Electrical Gurus

Are there any electrical guys that understand why they did things the way they did it back in the day? What I'm talking about is on the 71 B&E body cars with an oil pressure gauge. There was what was called a resistance wire (service manual) that hooked to the sending unit and then went to the harness. With a ohm meter set to the 20 scale it shows a little over 30 ohms, The service manual shows this wire for both the Hemi and 440 cars and my 383 RR had this wire. Everybody that I have asked has never seen one, or doesn't know what it is for. I'm wanting to know what it did in reference to the operation of the gauge and is it required?

As I remember it was with the Rallye gauge cluster. It was part number 2983848. The senders were 249522 up to march 1971 and 3488609 after March 1971. Senders were the same on all car lines. So was the Rallye cluster only available on those engines? That's the only difference I see.

What year is your road runner ?

71.

On the rallye cluster, you could get it even on a 318 2 door. Satellite Sebring could be had with a rallye cluster and a 318.

Well if thats the case than the only reason I see is pressure. Its possible the motors in question had a very high pressure pump and using the resistor wire allows the gauge to read in the normal range. With out the wire I would assume the guage might be pegged at higher rpm's. That's the only logic that makes sense.

I think you are correct. My 71 Hemi GTX had this wire in the original harness. Somewhere along the harness and this wore got cooed.

When I went though the vehicle to clean it up I ordered a new harness and this “resistance” wire was not included. Stock motor with an accurate aftermarket gauge shows around 75-80 lbs. of pressure. The Rallye gauge is almost pegged at this pressure.

Mopar used two different oil pressure senders, one for the oil light and the other one which is much bigger in outside diameter is for oil pressure
They are distinctly different in appearances: scope:

Cab, this is strictly for the gauge type.

I've talked to both wiring harness suppliers and they never seen, or heard of this wire.

I am pretty sure I gave you the right answer.

All the stock Mopar engine wiring harnesses I've seen have the same wire (light Tan 18- or 20-gauge wire with a female slide on terminal) in both of them, no differences between them

You have not seen them all. Harness

I think your answer about calibration issues is generally correct.
The details we would have to figure out, but essentially there seems to be a change in calibration.
The aftermarket replacements, including the one I bought from BRE, read too high with the original '67 gage in my car.

Scroll through here for that.
https://www.forabodiesonly.com/mopar/threads/oil-pressure-gage-and-sending-unit-revisited.490485/
Note Redfish worked at Chrysler dealer for years - so I'm sure he's correct that some gages were tweaked to deal with a specific production issue but I think he's saying that was 67 only.
(The gage in my '67 worked correctly for years until the sender started spraying oil)

OP can do what I did to check the gage. Get some resistors, I just happend to have some 11 ohm in the box, and or a reostat/pot. Then hook them up to the sending wire as a simulation of the factory spec'd sender sender resistance.
For the 67 A body oil pressure gages. 80 psi is the top mark, and 40 the middle.
The sender should have 10-12 ohms at 80 psi. around 23 at 40 psi, and around 74 ohms at zero. My tests showed 46 ohms would bring the Barracuda's gage to the first mark (10 psi) and plotting that it looks to be correct but I can't guarentee that's the spec.

My temporary solution was to splice in a short length of resistance wire. It may result in a less precise response, but critically (for me) keeps maximum amperage through the gage in the range it is was intended.
I now have an NOS sending unit. If it tests correctly it will eventually go on the car,

That is exactly what my wire looks like, even down to the friction tape. Now, my car is a 383 car, so I don't know why it has one. I think the reason I'm wondering about all of this is cause the after market sending units now a days are crap compared to originals, so I'm just going to see how the gauge reads and then decide weather to us the wire, or not.

The senders, water temp, oil pressure and fuel level read ohms not voltage, amperage or Watts, no current going through them, I THINK

Basically the senders have a built-in variable resistor circuit that reads between the 5V and ground..
Since the gas, water temp, oil pressure gauges are essentially a 5V DC meter by adjusting the resistance(ohms) to ground changes the voltage read by the meter..

Just my $0.02...

the gauges have -5 volts to them to make them read the ohms, not to the senders

You really don't know how electrical circuits work

The gauges and senders are both have voltage on them. As the resistance of the sender changes, the voltage dropped across it varies, changing the voltage left for the gauge to drop, that's what moves the needle on the gauge,

CAB...
See the attached schematic as it better explains how a basic fuel gauge circuit works.
For the Mopar systems, the 5V is supplied by the voltage limiter device is found on the gauge cluster PCB or sometimes built into the fuel gauge of most Mopars in the 60s/70s/80s.


Just my $0.02...

OK. You lost me. My illustrations on FABO show a test with a resistance wire spliced into a reprop harness, and tests of sending units pressure vs resistance.
Any car with an oil pressure gage (included in the rally instrument cluster) would get the oil pressure sending unit. Which sending unit depended on the year and possibly model. If there was a special sending unit or gage calibration for particular hi-perf engine option, it is probably listed in the books or the TSB.

For those wondering how the gages work with the senders, Chrysler's Mr Tech can help out
http://www.web.imperialclub.info/Repair/Lit/Master/227/Page01.htm

Up Date: I found my original sending unit and on the hex part where you put a wrench to tighten it is stamped on one flat80 and on another flat is stamped OK and Introl on the housing, which is I guess who made it.

For some its making a mountain out of a mole hill.

Looking closely at your diagram there are no wires, voltage supply, hook to the indicator needle, correct
That being so the coils read the ohms from the sender and pull the needle towards them when they have the highest or lowest ohms to make them work like a magnet, correct
As already posted any voltage sent into the gas tank sender could and would cause an explosion in the tank when the ignition key was turned on activating the sender when jumping across the parts to make the connection to read it, correct
Ohms don't spark

Lets get back on topic, why did some cars have these wires and others didn't.

You were given that answer.

Maybe someone after buying the car put it on later.
No one on here has heard of it or seen one on any car so far
The only fuse able link or safety wire I have heard of on any Mopar up through 1974 had them on the main starter battery relay feed from the starter main battery cable

Some applications needed a certain-resistance in a portion of a circuit.
Adding a high-resistance wire will accomplish that.
Usually a nickel conductor type wire: i.e. fusible wire.
Certain models used such a wire in rear defroster circuit, believe some a-bodies..
It introduced a voltage drop (along it's path), thus lower voltage (speed) at the defroster motor terminals.
Same reason you see resistors hanging off 3-speed wiper switches.

Not true. I have seen them. For what its worth they are not needed. Most thought they were some type of extension wire not a resistor wire. These were a 2 year only part as I recall. 70-71 e-body and 71 b-body. Were used with the hemi and 6 pack 440. The cars gauge and senders were the same no mater what motor. The hemi and 6 pack 440 must of made more oil pressure so the resistor wire was added to keep the gauge in the normal range. However with that wire the gauge would read lower through out the whole range. There is no mention in the FSM that I have seen and they are listed in the parts manual. I posted the part number and I also posted a link with one that was sold. I do not know what more the poster of this thread needs to hear.

Incorrect.
Chrysler's Tech explains how it works here: http://www.web.imperialclub.info/Repair/Lit/Master/227/Page01.htm

Or for those who prefer audio - visual.
Here's the fillmstrip that accompanied the booklet.

My car is a 383 car and it has one, also the FSM clearly shows it in the engine wiring digram, it even says resistance wire.

Ok so some how it has one whats the big deal? It either was a factory blunder or some one added it. Either way so what. If it were me I would run without it. If it was a by the numbers and a full restoration of a Hemi or 440 six pack I would leave it. In the FSM i only looked under diagnosis and it made no mention that I found. In the wiring diagram I will bet it mentioned HP and hemi cars. Unless you followed that car its whole life from the day it was born you will never know.

No big deal, but no body can explain its purpose. It’s not a factory mess up cause the wire coming out of the harness has to match the connector of this wire, Where usually a wire comes out of the harness and goes straight to the sending unit. I can state that the car has not been messed with before. If the picture loaded, this is the wire. I’ll try it with and without, I’m bringing it up cause this is some forgotten mopar thing that nobody knew about, even the experts.

It is interesting subject. My 70 383 b body RR has added mechanical gauge along with the pressure unit for the instrument panel. All original to the car. My water temp readys very accurately as it rises from cold to 180. My pressure gauge rises to 20 psi and hovers there, while the mechanical is around 50 psi. Idle or high rpm the mechanical changes, the electrical does not. I think I am going to try changing the sender to see if it changes how the electrical works.

But for the OP ? It doesn't make sense to me.

The oil pump for a 440-6 or hemi was the same big block pump except it had a heavier relief valve spring so bypass did not occur until a higher pressure. The sender called for is common through the years, but the 7-71 parts book calls out this added harness for 440-6 and hemi only, yet the gauge would be the same for a 383. Adding a resistor to prevent pegging ok, but that means your gauge is offset in value and does not accurately present pressure. Even so a hemi or 6pack should not have much higher pressure unless the motor spec were much tighter to give less leakage. Which I don't think they were. Are we to believe ralley gauges are motor specific? I don't think they are.