MGB Oil Pressure Gauge Gives Constant Reading -- Doesn't Seem Right

[joelodom]

I can't find a similar thread so starting a new one...

The oil pressure gauge in my newly-acquired 1970 MGB doesn't seem right. It reads zero when the car is off -- even if I turn the key to on without starting the engine -- but the gauge slowly creeps up to about 50 PSI when I start the engine then it NEVER changes, regardless of RPM or temperature. Seems like the sensor may be stuck or something? Any ideas?

 

[YakkoWarner]

I am pretty sure 1970 used the electrical sensor for the gauge - earlier and then again later years used a physical pipe coming from the engine into the dashboard with a completely mechanical gauge (I kind of prefer the electrical setup because having hot oil dead center over my lap doesn't sound fun). You are correct in that it should vary with RPM. Sensor or wiring is a possible problem. I'd disconnect the wire while running (it won't damage anything) and see if the gauge moves - in theory it should go to zero, and if you ground the wire it should go off-scale high. If that doesn't happen, wiring is likely to be at fault. If that does happen, sending unit is suspect (but could be a bad ground or connection on the unit as well).

 

[DrEntropy]

Only two things there... gauge or sender. Yakko's right, yours should be a 12V electrical, no voltage stabilization. And to be contrary, I prefer the hard plumbed system. In well over fifty years of ownership, never had a lap full of hot oil... mehhheh.

Bookmark this page: >ADVANCE AUTO WIRE< and it can save you a ton of time!!

 

[joelodom]

Cool. Thank you. I just ordered a maintenance manual so I can see where the sensor (sending unit?) is. Is there just one wire between the sensor and the gauge and they share the common ground with the rest of the car? So I disconnect the wire at the sensor side to see if the gauge goes to zero then touch it to the ground and see if the gauge goes off-scale high? If the gauge responds as expected then it's the sensor that I need to replace?

 

[DrEntropy]

Most likely, you got it! Only other "fly in the ointment" may be the voltage wire supplying the gauge. Tho with the description you gave, that's unlikely.

Got a VOM? Best investment for electron chasing and they're CHEAP now.

 

[joelodom]

Is this it? One wire and looks similar (but a bit different) than the part I found from a supplier.

 

[joelodom]

I confirmed that's the sending unit by unplugging the wire and watching the pressure gauge go to zero. Then my son and I shorted the wire to the frame and watched the gauge climb. The better he made the electrical connection, the higher the pressure reading, so I think the gauge is okay. I ordered a new sending unit.

To replace the pressure sending unit, it looks like I just unscrew the old unit with a wrench and screw the new one in until it's tight, but what do I use to seal the threads? Is there a recommended sealing compound? Teflon tape?

Thanks again for the help. I want to know if the oil pressure is misbehaving!

 

[DrEntropy]

To replace the pressure sending unit, it looks like I just unscrew the old unit with a wrench and screw the new one in until it's tight, but what do I use to seal the threads? Is there a recommended sealing compound? Teflon tape?

That's about it. Teflon tape will be fine.

 

[BobHaskell]

I am pretty sure 1970 used the electrical sensor for the gauge - earlier and then again later years used a physical pipe coming from the engine into the dashboard with a completely mechanical gauge (I kind of prefer the electrical setup because having hot oil dead center over my lap doesn't sound fun). You are correct in that it should vary with RPM. Sensor or wiring is a possible problem. I'd disconnect the wire while running (it won't damage anything) and see if the gauge moves - in theory it should go to zero, and if you ground the wire it should go off-scale high. If that doesn't happen, wiring is likely to be at fault. If that does happen, sending unit is suspect (but could be a bad ground or connection on the unit as well).

Re: earlier and then again later years used a physical pipe into the dashboard. Our '80 MGB has an oil pressure sensor in the engine compartment next to the windscreen washer pump, no pipe to the gauge.

 

[YakkoWarner]

I confirmed that's the sending unit by unplugging the wire and watching the pressure gauge go to zero. Then my son and I shorted the wire to the frame and watched the gauge climb. The better he made the electrical connection, the higher the pressure reading, so I think the gauge is okay. I ordered a new sending unit.

To replace the pressure sending unit, it looks like I just unscrew the old unit with a wrench and screw the new one in until it's tight, but what do I use to seal the threads? Is there a recommended sealing compound? Teflon tape?

Thanks again for the help. I want to know if the oil pressure is misbehaving!

Inquiring minds want to know:

Where did you FIND a replacement sender??? For a very long time those were unavailable - if someone is making new ones I'm thinking I should grab one myself while the opportunity exists. I THINK mine is OK - havn't been able to run the engine for a decade though so might be worth having one on hand.

I don't think it needs any thread sealant. The fitting where the hose attaches is more like a brake fitting or compression type connection.

 

[DrEntropy]

I don't think it needs any thread sealant. The fitting where the hose attaches is more like a brake fitting or compression type connection.

Just added lubricant in case it needs removing at some later date.

ISTR the threads are a pipethread-like taper on that sensor. Those sensors were indeed hens' teeth, same for Alfa Romeo's early Jaeger ones. If available, a spare would be wise.

 

[joelodom]

I found a couple places online that sell oil pressure senders for a 1970 MGB. Paid about $140. It wasn't a hard search and this morning at a car show someone mentioned the seller as reputable, so I think it'll be good.

 

[joelodom]

Well, I installed the new oil pressure sending unit this morning and... the gauge now shows constant 80 PSI instead of 50 PSI. Sigh. Next I'm going to check the output resistance of the unit at different RPM. Does anyone know of a chart that gives the oil pressure for a given resistance?

 

[Mickey Richaud]

Well, I installed the new oil pressure sending unit this morning and... the gauge now shows constant 80 PSI instead of 50 PSI. Sigh. Next I'm going to check the output resistance of the unit at different RPM. Does anyone know of a chart that gives the oil pressure for a given resistance?

Since you have an electric gauge and sending unit, I believe, if I'm not mistaken, you should have a voltage stabilizer inline. Could be missing/faulty. Check wiring diagram #9 here: http://www.advanceautowire.com/mgb.pdf

 

[maynard]

Is it the right unit? You may have one for an idiot light.

 

[joelodom]

Is it the right unit? You may have one for an idiot light.

Good thought, but it was sold from a reputable retailer as a replacement for the original unit. Today I drove the MGB and the needle actually does move in a semi-believable way, and the resistance readings (video of my test) change in a believable way that align with what the gauge says, so I think it will at least give me a heads up if the pressure drops below what I normally see.