Speed switch - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mabu · 10-29-2009, 08:25 AM

I have searched but cannot find anything, can someone please tell me where the speed switch is located? (Not speed switch solenoid)

68 TT · 10-29-2009, 08:42 AM

I haven't hear anybody talk about a speed switch before. Are you talking about a part of the cruise control system?

mabu · 10-29-2009, 08:53 AM

Truck is a 1980 originally 6 cyclinder now has a 350 but missing a bunch of things. Including the speed switch solenoid. It is a California truck so I need all the smog junk to be on the truck. In the wiring diagrams it shows a speed switch with wiring going to the speed switch solenoid located on the carburetor.

mabu · 10-29-2009, 11:23 PM

Anyone?

TravisMartin · 10-30-2009, 03:18 AM

The only solenoid I can think of on an '80 carb would be a fast idle solenoid to compensate for the air conditioning compressor. But I'm not sure if that's what you're talking about. On my 3/4 ton '82 with the HD emissions 350, they had a vacuum operated fast idle with a solenoid valve to provide manifold vacuum to it when the compressor runs.

I bought an '80 new but it's been way too long from me to remember that detail.

Where did you get the diagram you're working from? Can you scan it and post it? I have a factory shop manual for an '82 but it's loaned out right now; I can look when I get it back and see if the '82 had something similar.

Travis

TravisMartin · 10-30-2009, 03:24 AM

I just had another thought; perhaps it was TCS (transmission controlled spark) that you're looking at. I don't recall whether an '80 would have that or not; perhaps someone else will clarify.

In a nutshell, it's a system where a switch on the transmission closes in high gear and turns on a solenoid mounted on the intake near the carb that switches vacuum ports to the distributor's vacuum advance.

This system was installed commonly on the '70 Chevelles I used to fool with but I don't remember whether it was still around by '80.

Another possibility...I definitely remember the '80 having a lockup torque converter, in fact I think '80 was the first year for it, and it was controlled by a vacuum operated switch on the firewall.

But to my knowledge none of these were labeled a "speed switch" so more info would sure be helpful.

mabu · 10-30-2009, 08:29 AM

Wiring diagrams from a post on this site:
http://67-72chevytrucks.com/vboard/s...wiring+diagram

http://www.73-87chevytrucks.com/tech...80_SBC_eng.jpg
also haynes book shows the same.
If it is only related to the ac or cruise control it would be nice, I could ignore it for now.

mabu · 10-30-2009, 09:37 PM

?Anything you see?

TravisMartin · 10-31-2009, 02:01 AM

I really don't know what to make of it; don't recall ever seeing such a thing.

The schematics you found do tell me this much: the "speed switch" is drawn as a relay that is connected to the tachometer output of the HEI distributor. That would imply that it changes state at some engine RPM. You can see that its contacts are connected to the "speed switch solenoid" so the solenoid activates either above or below some RPM; it doesn't indicate which.

Now the mystery, at least to me, is...what the heck does it do? And is it necessary?

My best guess is it's something to do with emissions controls of the day, but that's only a guess. Maybe EGR? When did EGR become required?

Unless someone comes up with better information, I'd ignore it for now.

Travis

TravisMartin · 10-31-2009, 02:19 AM

I looked at the drawings for all the engines that were posted and notice that all have this "speed switch" except the 250CID six; even the 292 has it.

The more I think about it, the more I'm betting it's EGR control. EGR is the big vacuum operated valve on the intake manifold that bleeds some exhaust gas into the intake for reduced emissions. If I'm right, the "speed switch solenoid" will be a solenoid operated vacuum valve mounted on the intake somewhere. It'll have a couple of vacuum lines coming from it, one to the EGR valve and the other to the carb. There should be a vacuum hose schematic drawing on the air cleaner. But if your truck was originally a six, you'd need to look at someone else's drawing.

I know where there's a '78 454 truck parked that's still pretty much original; I'll go look at it and see if it tells me anything. The drawings you posted say they are for '77 to '80 so we'll see. I'll try to have some more info in a couple of days. Might even have some pictures if it looks like they'll help.

Travis

10-29-2009, 08:25 AM	#1
mabu Registered User Join Date: Oct 2009 Location: Rancho Cucamonga, CA Posts: 6	Speed switch I have searched but cannot find anything, can someone please tell me where the speed switch is located? (Not speed switch solenoid)

10-29-2009, 08:42 AM	#2
68 TT Still plays with trucks Join Date: Jul 2009 Location: Spokane, WA Posts: 3,556	Re: Speed switch I haven't hear anybody talk about a speed switch before. Are you talking about a part of the cruise control system?

10-29-2009, 08:53 AM	#3
mabu Registered User Join Date: Oct 2009 Location: Rancho Cucamonga, CA Posts: 6	Re: Speed switch Truck is a 1980 originally 6 cyclinder now has a 350 but missing a bunch of things. Including the speed switch solenoid. It is a California truck so I need all the smog junk to be on the truck. In the wiring diagrams it shows a speed switch with wiring going to the speed switch solenoid located on the carburetor.

10-29-2009, 11:23 PM	#4
mabu Registered User Join Date: Oct 2009 Location: Rancho Cucamonga, CA Posts: 6	Re: Speed switch Anyone?

10-30-2009, 03:18 AM	#5
TravisMartin Curmudgeon of Poor Means Join Date: Jan 2009 Location: Lubbock, Texas Posts: 113	Re: Speed switch The only solenoid I can think of on an '80 carb would be a fast idle solenoid to compensate for the air conditioning compressor. But I'm not sure if that's what you're talking about. On my 3/4 ton '82 with the HD emissions 350, they had a vacuum operated fast idle with a solenoid valve to provide manifold vacuum to it when the compressor runs. I bought an '80 new but it's been way too long from me to remember that detail. Where did you get the diagram you're working from? Can you scan it and post it? I have a factory shop manual for an '82 but it's loaned out right now; I can look when I get it back and see if the '82 had something similar. Travis

10-30-2009, 03:24 AM	#6
TravisMartin Curmudgeon of Poor Means Join Date: Jan 2009 Location: Lubbock, Texas Posts: 113	Re: Speed switch I just had another thought; perhaps it was TCS (transmission controlled spark) that you're looking at. I don't recall whether an '80 would have that or not; perhaps someone else will clarify. In a nutshell, it's a system where a switch on the transmission closes in high gear and turns on a solenoid mounted on the intake near the carb that switches vacuum ports to the distributor's vacuum advance. This system was installed commonly on the '70 Chevelles I used to fool with but I don't remember whether it was still around by '80. Another possibility...I definitely remember the '80 having a lockup torque converter, in fact I think '80 was the first year for it, and it was controlled by a vacuum operated switch on the firewall. But to my knowledge none of these were labeled a "speed switch" so more info would sure be helpful.

10-30-2009, 08:29 AM	#7
mabu Registered User Join Date: Oct 2009 Location: Rancho Cucamonga, CA Posts: 6	Re: Speed switch Wiring diagrams from a post on this site: http://67-72chevytrucks.com/vboard/s...wiring+diagram http://www.73-87chevytrucks.com/tech...80_SBC_eng.jpg also haynes book shows the same. If it is only related to the ac or cruise control it would be nice, I could ignore it for now.

10-30-2009, 09:37 PM	#8
mabu Registered User Join Date: Oct 2009 Location: Rancho Cucamonga, CA Posts: 6	Re: Speed switch ?Anything you see?

10-31-2009, 02:01 AM	#9
TravisMartin Curmudgeon of Poor Means Join Date: Jan 2009 Location: Lubbock, Texas Posts: 113	Re: Speed switch I really don't know what to make of it; don't recall ever seeing such a thing. The schematics you found do tell me this much: the "speed switch" is drawn as a relay that is connected to the tachometer output of the HEI distributor. That would imply that it changes state at some engine RPM. You can see that its contacts are connected to the "speed switch solenoid" so the solenoid activates either above or below some RPM; it doesn't indicate which. Now the mystery, at least to me, is...what the heck does it do? And is it necessary? My best guess is it's something to do with emissions controls of the day, but that's only a guess. Maybe EGR? When did EGR become required? Unless someone comes up with better information, I'd ignore it for now. Travis

10-31-2009, 02:19 AM	#10
TravisMartin Curmudgeon of Poor Means Join Date: Jan 2009 Location: Lubbock, Texas Posts: 113	Re: Speed switch I looked at the drawings for all the engines that were posted and notice that all have this "speed switch" except the 250CID six; even the 292 has it. The more I think about it, the more I'm betting it's EGR control. EGR is the big vacuum operated valve on the intake manifold that bleeds some exhaust gas into the intake for reduced emissions. If I'm right, the "speed switch solenoid" will be a solenoid operated vacuum valve mounted on the intake somewhere. It'll have a couple of vacuum lines coming from it, one to the EGR valve and the other to the carb. There should be a vacuum hose schematic drawing on the air cleaner. But if your truck was originally a six, you'd need to look at someone else's drawing. I know where there's a '78 454 truck parked that's still pretty much original; I'll go look at it and see if it tells me anything. The drawings you posted say they are for '77 to '80 so we'll see. I'll try to have some more info in a couple of days. Might even have some pictures if it looks like they'll help. Travis Last edited by TravisMartin; 10-31-2009 at 02:29 AM.



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