Series Land Rovers
Series III Oil Pressure Gauge - Troubleshooting a very odd device
Original Author: Alan Richer (OVLR)
The Land-Rover Series III electrical oil pressure gauge as provided is to an initial glance a very simple device - a gauge fed regulated voltage connected to a sender plumbed into an oil line.
However (and most unfortunately) this is not the case.
The sender internals are nothing as simple as a variable resistance - it's a variable duty cycle make/break unit that relies on slow gauge response to work at all. The sender contains a bimetal strip that is heated by a resistance element. The duty cycle is affected by the position of one of the electrical contacts (which is itself affected by the oil pressure of the vehicle).
This is a great setup till it breaks - and decides not to make again.
Metering the sender won't do you a lot of good - you need to watch it for make/break cycles and a test lamp might be best for this.
A functioning sender will show up as a flickering light if a test lamp is attached across it with the engine running. The higher the oil pressure, the more rapid the cycle and the shorter the pulses.
Diagnosing this arcane bit of British engineering isn't any more difficult than a problem with any of the other gauges, once you understand that. The issue here is that you can't simply meter the output of the sender to see if it is working - the entire circuit (gauge, sender and associated wiring) need to be connected and functional before you can ensure the sender's functionality (or lack thereof).
First thing I'd do is to ground the wire on the sender (which is a big can about the size of a doorknob, not the little switch one) and see if the gauge reads upward/pins. If it does, then this tells us that the basic circuit is functional and we can turn our attention to the sender. If not, then you'll need to work your way back through the circuit with a meter and find out where Mr. Lucas has let you down.
If +12 is getting to the sender and still nothing, dismount the sender from the fitting (making sure to plug the hole when you do) and examine the terminal that the wire is connecting to. I've seen these corrode between the terminal and rivet so that they are in physical but not electrical contact. Poking around with a meter at that joint may tell you what's up, and soldering the two together may fix the
sender.
To summarize, even though the design is at best a bit arcane there's no reason that a bit of common-sense electrical troubleshooting combined with knowledge can't get this functioning again as the designers intended.
AJR
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