Instrumentation gremlins

[John]

My 86 Excel SE has recently developed a couple of instrumentation issues. The oil pressure gauge is fine whilst driving, steady 4 out of 5, but at tickover it constantly jumps about and even flicks from zero to max.

The temperature gauge has also started playing up. If I put the headlamps on the gauge jumps from normal to just under the red zone, 100 degrees. Switch the lights off and it returns to normal! I've checked that the alternator belt is not too slack, that the temperature sender wire is in good nick and the connection is clean.

Has anyone seen either of these gremlins before?

Cheers John

[rbgosling]

Not got the same issues, but still issues. In my case the oil pressure looks like it's too high, and the coolant temperature is certainly too cold (I've compared the reading to one taken with an infrared thermometer, so it's definitely the gauge, not a cold engine).

In previous cars (MG BGT, Spitfire) these issues would usually be down to the Voltage Stabiliser, but the Excel doesn't seem to use such a device.

Brief tests suggest the voltage at the gauges is different at each gauge, but I can't work out why, although I haven't spent as much time on this as I should ('cos it's not exactly crucial).

I'll be interested to see what anyone suggests, and if / how you sort out yours!

[MetBlue]

My oil pressure gauge behaved as you described.
Assume you've check oil level ( mine was good).
I replaced gauge with a second hand one and it's been good since.
In my case, was definitely the gauge gone west.
Tony

[John]

Thanks guys, I had the dash out a few months back so I should check the connections first before assuming it's something worse. Engine uses very little oil and oil level is as it should be, so I suspect it's electrical.

I'll update this post if I mange to sort it.

[Hawaiis0]

Most likely a loose wire or poor contact. Mine does similar but comes and goes every time I move the dash.

[Lotus-e-Clan]

Also check all common earth connections. Particularly the major earth groups at the chassis, engine and pedal box.

These gauges (temp and oil) sink to earth through the sensor. If an earthing point is flaky, then the readings will be flaky. It's possible that both local sensor connectors (temp sensor and oil pressure sensor) are flaky in their own right, and this is the first thing to check and correct. Loose spades or fractured sensor wires are common.

Vibrations at idle are a different quality to vibrations at speed, plus alternator output is low at idle and high at speed, which shapes the voltage and current electrical forces through a flaky connector. Lights-on draws high current, which will favour their own earthing point and probably compromise a flaky earth elsewhere.

These gauges will have a common 12v supply at the dash too, so the same flaky connection, vibration, and alternator supply principles outlined above applies to those too.

[John]

Thanks Peter, I've never had reason to check any of the earthing points before so they could be showing their age. Time for an inspection I think.

[DavidOliver]

I have recently delved into electrical circuits including instruments.
I was lucky that Forum members found me the correct version of circuits as the S.E. is very different to the LC.
Regarding instruments, and bearing in mind that the fibreglass body requires direct wiring for earthing and the key point is the connection to the pedal box, next to the fuse box. Access for checking requires removing the plastic undertray and door pillar carpet finisher.
A true Lotus position tester.

Dave the cog.

[John]

I'll check that out Dave, just been looking at the excellent parts listing we have on here and I can see where I need to check - upside down with ankles over headrest!