Oil pressure reading lower

[graham.hilder@domain.elided (Graham Hilder)]

Regarding the topic of low readings on Alfa oil pressure gauges, on 6th =
May  Godfrey posted :

>>>>
Gauge sending units and readout heads often seem a bit twitchy,
particularly used ones......... Having once worked at an instrument =
rebuilder, the right thing to do is to have both the sending unit and the =
gauge calibrated together on the bench for accurate readings. It doesn't =
seem to matter whose gauges they are,
they are all like this far as my experience shows.
>>>>>

On this, I'd certainly agree: for assured accurate readings you'd have to =
calibrate the sender-gauge combination......  but this wasn't the original =
point at issue.  The original point wasn't the absolute accuracy of the =
system, but the problem of readings gradually falling ever lower as the =
sender ages, and being restored by forking out for a new sender now and =
again (and again, and again, every few years). Mind you, it's true that =
you could equally well restore the correct readings by recalibrating the =
old sender. However, because I'm a pernickety sort of a character, I'd =
like to discover the reasons WHY these jolly things do deteriorate over =
time, rather than just accepting that they are a slack product and =
replacing or recalibrating them.=20

Now, concerning the possible reasons,  Fred posted that it was typically =
due to oil leaking past the seal and onto the rheostat. I posted back and =
said that none of the low-reading units I'd opened up for  inspection had =
had this problem  so it must be something else. Then, in an off-digest =
email to me, Fred confirmed that all those he'd opened did have oil in the =
rheostat housing and the resistance read higher (hence lower gauge =
readings). Well, no doubt about that, is there....... that's quite =
conclusive, in Fred's cases. However, in my cases, there really was no =
oil, and the rheostat was clean and of correct resistance..... but the =
diaphragm just wasn't moving the slider as far as in a new unit. All the =
linkages looked in excellent shape and seemed to be moving perfectly =
freely. So I think there's some other cause for decreasing readings, in =
addition to the one Fred has found. =20

Regards to all,
Graham H,
New Zealand.
(Alfasuds, GTV, Alfetta GT)

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