Vacuum Jumps around
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Cargo Master
Joined: Oct 2010
Posts: 2,339
Likes: 32
From: Lost In a Pit of Despair
Vacuum Jumps around
I hooked up a mechanical vacuum gauge today while I had the dash tore apart for some wiring, and when I took it out after everything was back together, I noticed that whenever I went to shift, the needle jumped up really quick then dropped back down.. I'm not quite sure whats going on. Is this normal? FWIW, its a 76 f250 300/np435
(pardon the shakey nighttime video. It was the best I could do holding/driving at the same time)
Thanks
Jameson
(pardon the shakey nighttime video. It was the best I could do holding/driving at the same time)
Thanks
Jameson
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
It's normal behavior with a manual trans, when you disengage the clutch to shift gears you're momentarily unloading the motor.... therefore, vacuum will spike.
less load = more vacuum
more load = less vacuum
How to Use and Interpret a Vacuum Gauge
less load = more vacuum
more load = less vacuum
How to Use and Interpret a Vacuum Gauge
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Your dad's correct, the "Low Fuel Economy" dash light, it basically just measured the manifold vacuum signal, when it dropped below a specified threshold it would illuminate the dash light, pretty useless really.
Post Fiend
Joined: Jan 2006
Posts: 5,037
Likes: 9
From: Kasson, Minnesota
I've got one of those "low fuel Econ" lights on my 75 supercab.
Cargo Master
Joined: Apr 2010
Posts: 2,600
Likes: 64
From: Billings, MT
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Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
If i'm not mistaken '75 was the only year they were offered in the F-series trucks, maybe late '74 too but i'm not certain...
I've read the low fuel economy lights were in response to the '73 OPEC oil embargo, but Ford ditched them after '75
I've read the low fuel economy lights were in response to the '73 OPEC oil embargo, but Ford ditched them after '75
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More Turbo
Joined: Dec 2010
Posts: 619
Likes: 0
From: Liberty Hill, Tx
I have a couple of these dashes, and my plan for them is when I get around to it I want to put a FI 300 from a 90 F-150 in one of these trucks and I want to use that light as my check engine light, therefor it will look stock and will have a check engine light
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
The switch that operates the light is mounted on the firewall and has a vacuum line to the engine.
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