The Stovebolt Site Forums The Shop Area Engine Shop Throttle Solenoids

I buried this in a long and rambling post in the lounge. People probably were snoring before they got to it. I'd appreciate any help.

"Does anyone know what energizes/deenergizes the throttle solenoid for a vehicle like mine (73 G10 with a 250 six cylinder and manual transmission) without A/C. The solenoid cycles on when the engine is started but only stays on a couple of seconds. There is a vacuum line on the solenoid and I guessed that when vacuum is applied, the solenoid shuts down. However, my testing has yet to prove this theory. I need to find out how to keep it on long enough to set the idle in accordance with the emissions sticker."

I took a look in two of my books and they didn't show any vac hose connected to the idle solenoid valve? I have a feeling the vacuum hose may in fact actually raise the idle slightly on deceleration "high Vacuum" to help clear the engine out and prevent loading up "smog thing".

Solenoid should stay on when ignition is on, it is to keep engine from 'dieseling' when you shut off the engine. there is no vacuum involved in the circuit

Any idea what the vacuum ports are for? there are two vacuum nipples on the solenoid. One is blocked and the other is connected to ported vacuum from the carb. Of course, both could be the product of someone's work over the years. I do know that the solenoid only stays energized for a matter of seconds. I set the idle by forcing the plunger out and adjusting it, then shutting down, restarting, and running to my tach/dwell meter for a check before the solenoid de-energized. Kind of the Keystone Cops approach to tune-ups.

Sixmaniac:

Your truck has "Transmission Controlled Spark"; an early emissions control gizmo that came out on the '72 GM trucks.

My '72 Chevy originally had it, along with the Rochester MV carb. Then some previous owner removed that carb and substituted an earlier Rochester B carb.
I have yet to get the vacuum advance to work correctly- it presently works "backwards": full advance at idle, and less & less advance as the throttle is opened.

Now, this is how the shop manual says the systems is supposed to work:
The system consists of a switch located on the tranny, a relay on the cowl, and a solenoid on the carb that also has a couple of vacuum nipples.
The vacuum hose goes from the carb base to the solenoid, then to the distributor.

The idea is that the distributor vacuum advance gets vacuum only when the engine is cold and in first or second gear.
When running in "high gear", the vacuum advance is "locked-out", and the solenoid is "energized", which causes that little plunger to move out slightly, and prevent the throttle from closing completely, thus (hopefully) preventing over-rich mixture on deceleration.

This is somewhat paraprased; the big point is, if it's been altered, you'll have a lot of trouble getting the engine timed.

My best suggestion is to get a factory shop manual for your truck: it has a pretty clear description of the TCS system and how to work on it. ($30.00 from Jim Carter or one of the other suppliers- IT'S WORTH IT !).

When the Spring breaks, I'm going to scrounge up the missing parts to put the TCS back on my truck, and save the Rochester "B" for an earlier vehicle.

Frank,
Thanks for your help. I think other folks probably thought I was crazy with my description of the solenoid. Before this vehicle, I had only seen the solenoids that automatically energize to maintain idle when the AC is on, or as described in earlier posts, stay on all the time and then shut down to slow the engine and prevent dieseling at shutdown. I've put HEI on the van and was really shocked when I couldn't find a traditional ported vacuum outlet on the carb. All three ports are more like manifold vacuum with a strong "signal" even at idle.