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Old 11-25-2015, 08:44 PM
AzDave AzDave is offline
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Default How to check a pressure switch.

At Veggiegarage here is how I check a pressure switch.

There are 3 wires.
Black Common (C)
Red Normally Open (NO)
Blue Normally Closed (NC)

With less than 55 PSI (Ford) or 10 PSI (Dodge) the C and NC will have a closed circuit. 0 ohms

With more than 55 PSI (Ford) or 10 PSI (Dodge) the C and NO will have a closed circuit. 0 ohms.

So with switch removed use an ohm meter. check for 0 ohms between C and NC and open circuit between C and NO.

Now I have a 1/4 npt fitting with an air fitting to my air hose. Air up compressor to desired pressure say 60 pounds for Ford and 15 pounds for Dodge. Apply pressure to the pressure switch and recheck.

With pressure you want 0 ohms between C and NO and open circuit between C and NC.

Check both sides of switch with no pressure and pressure. You can have a switch that has continuity between all terminals.

With timer cards the C or Black wire connects to the diesel power wire that did go to the diesel pump positive terminal.

The NO or Red Wire goes to the green light in the Veggie Switch.

The NC or Blue wire goes to the Diesel pump positive terminal.

Hope this helps. Dave at Veggiegarage.com
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  #2  
Old 11-27-2015, 03:25 PM
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Beautiful write up Dave. Thank you. I get a lot of calls that end up being a bad PS. Now I can direct them here.

Many people don't understand what the PS does. The PS directly interrupts power to the D2 pump. Then, it sends a signal to the SSB when it does this. So, less than 55/10psi - power goes to the pump. Over 55/10psi, 12v signal is sent to the SSB.

Also, the PS does not see D2 pressure - only VO pressure. (it is before the V3/V4 check valve)

Quote:
With less than 55 PSI (Ford) or 10 PSI (Dodge) the C and NC will have a closed circuit. 0 ohms
Without pressure testing, if this circuit is open with system/engine off - the switch is bad. This can also be tested with a test light by turning the key on and checking for 12V (within 30secs of turning the key on).

If this ^^^ happens, use your jumper plug to bypass the PS and send power to the pump all the time. You may consume some D2 when switched over to VO when using the jumper. This is because the D2 pump will be on and the fuel with the most pressure will make it to the engine. So, if your D2 pressure is 55psi (stock) and VO pressure drops below that when accelerating, etc - the engine will get D2 until the pressure comes back up.
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Old 11-29-2015, 09:11 PM
liska liska is offline
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I tested the PS in the manifold without removing it yet, too cold today to be in the garage. When I tested the C (black wire) and NC (blue wire) my multimeter and my reading was between 0.2-0.4 (sometimes 0.5) Ohms.

Before I pull the switch I wanted to ask if the above readings indicate a faulty switch, if so I wont bother pulling it and testing under pressure, I will just replace it.

If the reading MUST be 0.0 ohms and I get any reading besides 0.0 then it is faulty, correct?

Let me know, thanks.
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Old 11-30-2015, 01:55 PM
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skyskijason skyskijason is offline
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Quote:
Originally Posted by liska View Post
I tested the PS in the manifold without removing it yet, too cold today to be in the garage. When I tested the C (black wire) and NC (blue wire) my multimeter and my reading was between 0.2-0.4 (sometimes 0.5) Ohms.

Before I pull the switch I wanted to ask if the above readings indicate a faulty switch, if so I wont bother pulling it and testing under pressure, I will just replace it.

If the reading MUST be 0.0 ohms and I get any reading besides 0.0 then it is faulty, correct?

Let me know, thanks.
I'm not sure how much resistance is too much?

Can you check if 12v is making it through the switch?
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Old 12-01-2015, 12:12 AM
liska liska is offline
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Quote:
Originally Posted by liska View Post
When I tested the C (black wire) and NC (blue wire) my multimeter and my reading was between 0.2-0.4 (sometimes 0.5) Ohms.

If the reading MUST be 0.0 ohms and I get any reading besides 0.0 then it is faulty, correct.
OK so I pulled the PS and tested it under pressure ~15psi. I got a reading of 0.2 ohms.

So I go back to my original question, if I get any other reading besides 0.0 ohms does that mean the PS is bad?
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Old 12-03-2015, 02:21 PM
liska liska is offline
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Can anyone chime in if the resistance readings I am getting on my PS are within standards or does it have to be 0.0 ohms?
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Old 12-09-2015, 03:26 PM
AzDave AzDave is offline
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0.2 or 0.5 ohms is in your leads or connections. Close enough to zero.
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Old 03-13-2016, 07:51 PM
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Quote:
Originally Posted by skyskijason View Post
Without pressure testing, if this circuit is open with system/engine off - the switch is bad. This can also be tested with a test light by turning the key on and checking for 12V (within 30secs of turning the key on).
Could you please clarify which wire would be getting 12V if faulty, and how to get a test light on the wire if the plug is connected? Not sure where you can get a test light in there if there is no exposed wire!
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Old 03-13-2016, 08:34 PM
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Quote:
Originally Posted by biostroker View Post
Could you please clarify which wire would be getting 12V if faulty, and how to get a test light on the wire if the plug is connected? Not sure where you can get a test light in there if there is no exposed wire!
Use a test light that has a sharp pointed tip to stab the wire. Careful not to stab your finger! Dont ask me how I know that lol...
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Old 03-13-2016, 10:19 PM
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Quote:
Originally Posted by Clay View Post
Use a test light that has a sharp pointed tip to stab the wire. Careful not to stab your finger! Dont ask me how I know that lol...
Ha! That's the only way I could think of. Thanks for clarifying!
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