Rallye Dash - No Oil Pressure

[Rapidtrent]

So, I got my '71 Hemi Challenger out of hibernation (Climate and Critter Controlled) only to find that the Oil Pressure Gauge wasn't registering. The engine sounded normal, but I wasn't about to take chances. Back in the day - my attention would be given to the Oil Pressure Sending Unit as they are prone to failure and well this one might be 55 yrs old so. But in my mind, there were other possibilities including wiring, gauge failure, fuse or an engine mechanical despite no tattle-tail sounds. Process of elimination; My good friend Blair, who runs one of the top tier Mopar restoration shops on the East Coast suggested that I test the wiring before going all-in on a Sending Unit replacement. This was brilliant advice. I disconnected the OPSU connector and ran a test light (for resistance) to the negative terminal. Turning the ignition key to run (not start) the gauge came to life! This eliminated any wiring or fuse (are they fused or just fusible linked) possibilities. My focus shifted to the OPSU which I tried to resurrect by cleaning and tightening the contacts and threads to enable grounding (even through the Teflon tape). But to no avail unfortunately, the old unit had given up the ghost. Upon replacement and starting, the dash gauge came to life (again) reading close to the "H" while warming up. Interesting observation, the test light was blinking while I was conducting the wiring test. Not a continuous light as I expected. Unfortunately, I can't upload the short video I made. Here's a couple of Pics. Next Project - Fuel Gauge!

 

[pschlosser]

I welcome seeing an image of the instrument cluster while the engine is running. I'm working a theory the voltage regulator behind the dash has failed.

 

[Copilot]

The test light was dimming because the limiter applies 5 volts to the gages. It does it by turning the 12 volts on and off, if constant it would fry the gages. The new electronic voltage limiters apply a constant 5 volts to the gages, much better system.

 

[Rapidtrent]

I welcome seeing an image of the instrument cluster while the engine is running. I'm working a theory the voltage regulator behind the dash has failed.

So that 3rd picture was taken while the engine was running.

 

[Rapidtrent]

I welcome seeing an image of the instrument cluster while the engine is running. I'm working a theory the voltage regulator behind the dash has failed.

I had to capture a screen shot from a video -

I'll mention that I have another pic at idle where the alternator is discharging slightly. The Temperature Gauge is working btw. The source vid was taken shortly after starting.

 

[pschlosser]

On rallye instrument clusters, the temperature, oil and fuel gauges feed off of a common part, a voltage limiter, attached to the back of the cluster. When that sucker fails, all three of the gauges fail together.

While your fuel and temperature gauges (in the photo) may not be working (unless the engine is cold) the oil pressure gauge appears functioning, so my theory its the voltage limiter is kaput.

 

[tonysrt]

I'm surprised you have a hemi and don't have a mechanical oil pressure gauge. The only real way to know what pressure you actually have.

 

[Xcudame]

Alternator discharging slightly at idle is common for our Mopars. I agree the 5 volt regulator could be (or has) going bad. Buy some inexpensive oil and water temp gauges to connect temporarily. That will help confirm all is good with the engine and the voltage regulator is the culprit. 426 Hemi is just to valuable and rare to take changes! I like you don't take chances with it!

 

[moparleo]

Why buy a low quality set of anything ?
The car is not restored,at least to atop . oem standard so why worry about an extra set of Gauges that can be mounted almost anywhere without adding any extra holes anywhere.
I would install an "t" fitting in the oil pressure threaded hole that the Gauge/ sending unit is connected to and also connect a manual gauge to the same port.
Now you have a smart gauge as well as an idiot light and a gauge that does not probably give an accurate reading of the oil pressure any way. 3 way redundancy.
And In diagnostics, rule number one is to never assume anything. Always start with the basic, simple stuff.
People tend to overthink things.
Electrical issue ? Always battery first: charged, properly grounded, clean connections ?
Electrical anything else ? Number one issue, especially if it has 50 year old wiring is clean grounds, and connections.
KISS

 

[Challenger RTA]

Simple test. Unplug the sending unit. If the reading drops the gauge is good. More than likely it's the sending unit. The higher the resistance the lower the gauge reads and vise a versa. There is a possibility of the gauge wire shorted to ground somewhere.
Check to see what the resistance is for the sending unit. 41 ohms. If it's lower than that it will give you a high reading. That's what your getting. Shorted out.

If it's open you will get no reading on the gauge. If it's open there will be no reading on the meter or the oil gauge. That is what your are not getting.

 

[Xcudame]

Nothing wrong with using an auxillary gauge to verify there's nothing wrong with his 426 Hemi. It's sounds like we have the same idea @moparleo perhaps I didn't provide enough description. I didn't mean gauges for his dash, but under the hood to trouble shoot. I have in my arsenal, auxillary oil pressure, fuel pressure, water temperature and vacuum gauges to use. I agree, the factory dash gauges are not that accurate. Spending some money on diagnostic equipment for any engine is a smart thing. Even more so for a 426 Hemi as they are rarer.

 

[Rapidtrent]

So that 3rd picture was taken while the engine was running.

No it wasn't running but I had "jumped" the Oil Pressure Sending Unit with a test light and with the ignition in the "On" position. The test light provided some resistance for the circuit.

 

[Rapidtrent]

View attachment 154077

On rallye instrument clusters, the temperature, oil and fuel gauges feed off of a common part, a voltage limiter, attached to the back of the cluster. When that sucker fails, all three of the gauges fail together.

While your fuel and temperature gauges (in the photo) may not be working (unless the engine is cold) the oil pressure gauge appears functioning, so my theory its the voltage limiter is kaput.

Sorry for any confusion - the engine was not running. I had the ignition in the "On" posistion while I "jumped" the Oil Pressure Sending Unit with a test light. Doing this the Oil Pressure Gauge on the dashboard registered a reading as the test light's resistance matched up with what a functioning sending unit would provide in the normal range. So please ignore the Tach, Fuel and Temp gauge readings from that picture.

Update - so I replaced the Oil Pressure Sending Unit after determining that the wiring was functional. The Temp gauge is functioning too! Now the Fuel Gauge is the only non-functional gauge.

 

[Rapidtrent]

Why buy a low quality set of anything ?
The car is not restored,at least to atop . oem standard so why worry about an extra set of Gauges that can be mounted almost anywhere without adding any extra holes anywhere.
I would install an "t" fitting in the oil pressure threaded hole that the Gauge/ sending unit is connected to and also connect a manual gauge to the same port.
Now you have a smart gauge as well as an idiot light and a gauge that does not probably give an accurate reading of the oil pressure any way. 3 way redundancy.
And In diagnostics, rule number one is to never assume anything. Always start with the basic, simple stuff.
People tend to overthink things.
Electrical issue ? Always battery first: charged, properly grounded, clean connections ?
Electrical anything else ? Number one issue, especially if it has 50 year old wiring is clean grounds, and connections.
KISS

Thanks for the advice, I agree start with the basics. I spared everyone from reading that. The test light "rig" that I shared isolates the Oil Pressure Unit from the rest of the circuit. The sending unit was failure prone back in the day and as mine appeared to be from back in the day I chose this approach and shared it with the audience. Further, rather than "reseeding" or cleaning all of the bulkhead and joint connectors, I offer this approach (test light rig) while at the same time advocate cleaning battery terminals, starter connection etc. But that was already accomplished. I should also stress that while the engine was running everything sounded normal, that is no clackity clack etc. Otherwise, it would have shut down immediately and some prayers would be uttered. Thanks again.

 

[mrmopar340]

Sorry for any confusion - the engine was not running. I had the ignition in the "On" posistion while I "jumped" the Oil Pressure Sending Unit with a test light. Doing this the Oil Pressure Gauge on the dashboard registered a reading as the test light's resistance matched up with what a functioning sending unit would provide in the normal range. So please ignore the Tach, Fuel and Temp gauge readings from that picture.

Update - so I replaced the Oil Pressure Sending Unit after determining that the wiring was functional. The Temp gauge is functioning too! Now the Fuel Gauge is the only non-functional gauge.

Check the ground strap on the fuel line between the tank outlet and the fuel line. There is a rubber peice in there to allow the line to flex without breaking. There is a metal bridge between the two peices. If the strap isn't there it won't read properly. Try this first before you go into pulling the tank.