OP Gauge Conundrum . . .

[tekenaar]

A real poser here . . . a week or so ago, my "Rallye" VDO OP gauge, vintage 4.75, began reading a bit higher than what it had in the past, then went through several days of gauge "flicker" and finally went to "pegging when ignition on" stage.

No problem, I thought, check for short in wiring or grounded sender terminal! . . . not so fast, grasshopper, at least not this time. Sender resistance test read normal and there was no short to ground in the sender "G" wire, but OP gauge still consistently pegged when ignition key was turned on . . . even went so far as to disconnect lead from sender completely, but gauge still pegged.

Hmmm, perhaps it's the gauge itself! Tested resistances between the three gauge terminals - sender, 12V, and ground - and none were shorted, i.e. normal readings! Further investigation showed that the gauge pegged as soon as a ground was connected. Even with the "gauge ground" terminal disconnected, just plugging in the light bulb would peg the gauge. Well, since ground is the gauge reference point, it won't function at all without it . . . no readings.

OK, I thought I had covered all the bases, but the problem still persists! . . . well, "in for a penny, in for a pound!" as the Britts are wont to say! Ordered a new VDO OP gauge from "egauges" and . . . WHAT THE #@%&#!! . . . did the same thing, needle pegs slower, to be sure - better damping on new gauge - but it does the same thing! Checked resistances on the new gauge and they're within 10% the same as my original gauge! Must admit, I'm stumped . . . did electrical principles somehow change between two weeks ago and now?!!

[First opel 1981]

Originally Posted by tekenaar A real poser here . . . a week or so ago, my "Rallye" VDO OP gauge, vintage 4.75, began reading a bit higher than what it had in the past, then went through several days of gauge "flicker" and finally went to "pegging when ignition on" stage. No problem, I thought, check for short in wiring or grounded sender terminal! . . . not so fast, grasshopper, at least not this time. Sender resistance test read normal and there was no short to ground in the sender "G" wire, but OP gauge still consistently pegged when ignition key was turned on . . . even went so far as to disconnect lead from sender completely, but gauge still pegged. Hmmm, perhaps it's the gauge itself! Tested resistances between the three gauge terminals - sender, 12V, and ground - and none were shorted, i.e. normal readings! Further investigation showed that the gauge pegged as soon as a ground was connected. Even with the "gauge ground" terminal disconnected, just plugging in the light bulb would peg the gauge. Well, since ground is the gauge reference point, it won't function at all without it . . . no readings. OK, I thought I had covered all the bases, but the problem still persists! . . . well, "in for a penny, in for a pound!" as the Britts are wont to say! Ordered a new VDO OP gauge from "egauges" and . . . WHAT THE #@%&#!! . . . did the same thing, needle pegs slower, to be sure - better damping on new gauge - but it does the same thing! Checked resistances on the new gauge and they're within 10% the same as my original gauge! Must admit, I'm stumped . . . did electrical principles somehow change between two weeks ago and now?!!

On the Opel pressure gauge the signal is ground based. When the oil pressure rises, the sending unit provides less and less resistance. When there is no (technically there is always resistance) resistance then the needle will peg. This would lead me to believe there is a short and you tested incorrectly.
The short could be in several areas.
1) Oil sender is bad which allows no resistance
2) Wire, somewhere, is shorting (technically grounding completely). Check for cracked wires since the fuel sender unit is so close to the exhaust
3) Oil pressure gauge is grounding improperly somewhere (unlikely)

My guess is the wire itself. Run a temporary wire from the oil sender to the gauge. If the gauge still pegs then that will eliminate that one. Since the sender operates on a ground circuit, this is a pretty harmless test.

HTH

[hrcollinsjr]

Originally Posted by tekenaar No problem, I thought, check for short in wiring or grounded sender terminal! . . . not so fast, grasshopper, at least not this time. Sender resistance test read normal and there was no short to ground in the sender "G" wire, but OP gauge still consistently pegged when ignition key was turned on . . . even went so far as to disconnect lead from sender completely, but gauge still pegged.

Otto,
I'm trying to read between the lines. You did test the sender itself, correct? Since you have an extra gauge, hook it up directly to the sender under the hood.

Hmmm, perhaps it's the gauge itself! Tested resistances between the three gauge terminals - sender, 12V, and ground - and none were shorted, i.e. normal readings! Further investigation showed that the gauge pegged as soon as a ground was connected. Even with the "gauge ground" terminal disconnected, just plugging in the light bulb would peg the gauge. Well, since ground is the gauge reference point, it won't function at all without it . . . no readings.

Try using a different ground and power source. Did you somehow get so involved testing everything else that you forgot it. You know way more about electrical stuff than I do but I use the KISS principle and systematically eliminate each part of the equation. With DC systems this usually works for me.

HTH,
Harold

[tekenaar]

Originally Posted by tekenaar A real poser here . . . a week or so ago, my "Rallye" VDO OP gauge, vintage 4.75, began reading a bit higher than what it had in the past, then went through several days of gauge "flicker" and finally went to "pegging when ignition on" stage. No problem, I thought, check for short in wiring or grounded sender terminal! . . . not so fast, grasshopper, at least not this time. Sender resistance test read normal and there was no short to ground in the sender "G" wire, but OP gauge still consistently pegged when ignition key was turned on . . . even went so far as to disconnect lead from sender completely, but gauge still pegged. Hmmm, . . . did electrical principles somehow change between two weeks ago and now?!!

. . . of course everyone knows that a grounded OP sender wire will peg the OP gauge and I was so focused on that source of the problem, that I completely forgot that an open sender wire (broken, disconnected) will do the same thing!

Rather than the "reference" being some resistance to balance the meter circuit to give a reading based on changing value, an open "G" lead causes the gauge to "read" the full 12V to ground with no resistance in the "reference" circuit. The long and short of it is that either infinite (open) or zero (short) resistance in the parallel "reference" circuit of the gauge will cause it to peg the needle.

I looked in one of my ancient "Motorbooks" from the '50s right after I posted to refresh my gauge operation memory as they still have very detailed illustrations of the circuitry inside gauges, which is difficult to find in any Motor Manuals nowadays. All the "out of the car testing" of the gauge with a stand-alone 12V source was sadly also done without a reference resistor on the "G" terminal . . . DOH!

Sometimes you just need to say a problem outloud to put you back on the right track. Thanks for all the inputs regardless!

[First opel 1981]

Well I learned something today. I had no idea a broken wire would peg the gauge as well. That would only be on the GT, right? Because it has the double sender and all?

[tekenaar]

Originally Posted by First opel 1981 Well I learned something today. I had no idea a broken wire would peg the gauge as well. That would only be on the GT, right? Because it has the double sender and all?

. . . any Opel model with "Rallye" gauges, as well!