Scott Fisher <sfisher@wsl.dec.com> sez:
|> Re: my high oil pressure.
|>
|> I don't mean to be a stickler, but remind me of things like how
|> fresh the bearings are, when the problem arose, etc.
The problem started somewhat recently on a well used but hopefully
sound engine. I'm not sure exactly when the problem arose.
I will bet you a sixpack of Samuel Smith's Nut Brown Ale that it's the
gauge or the wiring between the gauge and the sending unit.
|> Which kind of sensor do you have?
The sending unit looks like a top with a little metal tab sticking out the
top. I swapped sending units last night. No change was apparent.
Ah, so it's an electrical gauge. (Some years used a mechanical gauge.)
Then it's the gauge or the wiring between the gauge and the sending
unit. :-)
meaddata.com is in Dayton Ohio. Maybe I'll stop buy California on my
way back from Utah. I'm heading to mjb's Fat Chance Garage to get my
Weber tuned :-) I'll see if I can find one here, though.
Seriously, hook up a known good mechanical gauge. Stewart-Warner makes
them, AC of England makes them; those are the two brands I have in my
garage now. (Well, no, I took one into the house so I could match the
fitting, and the other one is resting calmly in the radio slot in my B...)
There is nothing quite like hooking up a mechanical gauge and having it
read 60 psi at 2000 RPM and 45 psi at a hot 750-rpm idle to make your
heart feel light. Or to make you scream imprecations at the unquiet
ghost of Joseph Lucas.
|> It's been my experience that MG engines are about fifty to a hundred
|> times more reliable than Lucas gauges. Chew on that thought while you
|> contemplate rebuilding your bottom end.
Well, considering my speedo reads high and my fuel gauge low, it wouldn't
knock me over with surprise if my oil gauge reads high.
Here's what the problem is, I bet.
Your car and mine have the same kind of oil pressure gauge, the top-shaped
sending unit with the metal tab. You have a wire running from the tab into
the wiring harness, yes? And there are wires, and not tubes, running to
the back of the gauge, yes? All this sound familiar?
Well, here's how that gauge works. Basically, the sending unit that lives
on your engine block gets an oil feed from the braided hose coming off the
back of the block, behind the distributor. (I did all this last night so
it's fresh in my fingernails. :-) Oil pressure pushes on a diaphragm inside
the sending unit, which varies the resistance. Voltage runs TO the gauge,
across the windings for the needle, and then to that little tab on top of
the sending unit, which grounds to the engine block. Resistance inside the
sending unit determines where the needle points; if you open the circuit
(for instance, by pulling the wire off the tab), you should get 0 on the
gauge, while if you short the circuit (for example, by connecting a wire
from the tab to the engine block), you get 100 psi. I checked that out a
few weeks ago.
Or to put it succinctly:
Open circuit: false low. Short circuit: false high.
Q.E.D., d00d.
Since you have already replaced the sending unit, I will bet you the
extra sending unit you seem to have that there is in fact a short somewhere
in either the gauge or the wiring from the gauge to the sending unit.
If it is stuck closed, it would seem to me that I won't be able to get it
to come out, since something besides spring tension is holding it in.
Yet another reason to suspect the gauge or the wiring between the gauge
and the sending unit. Hint: there's a REASON for all the jokes about Lucas.
The thing about MGs is that for the most part, there are known, predictable
ways in which things on them fail. 90% of those failures are electrical,
most of those having to do with connectors that shake loose or rust out.
The engines are basically sound, sturdy, and reliable. For you to have a
genuine case of 100 psi oil pressure would require some really bizarre,
arcane, and highly unlikely combination of failure modes. That's not to
say it can't happen, just that it's about as likely as that the next
President will balance the budget in his first year in office. :-)
|> Keep the faith!
I'm trying. I just hope I don't start having nightmares about cranks
exploding or oil filters blowing through the hood :-O
Short between the gauge and the sending unit. Short between the gauge
and the sending unit. (While I've got a failed sending unit, because
the oil pressure checks out as PERFECT on the mechanical gauges but it
occasionally drops on the electric gauge... intermittent open between the
gauge and the sending unit, and I've verified that the connections from
the gauge to the tab on the sending unit are good.)
If only it were easier to get to the gauge. Is yours right above
the steering column? I can't do much more to mine than peer up at the
back of the gauge; I sure don't know how I'd get it out of the dash if
the gauge were broken.
--
"Do you ever wish you had a joystick with a big red button on it so
you could just nuke the person when you send a reply?" -- Kim
Scott Fisher/sfisher@wsl.pa.dec.com/DEC Western Software Labs/Palo Alto, CA
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