86 300 SDL. Upon starting the fuel gauge goes to Full plus, the temp goes to top as soon as thermostat opens and the oil pressure gauge bounces when being driven, at idle it sits on the bottom.
I pulled the dash unit, removed all plugs and re-plugged firmly, no change.
How do I make sure that the ground works for this unit.
This problem started when the shop replaced the dash bulbs and replaced the thermostat.
Before that time the guages were good.
Love my 126
Thanks
John

The gauges normally work with positive voltage applied to one side of the gauge in the instrument cluster and the sensor varies the potential to ground on the other side of the gauge. That is, the sensor would normally be changing resistance to the ground connection at the sensor. Gauge works like a volt / current meter, sort of.
Do you have the FSM? So you can check the connections.
What was done to the car?
Sounds like you have lost or are losing connection to sensors.
Edit:
There is a ground connection behind the instrument cluster, left side, that feeds pin 3 ground (brown) of the 8 pin connector on the cluster.
It is just a bunch of brown wires with lugs and a bolt through the lugs to the body.
End Edit.

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1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
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I don't know what the "FSM" is. The car is very original, no add ons or upgto the shop one month ago , when I purchased it to obtain a mech. inspec. for the insurance company. At that time I had them put new dash light bulbs in and it came back like this.
I will try adding a ground to see if that solves the problem.
. I am still unsure of the solution but will move forward with your help.
Thanks Again,
John

__________________
Johnny K

1986 300 SDL

FSM means factory service manuals.




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