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Electrical Gurus #3072651
08/30/22 03:32 PM
08/30/22 03:32 PM
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71birdJ68 Offline OP
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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?

Re: Electrical Gurus [Re: 71birdJ68] #3072666
08/30/22 04:09 PM
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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.

Last edited by NITROUSN; 08/30/22 04:10 PM.
Re: Electrical Gurus [Re: 71birdJ68] #3072669
08/30/22 04:13 PM
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Originally Posted by 71birdJ68
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?


What year is your road runner ?


running up my post count some more .
Re: Electrical Gurus [Re: JohnRR] #3072673
08/30/22 04:17 PM
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71.

Re: Electrical Gurus [Re: NITROUSN] #3072676
08/30/22 04:21 PM
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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.

Re: Electrical Gurus [Re: 71birdJ68] #3072727
08/30/22 05:53 PM
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Originally Posted by 71birdJ68
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.

Re: Electrical Gurus [Re: NITROUSN] #3072739
08/30/22 06:40 PM
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Originally Posted by NITROUSN
Originally Posted by 71birdJ68
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.

Re: Electrical Gurus [Re: A727Tflite] #3072752
08/30/22 08:24 PM
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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 twocents
They are distinctly different in appearances: scope:

Last edited by Cab_Burge; 08/31/22 01:32 AM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Electrical Gurus [Re: Cab_Burge] #3072777
08/30/22 09:54 PM
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Cab, this is strictly for the gauge type.

Re: Electrical Gurus [Re: A727Tflite] #3072780
08/30/22 09:56 PM
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I've talked to both wiring harness suppliers and they never seen, or heard of this wire.

Re: Electrical Gurus [Re: 71birdJ68] #3072783
08/30/22 10:12 PM
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Originally Posted by 71birdJ68
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.

Re: Electrical Gurus [Re: 71birdJ68] #3072808
08/31/22 01:39 AM
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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 shruggy scope

Last edited by Cab_Burge; 08/31/22 01:40 AM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Electrical Gurus [Re: Cab_Burge] #3072830
08/31/22 08:40 AM
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Originally Posted by Cab_Burge
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 shruggy scope


You have not seen them all. Harness

Last edited by NITROUSN; 08/31/22 08:41 AM.
Re: Electrical Gurus [Re: NITROUSN] #3072837
08/31/22 09:08 AM
08/31/22 09:08 AM
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Mattax Offline
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Originally Posted by NITROUSN
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.


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,

Last edited by Mattax; 08/31/22 09:22 AM.
Re: Electrical Gurus [Re: NITROUSN] #3072851
08/31/22 09:47 AM
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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.

Re: Electrical Gurus [Re: Mattax] #3072922
08/31/22 02:12 PM
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The senders, water temp, oil pressure and fuel level read ohms not voltage, amperage or Watts, no current going through them, I THINK whistling scope


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Electrical Gurus [Re: Cab_Burge] #3072926
08/31/22 02:22 PM
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Originally Posted by Cab_Burge
The senders, water temp, oil pressure and fuel level read ohms not voltage, amperage or Watts, no current going through them, I THINK whistling scope


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... wink

Re: Electrical Gurus [Re: Sinitro] #3073062
08/31/22 09:13 PM
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the gauges have -5 volts to them to make them read the ohms, not to the senders scope


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Electrical Gurus [Re: Cab_Burge] #3073072
08/31/22 09:26 PM
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Originally Posted by Cab_Burge
the gauges have -5 volts to them to make them read the ohms, not to the senders scope


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,

Re: Electrical Gurus [Re: Cab_Burge] #3073114
08/31/22 11:44 PM
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Originally Posted by Cab_Burge
the gauges have -5 volts to them to make them read the ohms, not to the senders scope


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... wink

Fuel_Guage_Schematic.jpeg
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