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Problems with oil pressure switches

Thanks, buddy, for continuing to educate me about relay operation. I guess i always thought a 12v relay was just that, a 12v relay, nothing special, all the same. Now I find that there are differences and they make a difference. I checked the one that I had on the truck and it has a resistor in parallel with the solenoid. The resistance reading was the same both directions. Apparently a resistor can diminish the voltage spike from the collapsing coil field but not eliminate it. So I think I will check around and see what other relays I might have and use an external diode and resistor. I feel a little better now that I have a more solid idea what may have been frying the pressure switches. I won't bother me as much to buy another one if it will actually last for a few years.
 
That is a little odd, because you should have less resistance with a resistor in parallel, it would be more like 60 ohms, unless its high like 10,000 ohms, which it shouldnt be. Maybe that resistor already fried itself open.

Mine does not have any diode and been working well for a couple years. If you try that and want to add the diode and resistor later, I suggest at least a 6amp 50V diode and around 220 ohm 1W resistor.
 
That's the drawback of the sealed unit and measuring across a parallel circuit, you're only going to be able to measure the least resistance. Also it troubles me some that I can't verify that there are voltage spikes frying the OPS, it would take an o-scope to see that. Another thing is that other people have used the relay without a diode successfully even though in theory, they would have voltage spikes. I'm still a little shy about the trial and error aspect of trying a different relay and a new OPS and just hoping for the best. The cost could be worse than $26, but I am rather cheap. The other choice it put in a "T" and different pressure switch along with the different relay. Cost is about the same, but a little bit more work.
 
With the resistance in parallel, you should be able to measure the parallel resistance, because some voltage will make it through both resistors. They are both loads and will draw from the meter, and the combination of voltages across will allow the meter to read a lower resistance value. An 80 ohm and 220 ohm resistance in parallel should read 59 ohms, which would draw a little more current, so thats why I would use the diode too. However, its only 0.24 amps vs 0.175 amps at 14V, so that extra 65 milliamps isnt going to really kill anything, but it is what it is :) and why most relays today have gone away from the diode and just use a resistor.

Although most Bosch are 80 ohms, I wonder if yours doesnt use some higher resistance coil so that the parallel resistance is the same as normal relays where it was spec'd to 80 ohms on equipment.
 
Yep, or a Borg Warner, I've had good life from them and now that the load is off it it should last a good while.
 
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