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Dodge Repair/Battery Drains: '91 Spirit tachometer?


QUESTION: In a 91 Dodge Spirit RT, TIII motor, I have battery drain.  I found a 100mA or so leak.  It was found to be in the instrument panel, specifically the tachometer module.  How do I diagnose this problem to make it stop draining?  The tachometer works fine.  There's a variable resistor on the module.  I can measure a resistance between the 12V supply port and ground, about 2 Mohms.

The wiring diagram for the instrument cluster is virtually nothing in the '91 manual other than to describe the wires/purposes of the pins of the two plug sockets. I wonder why the tachometer module with the ignition "off" is drawing 100 mA while at the same time the tach is working. When the ignition of "off" the tach module would likely not draw that much current, and if you measure 2 meg ohms at pin A then that is clearly not going to draw 100 mA, rather a few micro-amps.

In any case the manual suggests the following for testing the tach circuit when the tach is NOT working:
Check for 12v on pin A of the black plug
With ignition 'on' check for 12v on pin C
Check for voltage at pin B which should be at least 1V. If less than check continuity between B and pin 43 of the pcm.
If these is less than 1V at pin B and continuity to 43 is present then replace pcm.
If all tests are good, replace the tachometer drive module is the manual's final wording.

Please tell me how you did the measurement that showed a 100 mA draw by the tachometer module. Probably the current is going through the ignition off draw fuse which is connected to many other things than the tachometer.
I would presume if it is indeed drawing 100 mA then you would want to replace it even though the odometer works.
As to the resistance between the 12v supply port and ground being 2 M ohms, then that would suggest a very small current draw. But was that on pin C or pin A of the black plug? If it was on pin A, then that is not the cause of the battery drain, it is too high... it would only draw  6 micro amps and would be trivial. Pin C should not have 12V on it when the ignition is off so it would not be the cause of the 100 mA battery drain. So are you sure that the tachometer is the location of the drain?


---------- FOLLOW-UP ----------

QUESTION: Thank you for the prompt and thoughtful reply.  

The current draw is seen at the battery terminal when everything is off.  This current drain is also seen across fuse #2 which feeds power to the dome, hood, trunk, glove box, vanity mirror lamps, power mirror motors, mini trip computer, map lights, illumination of the ignition key hole, part of the radio (clock?), and to this pin A on the black plug for the instrument panel.  

Strange enough that is pin A is hot all the time, it goes to the tach module.  While monitoring the 108 mA current across the Fuse #2, it drops to 5 mA when I unplug the module.  If I pull the tack out from the energized module it drops from 108 to 87mA.

I forgot E=IR.  So running 108 mA there is a 111 ohm leak somewhere.

The pin C shows battery voltage when ignition is on.  All measurements above are ignition off.  And pin B is the AC engine speed signal.

Maybe I have a leaking capacitor in there??

Hi Jim,
It does appear that the module for the tachometer is the cause. A leaky capacitor is the most likely cause. So start with the electrolytic caps and check them out visually and then if none appear leaking unsolder them from the circuit board and test them one at a time beginning with the largest.

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Roland Finston


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