Coolent temperture gauge problem

Could you please help with the following problem:

A coolent temperture gauge on a motor car, fluctuates between 70-90 degrees farenheit, this to due to a voltage fluctation from an engine ECU (Electronic Contol Unit).

Have been advised that a capacitor of 2.2 micro fharads in the supply line form the ECU, which is a 12V DC. Would stop
voltage fluctuations, this has been tried but has been unsuccesful, The voltage fluctations are between 8.5V-6.7V, from the engine ECU.

Could anybody advise of filtering or voltage stabilisation of this circuit. a new gauge has been fitted and a new coolent temperture sensor

 

[don01]

Hi,
You advise that the voltage fluctuation is from the ecu??? does the temp indicator run through the ecu?? If it does then the ecu probably uses this to tune the engine ( to compensate for hot to cold to give the "optimum " (I quote manufacturers not reality) mixture.
If this is the case then get hold of a manual for the specific ecu or one of those books in the car parts shops that give you details of ecus and how to access them with a computer.

I suspect the real problem lies around your voltage regulator cct. Does the voltage at say the dashboard lamps fluctuate as well? You could build a simple electronic cct using say an op amp driving a transistor or a transitor cct. Use an RC cct in series with the signal to ground and then use this to bias the transistor. Say a BD139 should have enough grunt. Consider a heat sink and power loses since this is an analogue cct.
If you voltage fluctuates then forget it, fix the regulator.

What type of car??
Let us know how it goes
Don

 

The car is an alfa romeo 156

 

[jbartos]

Suggestion: It probably would be better to have it diagnose by a specialist since there may be other issues such as interference, bad contact, etc.