#1  
Old 10-08-2018, 03:16 PM
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Default Hood Scoop Controls

The original hood scoop wiring and vacuum controls were mucked up by a previous owner. I'd like to restore the "hood air valve" to proper operation.

Read Rocky Rotella's article about shaker scoops in Hot Rod, online. So, if I understand correctly, the vacuum controlled relay inside the scoop is fed 12V through a passenger side cylinder head located thermostatically controlled switch. That relay, in the scoop, is fed manifold vacuum so that the air valve only operates when the engine is over 140 degrees and manifold vacuum is at a minimum. The relay feeds 12V to the air valve solenoid when the engine is warm and throttle is depressed. Makes sense.

The air valve solenoid, vacuum switch and scoop mounted bulkhead connector are in place, in my car. But that's where I lose it.

Looked in my 74 Service Manual and saw vague references to the "hood air valve", but not much else. The thermostat is allegedly fed from a point "A" with a yellow wire. The other side of the thermostat goes to the scoop vacuum controlled relay via another yellow wire, along with a ground, to the 2-pin "T" form connector.

I can't find point "A", the thermostat or yellow wires on my car. Some previous owner made a crappy little harness from two 1/4" quick-connect flat blade connectors and a couple of pieces of #14 AWG.

So my questions are thus:

  • Where is the 140 degree thermostat located? Exhaust side, or rear of the head?
    • Where is the source of 12 volts, point "A" (where lots of under hood engine wires connect)? It's not at the alternator, but I suppose it could be a soldered connection inside of the engine wiring harness.
  • Does the switch actually have two yellow wires connected to it? The thermostat on the back of my passenger head has two different colored wires, neither of which is yellow.
  • Has anyone experimented using an oil pressure controlled switch on the negative side of therelay wiring circuit? It would be necessary to find a normally open pressure switch to fit 1/4" or 1/8" pipe threads in the engine (at the distributor). The typical oil light switch is normally closed; i. e. closes contacts on pressure drop. I'd like that better than the thermostatically controlled switch.
    • Is the scoop bulkhead connector available anywhere?

Thanks!

Stuart
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  #2  
Old 10-09-2018, 12:23 PM
SD455DJ SD455DJ is offline
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Slick - Does this help any?

Dennis

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Old 10-09-2018, 12:24 PM
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...helps if I add the picture...geesh!
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Old 10-09-2018, 07:00 PM
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SD455DJ,

Thanks, that identifies the source as the radio fuse. But where is the thermal switch located? I see a sensor of some sort on the rear of the passenger side head, it has two wires, but none are yellow. It otherwise seems to fit the bill....

Stuart

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Old 10-09-2018, 07:17 PM
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Stuart - I'll try to get over to take a few pictures of my 74 (hope it was done correctly) and post them for you. It may take me a few days to get a chance to get over to where the car is to do that, if you can wait.

Dennis

  #6  
Old 10-09-2018, 09:59 PM
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Quote:
Originally Posted by SD455DJ View Post
Stuart - I'll try to get over to take a few pictures of my 74 (hope it was done correctly) and post them for you. It may take me a few days to get a chance to get over to where the car is to do that, if you can wait.

Dennis
Thank you Dennis, that would be great. I won't be able to get back to my car until Saturday anyway.

Stuart

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Old 10-22-2018, 03:16 PM
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Slick - Here are the pictures of the connections of my hood scoop...the electrical coming of the engine wire loom running across the driver side valve cover to the bottom of the scoop body and the vacuum line originating at the carb's main body and running to the nipple on the underside of the scoop base (3) next to the electrical plug. There were no other connectors of either type except inside the scoop to the solenoid actuator.

I hope this helps.

Dennis
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Old 10-23-2018, 11:06 AM
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Quote:
Originally Posted by SD455DJ View Post
Slick - Here are the pictures of the connections of my hood scoop...the electrical coming of the engine wire loom running across the driver side valve cover to the bottom of the scoop body and the vacuum line originating at the carb's main body and running to the nipple on the underside of the scoop base (3) next to the electrical plug. There were no other connectors of either type except inside the scoop to the solenoid actuator.

I hope this helps.

Dennis
Thanks for the photos Dennis.

First of all, let me say that your engine is so pretty it makes my eyes hurt!

My scoop connections are the same, I just lack that connector/harness from the engine harness.

Here is my issue: Where does the white wire go?

I know that the black wire is grounded. The white wire connects, at the load end, to the vacuum switch inside of the scoop. On the OTHER end, it connects to the radio fuse, BUT with a thermal switch in the circuit SOMEWHERE. Do you have any pictures of where that switch is, what it looks like, etc.?

As I said, in my 1974 Pontiac Assembly Manual and 1974 Pontiac Service Manual, they reference the switch, along with references in Rocky Rotella's Hot Rod article and the schematic posted earlier in the thread.

I can't find this switch on my car, nor what it looks like or where it installs.

If I could find a "make-on-rise" oil pressure switch, it might be a better solution to the issue of the scoop opening as soon as the ignition is activated, but before vacuum is present with the engine running.

Thanks,

Stuart

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Old 10-23-2018, 11:48 AM
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Stuart, I'll continue to see if we can find anything. In the meantime, here is the switch on the back of my passenger head with blue & black wires.

Dennis
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  #10  
Old 10-23-2018, 12:28 PM
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Danny,

I have the same switch. Though that might be it, but the service manual indicates that is an emissions switch and the colors are wrong. The sensor with one wire that's in (should be) your passenger head is also emissions. It activates vacuum advance if the engine gets too hot IIRC.

I could easily be wrong about all of this, those manuals are tough to read.

Stuart

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Old 10-23-2018, 12:31 PM
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According to the schematic you posted, both of the wires on the "thermal feed switch" should be yellow. The wires to the scoop connector should be yellow and black, if that drawing is correct.

Stuart

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Old 10-23-2018, 05:38 PM
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Stuart, I'll look again for more yellow wires...

Dennis

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Old 10-25-2018, 11:07 AM
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Quote:
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Stuart, I'll look again for more yellow wires...

Dennis
Hey Dennis, I think I found the answer, but it raises more questions.

Have a look at this document listing the scoop control parts.

The "thermal delay switch is on the back of the passenger cylinder head BELOW the emissions sensor that you posted a picture of in your last posting.

I'll have to look there on my car to see if it exists.

It has an associated gasket, so it may somehow fit into the rear casting (freeze) plug in the head.

Still wondering about those yellow wires, and why is the idle stop solenoid referenced in the hood scoop/induction control schematic?

Very interesting!

Stuart
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  #14  
Old 10-25-2018, 01:09 PM
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Very interesting Stuart. I'll have to see what my car has the next time I'm over there. It does look like something either in the head freeze plug or possibly bolted on the back with a bracket to the block/bellhousing???

Dennis

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Old 10-26-2018, 04:57 PM
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Stuart, Here is a better picture of the lower heat thermal switch with the YELLOW wires to it. The switches look similar, but I'm not sure they are the same part number. The wiring harness is an exact factory replacement from M&H of the original. I hope this solves the mystery.

Dennis
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  #16  
Old 10-27-2018, 08:26 AM
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Thank you Dennis! Great picture and mystery solved!

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Old 10-27-2018, 12:14 PM
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Glad to help.

Dennis

  #18  
Old 10-27-2018, 01:39 PM
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So, that's going to work with the automatic tube and dipstick?
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Old 10-27-2018, 10:18 PM
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It should. They're supposed to be on either stick or auto cars. Plan to check mine tomorrow. I'll let you know. If the dipstick is an issue, no reason not to fit it to the rear of the driver's side head.

  #20  
Old 10-30-2018, 05:06 PM
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Just revisiting. I was at one of the storage areas today and snapped a picture or three of the 350/manual 3 speed head and wiring. Showing position and matching AC 140M with 2713 date code.
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