getting power. I connected to the 12v coil positive (hope that is ok?)

As I understand the solenoid is supposed to actually control the idle speed and the actual idle screw is only supposed to make contact at shutdown when the solenoid no longer has power.

My solenoid doesn't seem to have enough power to push the throttle idle beyond 800 rpm. That is about what the idle is supposed to be but is my solenoid bad? I put some brake cleaner on it and a little lube so I will see if it improves. Also is how I adjusted it correct?

My 72 has the original solenoid. Works fine. When key is off it retracts. Key on it pops forward opening the throttle to idle speed of your choice. You can adjust the idle while running by turning the square on the end of the shaft. CCW decreases idle and CW increases idle. I find it quite handy while setting dwell, timing, and best lean idle. Of course I don't do that everyday. I guess it's each person's preference.

Dashman · 03-01-2026, 11:17 AM

The positive coil wire does not have 12 volts. Have you measured 12 volts at the coil or the solenoid? Why not use the wire diagram to get 12 volts from the original source?

**IT Cowboy** · 03-01-2026, 12:11 PM

Quote:

Originally Posted by Dashman

The positive coil wire does not have 12 volts. Have you measured 12 volts at the coil or the solenoid? Why not use the wire diagram to get 12 volts from the original source?

I have an aftermarket coil and distro.

**kwmech** · 03-01-2026, 01:27 PM

Majority of these solenoids were on AC trucks and didn't engage without AC on (of course) and the slight depression of the gas pedal to let it actuate. They will not usually just step it up on their own. Base idle is set by the screw on the carb and solenoid adjusted by the hex. Only about a 150-200rpm idle difference

howzzzit · 03-01-2026, 03:10 PM

Quote:

Originally Posted by kwmech

Majority of these solenoids were on AC trucks and didn't engage without AC on (of course) and the slight depression of the gas pedal to let it actuate. They will not usually just step it up on their own. Base idle is set by the screw on the carb and solenoid adjusted by the hex. Only about a 150-200rpm idle difference

My 72 has a/c and this solenoid. Solenoid retracts key off and extends key on without a/c being on. Operation in the service manual.

**IT Cowboy** · 03-01-2026, 08:50 PM

Quote:

Originally Posted by howzzzit

My 72 has a/c and this solenoid. Solenoid retracts key off and extends key on without a/c being on. Operation in the service manual.

Some old rumors said that it was for AC kick up but I dont think that is true. It was to prevent dieseling when shotting down especially with the fuel in the 70's

**Rust_never_sleeps** · 03-01-2026, 09:19 PM

Quote:

Originally Posted by IT Cowboy

Some old rumors said that it was for AC kick up but I dont think that is true. It was to prevent dieseling when shotting down especially with the fuel in the 70's

Easy to sanity check that from the wiring diagram

**c30** · 03-02-2026, 09:53 AM

I always thought that it is there for idle kickup when the ac is on.

**71CHEVYSHORTBED402** · 03-02-2026, 12:07 PM

Seems AI may be right for a change.

"An idle control solenoid on a Rochester Quadrajet acts as an adjustable throttle stop to maintain proper engine idle speed when energized (usually by ignition) and allows the throttle plates to close completely when off, preventing engine "run-on" or "dieseling". It often boosts idle RPM to handle electrical loads like air conditioning."

Seems your 72 would be like my 71 was, all kinds of little wires running off the TCS assembly and such to the solinoid. I tossed the TCS, and am onboard with Truckster's opinion, my solenoid found a forever home in a box.

Curious if yours is A/C and has a Anti-dieseling relay in the cab?

**IT Cowboy** · 03-02-2026, 12:26 PM

Quote:

Originally Posted by 71CHEVYSHORTBED402

Seems AI may be right for a change.

"An idle control solenoid on a Rochester Quadrajet acts as an adjustable throttle stop to maintain proper engine idle speed when energized (usually by ignition) and allows the throttle plates to close completely when off, preventing engine "run-on" or "dieseling". It often boosts idle RPM to handle electrical loads like air conditioning."

Seems your 72 would be like my 71 was, all kinds of little wires running off the TCS assembly and such to the solinoid. I tossed the TCS, and am onboard with Truckster's opinion, my solenoid found a forever home in a box.

Curious if yours is A/C and has a Anti-dieseling relay in the cab?

Mine is AC but no relay I know if. My understanding is that the solenoid maintains the idle speed and when there is no power during shutdown it allows the carb to close all the way starving it of air and preventing dieseling.

I was told to set the idle speed with the solenoid and then put the idle screw right up to touch the plate and then back the idle screw off 1/2 turn.

**IT Cowboy** · 03-02-2026, 12:34 PM

My solenoid isn't retracting like it should, I think the spring broke. Would anyone be willing to put their old one in small box and mail it to me

Stanco · 03-05-2026, 12:05 PM

The solenoid has a high current draw and was powered by a 12 gauge pink wire connected to the ignition terminal of the fuse panel. Connecting it to the ignition coil is a bad idea.

The solenoid is part of the TCS system. In an effort to reduce emissions Chevrolet increased the mechanical advance in the distributor while reducing the initial advance. The TCS system shut down the vacuum advance until the transmission was in high gear. With the reduced initial advance they had to open the throttle more to get it to idle. This caused a dieseling problem on shut down hence the solenoid.

I removed my solenoid and recurved my distributor with 12° mechanical advance (24° at the crankshaft) + 12° initial for 36° total advance all in at 3200 RPM. I'm also running full manifold vacuum advance.

I also removed the rest of the TCS system including all the wiring. I used the 12 gauge pink idle stop solenoid wire to power the Petronix module I installed in the distributor. This large pink wire is controlled by the ignition switch and was perfect to power the Petronix module. The truck idles and runs much better with this tune.

02-28-2026, 10:36 PM	#1
IT Cowboy Senior Member Join Date: Jan 2025 Location: Arizona in a van down by the river Posts: 617	Anti-dieseling solenoid questions I connected my anti-dieseling solenoid on my Rochester carb tonight as it was not getting power. I connected to the 12v coil positive (hope that is ok?) As I understand the solenoid is supposed to actually control the idle speed and the actual idle screw is only supposed to make contact at shutdown when the solenoid no longer has power. My solenoid doesn't seem to have enough power to push the throttle idle beyond 800 rpm. That is about what the idle is supposed to be but is my solenoid bad? I put some brake cleaner on it and a little lube so I will see if it improves. Also is how I adjusted it correct? __________________ 1972 C10 Custom Deluxe Highlander 402 Freedom isn't safe, it requires you to take responsibility for your own life, your own safety, and your own success. It carries with it the possibility of failure and the understanding that you're working without out a safety net. If you give the state the responsibility for any of these you also give up the freedom that accompanies the responsibility.

03-01-2026, 12:46 AM	#2
truckster Registered User Join Date: Jan 2001 Location: Orem, Utah Posts: 8,264	Re: Anti-dieseling solenoid questions Yours would probably be the only one in the known universe that hasn't been tossed. Set the timing and idle correctly and you won't need it. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson 1972 K/5 Blazer CST 4WD 1963 Corvair Monza Spyder Turbo convertible 1995 Camaro Z/28

03-01-2026, 11:17 AM	#4
Dashman Registered User Join Date: Apr 2021 Location: Santa Cruz, NM Posts: 122	Re: Anti-dieseling solenoid questions The positive coil wire does not have 12 volts. Have you measured 12 volts at the coil or the solenoid? Why not use the wire diagram to get 12 volts from the original source?

03-01-2026, 01:27 PM	#6
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,941	Re: Anti-dieseling solenoid questions Majority of these solenoids were on AC trucks and didn't engage without AC on (of course) and the slight depression of the gas pedal to let it actuate. They will not usually just step it up on their own. Base idle is set by the screw on the carb and solenoid adjusted by the hex. Only about a 150-200rpm idle difference

03-02-2026, 09:53 AM	#10
c30 Active Member Join Date: Feb 2015 Location: chicagoland Posts: 108	Re: Anti-dieseling solenoid questions I always thought that it is there for idle kickup when the ac is on.

03-02-2026, 12:07 PM	#11
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,689	Re: Anti-dieseling solenoid questions Seems AI may be right for a change. "An idle control solenoid on a Rochester Quadrajet acts as an adjustable throttle stop to maintain proper engine idle speed when energized (usually by ignition) and allows the throttle plates to close completely when off, preventing engine "run-on" or "dieseling". It often boosts idle RPM to handle electrical loads like air conditioning." Seems your 72 would be like my 71 was, all kinds of little wires running off the TCS assembly and such to the solinoid. I tossed the TCS, and am onboard with Truckster's opinion, my solenoid found a forever home in a box. Curious if yours is A/C and has a Anti-dieseling relay in the cab? __________________ Tony 1971 C10 Custom Deluxe SWB 402/400TH A/C I developed an assembly information kit for restoring my truck from nuts and bolts. It's written in Assy. order, short & simple, packed with all OEM hardware ID; castings; part ID; 100s of part numbers; wiring simple - Resource, and trivial too. Much info. applies to all 67-72 GM, A/C vehicles, esp. trucks W/O saying. Full search ability, including to 700+ images of illus., parts, charts, Assy., points of interest, cab, bed, & front clip cart plans- Specials and cores were serviced out and R excluded e.g. front/rear glass, body/paint, engine core, rebuilt hinges, steering box, trans, etc. The project was in-line with long former professions developing process, policy, specs, demo, written for novices, admin., policy, engineers, development, systems & test - Public & govt 2, gross. Sell soft copy cheaply, PM if interested.Build thread:https://67-72chevytrucks.com/vboard/...d.php?t=730025

03-02-2026, 12:34 PM	#13
IT Cowboy Senior Member Join Date: Jan 2025 Location: Arizona in a van down by the river Posts: 617	Re: Anti-dieseling solenoid questions My solenoid isn't retracting like it should, I think the spring broke. Would anyone be willing to put their old one in small box and mail it to me __________________ 1972 C10 Custom Deluxe Highlander 402 Freedom isn't safe, it requires you to take responsibility for your own life, your own safety, and your own success. It carries with it the possibility of failure and the understanding that you're working without out a safety net. If you give the state the responsibility for any of these you also give up the freedom that accompanies the responsibility.

03-05-2026, 12:05 PM	#14
Stanco Registered User Join Date: Dec 2023 Location: Rural California Posts: 51	Re: Anti-dieseling solenoid questions The solenoid has a high current draw and was powered by a 12 gauge pink wire connected to the ignition terminal of the fuse panel. Connecting it to the ignition coil is a bad idea. The solenoid is part of the TCS system. In an effort to reduce emissions Chevrolet increased the mechanical advance in the distributor while reducing the initial advance. The TCS system shut down the vacuum advance until the transmission was in high gear. With the reduced initial advance they had to open the throttle more to get it to idle. This caused a dieseling problem on shut down hence the solenoid. I removed my solenoid and recurved my distributor with 12° mechanical advance (24° at the crankshaft) + 12° initial for 36° total advance all in at 3200 RPM. I'm also running full manifold vacuum advance. I also removed the rest of the TCS system including all the wiring. I used the 12 gauge pink idle stop solenoid wire to power the Petronix module I installed in the distributor. This large pink wire is controlled by the ignition switch and was perfect to power the Petronix module. The truck idles and runs much better with this tune.



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