Oil pressure sending unit stays out, even with key on

Lubrication System (oil feed pump and scavenger pump, reservoir, filter, and lines)
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Kuda
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Oil pressure sending unit stays out, even with key on

#1

Post by Kuda » Wed Sep 21, 2005 4:44 pm

Here's a weird one: '49 Pan, converted to 12 volt (Cycle Elec gen. w/low voltage regulator), stock style dash. Tried to hook up pressure sending unit in addition to the 60 lb gauge. Install sending unit, wire it from acces. position on switch to terminal on sending unit. Light goes on with key, then goes out when started. Then stays out, even with key on, engine off. Pull unit, no continuity between terminal and body. Huh, must be bad unit. Buy new unit, test, good continuity. Install, light goes on with key, start, light goes out. And stays out. Pull unit, same thing, no longer any continuity between body and terminal. What the heck?!

-Kuda



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Re: Oil pressure sending unit

#2

Post by caschnd1 » Wed Sep 21, 2005 4:55 pm

Just a thought... is your sending unit rated for 6V? Maybe sending 12V through it is burning something out so that it no longer works.
-Craig

fourthgear
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Re: Oil pressure sending unit

#3

Post by fourthgear » Wed Sep 21, 2005 5:20 pm

kuda
I believe ( of course I don't have a wiring schematic in front of me) the bulb for the oil light is not grounded to the dash and there for has two wires from it . One should be in the center of the socket ( 12v power supply and the other is connected to the out side of the socket for ground and that is the one that goes to the sending unit .What it means is when you have no oil pressure you have continuity and you have a closed circuit and the light is lit . When you get pressure it opens the sending units switch and turns the light out. If you are putting 12V to the sending unit you are probably frying it . The filament acts as a resister and only lets a small amount of amperage through it and to ground , meaning the sending units switch. Like I said I don't have a wiring diagram in front of me and this is from my old brain type memory.Fourthgear...

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Re: Oil pressure sending unit

#4

Post by Kuda » Thu Sep 22, 2005 11:21 am

Huh. I'm startin' to guess that the dash ain't egg-sackly original. I strongly suspect you're right about the wiring, didn't think about it that way. Thanks for the tip, I'm off to the store for a two wire socket...

-Kuda

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Re: Oil pressure sending unit

#5

Post by FlatHeadSix » Thu Sep 22, 2005 2:23 pm

The sending unit is normally closed pressure switch which means it goes directly to ground until the oil pressure opens it. If you had it hooked to a hot lead from the ignition switch you probably smoked the unit as soon as you turned the switch on, good thing the sending unit blew first instead of frying the whole wiring harness.

When you put the 2-wire socket in the dash make sure that it is not grounded to the dash base or the light will never go out. The hot lead from the ignition switch goes to the center position on the socket, the lead from the sending unit goes to the socket's grounded side.

good luck
mike

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Re: Oil pressure sending unit: Update

#6

Post by Kuda » Mon Sep 26, 2005 4:06 pm

OK, now that I've had another look at it, here's the deal: It is a two wire switch, can't even see how it would ever work otherwise. Hot lead from ignition goes to bottom of bulb socket (lower tip of filiment), second wire goes from side of bulb socket down to oil pressure sending unit. Same deal: when ignition goes on, light goes on, goes out when oil pressure goes up. So far, all good. Touble is that after that first time, the unit will no longer ground when there's no pressure and the bulb stays off, permanently. The only thing I can think of is that maybe I'm asking for a 12v unit and getting a 6 volt unit. Anyone got a part number for a 12v oil pressure sending unit I can try to pick up? Thanks again,

-Kuda

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Re: Oil pressure sending unit

#7

Post by RussW » Tue Sep 27, 2005 3:56 am

Replacement units are listed to fit '41 - '84, covering both the 6 and 12 volt years. Like brake light switches, I wouldn't think voltage should matter if wired properly.
Where in the return line do you have it mounted? The stock style switch is triggered by VERY little pressure, about 3 lbs. Mine goes out just from kicking over the motor. The fitting on the oil pump for the sending unit allows very little oil to the unit on the cast pump, as I remember. I run an S&S pump, and they warn that the fitting hole on the pump for the sending unit should not be used as a return line, since it has little flow thru it. If you run too much oil at pressure to the switch, as on the same line as the 60lb. gauge, could you be mechanically breaking the switch with too much flow to it? Just a thought....

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Re: Oil pressure sending unit

#8

Post by Kuda » Tue Sep 27, 2005 1:02 pm

"Replacement units are listed to fit '41 - '84, covering both the 6 and 12 volt years. Like brake light switches, I wouldn't think voltage should matter if wired properly."
That's what I was thinking, but I hoped I was wrong...
"Where in the return line do you have it mounted?"
It's mounted to a T fitting (well, more like a Y fitting) with a 60 lb gauge. Even cold, the pressure rarely goes over 20lbs, and will drop to zero when hot at idle. Stock oil pump, recently rebuilt.
"The stock style switch is triggered by VERY little pressure, about 3 lbs. Mine goes out just from kicking over the motor. The fitting on the oil pump for the sending unit allows very little oil to the unit on the cast pump, as I remember."
It's a stock cast pump, but I haven't pulled it down yet to see if it's been drilled and updated to the later style. But it's definately the stock unit...
"I run an S&S pump, and they warn that the fitting hole on the pump for the sending unit should not be used as a return line, since it has little flow thru it. If you run too much oil at pressure to the switch, as on the same line as the 60lb. gauge, could you be mechanically breaking the switch with too much flow to it? Just a thought...."
I can't see how. It's mounted where the stock sending unit would go, I've just got a fitting on there to use both the gauge and light. I wanted the stock idiot light on there 'cause I can't see the gauge when sitting on the bike. Since it's all new, I thought a little extra protection was a good idea, at least until I "learn" the bike a little better. But I just can't figure out why the sending units are dying...

-Kuda

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Re: Oil pressure sending unit

#9

Post by RussW » Tue Sep 27, 2005 5:38 pm

It's a fairly simple setup and you're right, I can't see how it could screw up. The switch is just a diaphram setup that opens and closes a set of contact points with the pressure. If you are not frying them with inappropriate wiring, or breaking them physically with too much pressue, which I think would be impossible especially with a stock pump, then the only thing left is the switch itself. I would not be unheard of to get a few bad swithches in a row, all with the same fault, especially if you get the same brand from the same place. Get a new one somewhere else, maybe a Harley one, and plug it straight into the pump, no "y" fitting. If it works, then add the "y" again and see what happens. Process of elimination.

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Re: Oil pressure sending unit

#10

Post by fourthgear » Wed Sep 28, 2005 12:36 pm

kuda
You say you have a two wire switch? Thats not stock for panheads or Shovels for that matter. It should have one wire and it grounds to the motor which in-turn grounds back to Batt..I looked and tested my bulb sockets ( I'm rebuilding my dash ) even though they look like they are grounded to the dash , they are not . ( if they are the right ones )They are insulated between the ground and dash. If you have a two wire switch it would be something like a brake switch and maybe some one modified you switch set up . I would put it back to stock and use a stock type switch and make sure its wired to factory specs.

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Re: Oil pressure sending unit

#11

Post by Kuda » Wed Sep 28, 2005 12:55 pm

"You say you have a two wire switch?"
Yup, that's what I said, but it's not what I meant, sorry about that. I meant I have a two wire *socket*. And yes, it's insulated from the dash with a fiber washer. The sending unit (switch) is a single threaded pole that the ground wire attaches too. Sorry for the confusion...


-Kuda

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Re: Oil pressure sending unit

#12

Post by fourthgear » Wed Sep 28, 2005 6:33 pm

kuda
I'm confused most of the time any way. It does seem strange that you fried two switches , are you sure they are just stuck and not reseting with oil pressure drop?Try a different manufacturer of the switch if you can, I haven't looked to see if you can, but theres got to be one somewhere maybe try a stock one N.O.S. may have one.

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Re: Oil pressure sending unit

#13

Post by panhead » Wed Sep 28, 2005 7:08 pm

If you have a friend with a working one, swap them.

Kuda
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Got it!!

#14

Post by Kuda » Mon Oct 03, 2005 1:45 pm

Picked up a old used OEM sending unit at a swap meet, works perfect. Seems those aftermarket units just ain't worth a crap. Kinda funny that a 40+ year old switch works better than both of the new ones. <shrug> Anyway, thanks for all your help. Now on to the rest of the problems...

-Kuda

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Re: Oil pressure sending unit

#15

Post by DCoul » Thu Jun 21, 2018 5:55 pm

FlatHeadSix wrote:
Thu Sep 22, 2005 2:23 pm
The sending unit is normally closed pressure switch which means it goes directly to ground until the oil pressure opens it. If you had it hooked to a hot lead from the ignition switch you probably smoked the unit as soon as you turned the switch on, good thing the sending unit blew first instead of frying the whole wiring harness.

When you put the 2-wire socket in the dash make sure that it is not grounded to the dash base or the light will never go out. The hot lead from the ignition switch goes to the center position on the socket, the lead from the sending unit goes to the socket's grounded side.

good luck
mike
I know this is an old post but I thought I would put my two cents in.
I can confirm this procedure because I just went through this.
I hooked the wiring backward and fried the oil pressure switch. New oil pressure switch and the power to the black socket lead to the bulb filament. Red grounded socket wire goes to the oil pressure switch.
Thank you all for your input as it helped with the (hopefully) final wiring of my 1942 WLA bob job converted to 12V.

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