{Chrysler 300} Gauges and calibration

[John Grady <jkg@xxxxxxxxxxxxxxxxx>]

The recent series of postings on our gauges and that new device that reprograms them was really interesting . 

The aftermarket senders we get now are often incorrect for  our cars , but sold to us anyway . There is a major change in electrical design from balanced magnetic gauges up to 59 and then thermal bimetal driven gauges with a voltage regulator ( also thermal )  after , with totally different ohms of senders for E and F etc.  
The regulator type ( 60 up ) average out  a 12v pulse “on “ time  to get just under 6 v , average volts  , so a 6v “lantern battery “ can be applied direct to gauge heater terminals to test them. “A “ on gauge means this pulsing input pin  (or put on  steady 6 v )  The S means to variable ohms sender ,the other side of  which is grounded on the car . That is Also - on car ( ground) so to test without a sender , we put the - from 6V battery there on S . Gauge should then climb to F , H or Hi respectively . All 3 are the same electrically so the senders are too . Usually called out as about 10 ohms ( F) to 75-90( E)  ohms  . The L pin on gauge is the AC lighting pin . White wire with an orange hood goes there . 
On those dashes with thermal regulator in the oil gauge , 12 v goes IN on ignition pin (i) A is the pulsing output in this case to send power to Fuel and Temp “A” —and S is the oil sender on that one . Be extremely careful with jumpers etc as accidentally grounding A can blow up the regulator device . If you see pulsing volts on A it is ok . 

Ok., restoring all 4 , I found in a car trunk (F) in storage  —all had broken off fine wires to needle lighting . ( common— fine wire corrodes) 
While fixing that ( tricky stuff , use solder, clean terminal really shiny ,  fine radio litz  wire) , the opportunity came up to set it all up on bench and  measure some things .

 Because heating effect of electricity goes as the square of voltage , the resistance  of the  senders are not linear , nor is the gauge response . 

Cutting to it and keeping post reasonable (ok ok ) it turns out that 39 ohms to ground emulating the  sender brings it to about 1/4 scale , and 20 ohms brings it to half scale .
Knowing this , buy  a 20 ohm 2 watt resistor , put clip leads , you can sub for any of the three senders to ground and gauge should go to 1/2  scale . Makes it easy to find shorts opens or no good senders without ripping dash apart as you can see the gauge regulator etc are all ok ✅ in 3 minutes .
Note how 20 ohms is half scale ( half full) not the 45 you might expect with 90 being E .

So it does as expected , they messed with rates of change of resistance to get all this to dance together nicely… sort of. 
What to do about it being off is another long story !
Save your old gas tank sender do not mess with it  beyond gentle cleaning , never bend contact , it will rip up the winding . Replace float with brass tank from ford dealer , 2 for 18$ last time . Keep old temp senders off junked motors . Oil one is usually ok . 
Hope this helps .. 
J 


Sent from my iPhone

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