I've looked at the wiring schematics for both the ABS and the OP gauge...
and other than the Chassis (as ground for both) and the Battery(as 12 volt +
source) there's no meeting of the circuits.

However, (Old Timey) "Bad Grounds" will try to find "Earth" any way they can.

I'll almost be willing to bet there's something wrong with the OP circuit's ground
and It's using the Brake Pedal being depressed to find "Earth".
The Op sender uses the Engine Block as ground...
Hows your Engine to Chassis Grounding Strap?
'Reason I ask this is BECAUSE the actual ABS Hydraulic unit (Behind the Driver's
Headlamp) has it's very own grounding strap to the chassis.

There is a tiny tiny wire off the OP Gauge that feeds to the Indicators (I Lights)
Mayhaps it's participating in this Un Authorized grounding??

Check the main instrument ground behind the Instrument Cluster.

If there's no path from the OP Gauge to the Sender (Or if the Sender's OTL)
[Out To Lunch] the gauge might try another path?

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'84 300SD sold
124.128