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Hobbs Switch

2859 Views 12 Replies 6 Participants Last post by  SilverMk2
OK, my Spearo Turbo kit has a Hobbs switch that is supposed to turn the intercooler pump on at 3 psi. Right now, I can't tell if this thing is working or not, so I just use a switch to turn it on an off. However, I would like to get this working correctly. Is there an easy way to test if the Hobbs switch is working properly? Also, where do you get Hobbs switches if I just want to go ahead and replace it? Thanks.
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If you can make 3 psi in neutral (which I imaging shouldn't be a problem), just test it with a multi-meter, while someone rev's the motor a bit. As for replacing it, I would imagine most speed shops would have them...
I would run pump all the time to keep heat soak down.Mine runs all the time also increased reservoir capacity and installed larger pump.
You have an air to water intercooler? I didn't realize that. Is it the Spearco setup, or something else? I upgraded to a larger capacity pump, but have not added a resevoir. What did you use and where did you locate it? I was considering adding a Transmisson style cooler to add more capacity to the system. Right now I am probably at 2/3's of a gallon right now. You are running 15 psi right now, right? Very cool.

So you run your pump all the time. How long does a pump last you?
I have a Cartech air to water IC. I made a larger resevoir from 3" pvc and capped top and bottom off and added in and out fittings and a fill plug on top ,you can see the flow in the tank circulating.installed near power steering resevoir.The pump I use is for an RV water system that can pump 2 gal a minute,and is fairly large and noisy but has ran fine for over 7 years.I have about a 2 gal capacity.15 psi but need better fuel control(waiting for that great SS exchange) engine can handle 20+ way over built.Use a cooler with a cross flow pattern(EARLS) and not a single tube with a back and forth pattern.
youll need someone else to do it with you...but just either put you foot on the brake, slip the clutch and gas it til you hit 3psi while a buddy is on the end with a voltmeter on the 2 terminals to see if the hobbs switch closes the just pull the e brake and try and build enough boost to open it. Hobbs switches are usually like $40

Buy a MITYVAC and do it the safe way,a must have tool for working on turbo cars.
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aw ic

Any of you guys have some pictures of your air-water intercooler setups? I'd like to see them...


Jimi B
Try this link:

If that doesn't work, go to the Yahoo Groups MK2 list, got to Files, look down for the Demon folder, then click on the supraengine2 pic. That is the Spearco Glycol Charge Cooler in the middle. This is a really old pic from when Mike Wilkes owned the car. I recently had it recored because it was leaking coolant into the intake.
Wow, that is one sweet looking engine bay! I love how you redid all the hoses 8)

What turbo are you using, and how is it plumbed into the charge-cooler? It looks like the intercooler is sitting right on top of the turbo.....

Was that unit designed for the MK2, or is it a generic application?
Turbo is a TO4B and the intercooler was designed for the supras back in 84 by Corky Bell.Turbo sits right below intercooler,the reservoir is near air filter.Boost comes on fairly quick after 2000 RPM due to a short distance to the intake and can hit 15 psi before 3500 RPM depending on load.
I found this by accident at work but Allied electronics lists Hobbs pressure switches in 4psi w/ 1/8NPT thread. They have NO, NC, and DC3T forms. Prices are $14.84, $24.23, $30.45 respectively. Their website is supposed to be (1-800-433-5700). You might have to watch out some of these places have minimum orders (I know Digi-key is $20).
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