5 ohm resistor on temp sender wire?

[Driver Ed]

Hello, All-
Can anyone think of a reason the PO of my 64 Dart inserted a resistor (labeled 10w 5.1 Ohms 5%) in the wire from my temp sender? The resistor is a few inches from the sender, just hanging in mid air over the intake. My temp gauge started working intermittently yesterday. Needle looked fine, then gauge pegged low, then ok again. Happens after hard acceleration or bump, so figuring some sort of short. Sender wire is ancient, arthritic and been hacked a couple of times. I want to run a new wire to the bulkhead. I want to delete this resistor, but not if it is going to fry the instrument circuit board or cause other damage. All other gauges work great. Car is a 64 Dart GT. '77 318, Edelbrock Performer intake. Sensor/sender looks after market(stud and nut connector). Perhaps the sender is not right for my application and is being resistance corrected? PO removed an Electronic Ingnition set up and went back to points if that matters.
Thanks,
Driver Ed

 

[MadScientistMat]

You are either looking at an attempt to make the gauge read a lower temperature or the all time stupidest installation of the infamous "eBay chip mod".

 

[Vaanth]

Hello, All-
Can anyone think of a reason the PO of my 64 Dart inserted a resistor (labeled 10w 5.1 Ohms 5%) in the wire from my temp sender? The resistor is a few inches from the sender, just hanging in mid air over the intake. My temp gauge started working intermittently yesterday. Needle looked fine, then gauge pegged low, then ok again. Happens after hard acceleration or bump, so figuring some sort of short. Sender wire is ancient, arthritic and been hacked a couple of times. I want to run a new wire to the bulkhead. I want to delete this resistor, but not if it is going to fry the instrument circuit board or cause other damage. All other gauges work great. Car is a 64 Dart GT. '77 318, Edelbrock Performer intake. Sensor/sender looks after market(stud and nut connector). Perhaps the sender is not right for my application and is being resistance corrected? PO removed an Electronic Ingnition set up and went back to points if that matters.
Thanks,
Driver Ed

The resistor added inline with the lead from the temperature sensor back to the gauge was added for a few possible reasons:

Possibility 1) The sending unit does not have the correct resistance range, specifically, the resistance across the range is lower than the correct sending unit. The sending unit could be the wrong one for the application, or partially defective unit. The resistor was added to increase the resistance across the range, specifically +5 ohms at each point.

The original sending unit was Chrysler number 2426458 (superseded numbers: 2958320, 4051140, 4051141). The following image shows an example of this sending unit:

Verify that the sending unit resistance is correct, and removal of the resistor should allow correct readings at the gauge unless its calibration is off. See the following section for resistance values.

Possibility 2) The gauge calibration is off, and the resistor was added to offset the difference at the gauge. The gauges have mechanical adjustment to calibrate the needle reading independently. Although not necessarily proper, the calibration of the gauge can be adjusted to match the system.

Possibility 3) The gauge and sending unit were reading correctly, but the needle position is higher than is comfortable for the observer, so the resistor is added to lower the reading across the range. This is particularly evident on the "hot side" as the low sending unit resistance at the high end is now not as low, so the temperature looks cooler.

Dealer mechanics might have done this to remedy a customer complaint of the vehicle running too hot, although it actually wasn't.

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Checking the Sending Unit Resistance at Operating Temperatures

For testing the sending unit in operation, the following cardinal points and resistance values are of interest:

• Ambient temperature at about 77 degrees Fahrenheit (25 degrees Celsius), before running engine, gauge needle at lowest point, sending unit resistance = 360 Ω
• Engine at ~120 degrees Fahrenheit (49 degrees Celsius): The needle moves to Cold mark on the temperature gauge, sending unit resistance = 72 Ω
• Engine at ~190 degrees Fahrenheit (88 degrees Celsius: The needle moves to middle on the temperature gauge, sending unit resistance = 22 Ω (approximate range of ~18 to ~26 Ω)
• Engine at ~230 degrees Fahrenheit (110 degrees Celsius): The needle moves to Hot mark on the temperature gauge, sending unit resistance = 9 Ω

With the resistor removed, no damage to the gauge from the sending unit will occur if lowest sending unit resistance is ~7 ohms. The associated wiring and connections, in good condition, between the sending unit and the gauge adds a little resistance which should be less than 0.2 Ω.

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Checking the Gauge

To check cardinal points of the gauge, use the following resistance values:

Chrysler Specification:
Cold = 72 Ω
Middle = 22 Ω
Hot = 9 Ω
Gauge position tolerance at these points is listed in the service manual.

Miller Special Tool Specification (used by Chrysler for testing):
Cold = 75 Ω +/-5% -> 71.2 - 75.8 Ω
Middle = 22 Ω +/-5% -> 20.9 - 23.1 Ω
Hot = 10 Ω +/-5% -> 9.5 - 10.5 Ω

The Miller C-3826 tester can be used to test these points, or the point resistances can be made using fixed resistors or a combination thereof, variable resistors, decade resistance boxes.

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Regarding test lead resistance when measuring low resistance values

Minor test lead resistance is not much of a concern at higher resistances, but when measuring low resistances, such as the ground to ground continuity, low resistance of sending units, or ballast resistors, account for the resistance of the test leads connected to the measuring device. This can be done by shorting the leads together, noting the resistance, and subtracting it from the measurement readings, or use the meter's relative setting (if equipped) similarly with the lead shorting.

Some meters have a function to measure conductance and convert accordingly for low resistance values. Others, typically lab or some bench type meters can use a four wire ohms measurement to overcome any test lead effects.

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[slantsixdan]

Hello, All-
Can anyone think of a reason the PO of my 64 Dart inserted a resistor (labeled 10w 5.1 Ohms 5%) in the wire from my temp sender?

Might have to do with the change in temperature senders from '63 to '64. The early ('60-'63) senders have 13.5 - 15.5 Ω resistance at 220°F. The '64-up senders have 15 - 19 Ω resistance at 220°F.

Imagine two senders, one early and one late, in the middle of their respective resistance ranges. So, 14.5Ω for the early-type sender and 17Ω for the late-type sender. That's about a 16% difference, enough to visibly affect where the needle sits on the gauge face.

Assuming a gauge that reads correctly, using an early sender instead of a late sender will result in a higher reading, or using a late sender instead of an early sender will result in a lower reading.

The weird thing, tho, is that the same part number (2291 021) is specified for the '63 and '64 Dart gauges. So either that's a catalogue error, or it's not an error and they just didn't care that the needle would look like it was showing a lower reading on a '64 Dart than on a '63, both at the same actual engine temperature.

The early sender uses larger (1/4" dryseal pipe) threads, and a coin-shaped terminal; the wire slides on at right angles to the axis of the sender. The late sender uses smaller (1/8" dryseal pipe) threads, and has a stud-type terminal; the wire pushes on in line with the axis of the sender (or yeah, you can use a ring terminal and a nut to secure it, because the thread is studded…er…wtvr, you know what I meant). Easy enough to put a late sender in an early head by using a 1/4" to 1/8" pipe thread reducer bushing, but going the other way is harder. Not impossible, but more effort.

All of which to say: sometime in this car's past, someone might have installed some '63-down components, or maybe someone just installed a wrong sender, with less-than-stock resistance so the gauge wants to read high. Or maybe the gauge itself just drifted hotward over the years.

There's precedent for the use of a resistor as you describe; take a look:

I want to run a new wire to the bulkhead.

Good idea.

 

[Driver Ed]

Thanks everyone for the great insights. Funny about the factory service hack to mollify the customer. Anyway, my plan is:
1) replace the sketchy ancient wire without the resistor to get the gauge working.
2) start and run the car, radiator cap off, until the thermostat opens
3) check the temp of coolant and see what 'normal' operating temp looks like on the gauge
4) Decide at that point to swap sensor out, add back in the resistor or do nothing. I am the only one that drives and maintains the car, so even if the gauge shows hot or cool from reality, I'll know where safe is.
Cheers,
Driver Ed

 

[str12-340]

Is this an attempt to fix a malfunctioning factory voltage limiter?

Also a temperature "gun" is cheap and can make your estimates of reaching operating temperature a lot more accurate and you can also figure out the temp at a whole range of gauge readings. Harbor Freight is a good source.