Settled: Your Oil Pressure Gauge is Fake

[rbattelle]

To satisfy the skeptics who continue to hold on to the belief that their oil pressure gauge is somehow reading real pressures, I offer the following evidence to the contrary:



1. The 2003 Dodge Ram service manual page 8J-32:

Engine Oil Pressure Message - The instrument cluster circuitry restricts the oil pressure gauge needle operation in order to provide readings that are consistent with customer expectations. Each time the cluster receives a message from the PCM or ECM indicating the engine oil pressure is above about 41 kPa (6 psi) the cluster holds the gauge needle at a point near the middle increment within the normal range on the gauge scale.

2. The size of the sending unit on 2003-up trucks vs. the size of an actual oil pressure transducer. The unit on our trucks is too small to be a transducer.



3. The price of a new sending unit from Dodge relative to the price of a new sending unit from a 2nd gen truck (which has a real sensor). Price in 2003: $13. 50 from www.dodgeparts.com. Price in 1999: $210. 00 from www.dodgeparts.com.



4. Readings on the factory gauge versus an aftermarket electrical gauge. I have an Isspro R5621 0-100 psi electrical oil pressure gauge. It's readings are not consistent with the factory gauge.



5. This evening I tested the spare oil pressure sending unit that I have on-hand. The unit was purchased a couple months ago and never installed on any vehicle. I purchased it from www.dodgeparts.com.

I pressurized the unit using a Mityvac vacuum pump set on "Pressure", some nylon tubing, and some plastic fittings.

I measured the resistance across the sending unit using a fully-calibrated Fluke multimeter model number 189 set to measure resistance, auto-ranging mode.

At ambient pressure the resistance across the sending unit was 24 ohm.

The circuit remained closed at 24 ohm until the pressure gauge on the Mityvac pump indicated a pressure of 6 psig, at which point the resistance across the sender went to open circuit. The resistance remained at "open circuit" until pressure was decreased below 6 psi, at which point resistance immediately returned to 24 ohm.



What I'm saying here is: if you change something on your truck other than items which effect the engine's operating temperature, you will not notice any change in the oil pressure reading at the instrument cluster.



I hope that settles things... (why do I get the feeling it won't?).



-Ryan

 

[67guzzler]

Ryan -- I have no NO NOOOOO technical facts to support this claim. I'm just saying that when the engine and oil are cold and I start up, the pressure is much higher at idle (750 rpm's) as it's near the middle of the gauge when cold. It moves significantly past 1/2 way above 1500 rpm's when cold.



After warming up, the idle oil pressure is just barely inside the normal range to the left of center. Above 1500 rpm's it's just past 1/2 way.



So if it's fooling me, it's pretty sophisticated in how it's fooling me and is doing a darn good job.



But I 100% concede that you must be right. I'm just reporting what I see.

Engine Oil Pressure Message - The instrument cluster circuitry restricts the oil pressure gauge needle operation in order to provide readings that are consistent with customer expectations. Each time the cluster receives a message from the PCM or ECM indicating the engine oil pressure is above about 41 kPa (6 psi) the cluster holds the gauge needle at a point near the middle increment within the normal range on the gauge scale.

What does "middle increment within the normal range on the gauge scale" mean?

 

[robert4570]

One thing I noticed about these trucks is the instant oil pressure after an oil change .

My 99 took a fews seconds before the gauge reacted but the 05 is instant even though my oil filter was only 1/4 full. Which leads me to suspect its a control module sending the reading instead of a conventional method .

My guess FWIW.

 

[Matt400]

So if it's fooling me, it's pretty sophisticated in how it's fooling me and is doing a darn good job.

Not really, its pretty simple.

Once the oil pressure switch closes at 6 lbs thats the ECM's signal to go ahead and place the needle where the consumer would expect it to be based on coolant temp and RPM.



Hot temps and idle RPM's = a lower reading than Cold temps and 2500 RPM's

I ran into this some time ago when I was working on a circuit to fool the ECM into thinking coolant was at 220. I did this with a resistor and toggle switch so I could toggle the Vistronic fan on for added transmission cooling while backing a trailer for long periods of time. The unexpected side effect was when I told the ECM the coolant was at 220, it commanded a lower oil pressure thinking it must be thinner!

Now who is fooling who here

 

[Bertram65]

Seems like legitmate proof, though I swear the oil presure reads higher when right after I change it.

Ryan,

I think you should go try and join Jamie on Mythbusters, LOL. :-laf

 

[fox]

They could put their time into better things that inventing an ' IDIOT gauge'.

Like solving the driveling issues, or transfer those people to the VP-44 dept. !!

 

[67guzzler]

Not really, its pretty simple.

Once the oil pressure switch closes at 6 lbs thats the ECM's signal to go ahead and place the needle where the consumer would expect it to be based on coolant temp and RPM.



Hot temps and idle RPM's = a lower reading than Cold temps and 2500 RPM's

I ran into this some time ago when I was working on a circuit to fool the ECM into thinking coolant was at 220. I did this with a resistor and toggle switch so I could toggle the Vistronic fan on for added transmission cooling while backing a trailer for long periods of time. The unexpected side effect was when I told the ECM the coolant was at 220, it commanded a lower oil pressure thinking it must be thinner!

Now who is fooling who here

That's pretty wild that they'd go to such great lengths to fake the behavior of functioning pressure gauge. Seems pretty nasty of them, actually. A fake idiot light. What will they think of next?

 

[steved]

Well, I can see were a fluctuating gauge would cause customer complaints... and cause DC headaches tracking down something that isn't even an issue.



I assume you placed a real guage on the 1/8"NPT port on top of the oil filter housing? If this is the port, I might just get a "real" guage to check the fake one in the dash from time to time...



Oh, and you can get an oil pressure sending unit from Cummins for $28 for the 1999s.



steved

 

[BigGunZ]

That's OK. I've beat them at their own game... I use fake oil

 

[BILLUA]

I have a DiPricol oil presssure guage that I am installing along with 3 others when I get the chance... My bet is on Mr Batelle!



Cheers, Bill

 

[radar doctor]

Seems like i remember Flying in the old airplanes if the oil needle got just off of zero that we had great oil pressure. . so knowing that,,,, wouldn't you just be Glad you saw a positive movement of the needle???? Thanks RB for geting out the Ohm Meter. . That's a good feeling isn't it. . haha ... ... ... ... ..... DOC

 

[jwduke]

Ryan -- I have no NO NOOOOO technical facts to support this claim. I'm just saying that when the engine and oil are cold and I start up, the pressure is much higher at idle (750 rpm's) as it's near the middle of the gauge when cold. It moves significantly past 1/2 way above 1500 rpm's when cold.



After warming up, the idle oil pressure is just barely inside the normal range to the left of center. Above 1500 rpm's it's just past 1/2 way.



So if it's fooling me, it's pretty sophisticated in how it's fooling me and is doing a darn good job.



But I 100% concede that you must be right. I'm just reporting what I see.



What does "middle increment within the normal range on the gauge scale" mean?

Jay you have a good point, as does Ryan. But here's a thaught "outside of the box". It could be possible or is the way it was programed into the computer, that with all of the sensers, probes, & other bologny that OBDII is required to do, that in the computer, it is programed to tell the oil pressure gauge what to tell you, under different operating conditions.



Here's a thaught for everyone, I drive a Freightliner M-2 with a worthless MBE-4000 (Now, there is as Paul Harvey says "And now we know the rest of the story" situation). There is absolutly no fuses or nothing in this truck, the ECM handles it ALL. This stupid thing, if you kill it, the ECM, will tell the oil pressure gauge that it has 60 lbs oil pressure, when we all know a engine that isn't running has no oil pressure, unless it has some sort of pre-luber, which this over priced pile of rolling plastic, doesn't have.



Anything is possible here... ... ...

 

[67guzzler]

Just to clarify -- I was NOT contradicting Ryan's analysis one bit. I was just commenting with amazement how DC would go to the trouble to make a dummy light behave like a gauge. Seems a true gauge would have been easier in the first place -- I guess it would have been more expensive too.



Most of the vehicles I've owned have had oil pressure gauges so I'm used to watching the cold vs. warm behavior. I've also added aftermarket oil temperature gauges and have seen with accurate pressure gauges the effect heat has on viscosity -- this is plainly obvious at idle.



So I have to admit that this gauge simulator is pretty good at spoofing me! My guess is some engineer thought it nifty to tap into the much cheaper dummy light sensor and then write some software to make the needle behave the way it does. It's pretty cheap of them I think.

 

[jwduke]

Jay, I didn't think you were contradicting Ryan. But, never the less the oil pressure gauge is pretty lame for a $40,000+ vehicle!

 

[nickleinonen]

I was NOT contradicting Ryan's analysis one bit. I was just commenting with amazement how DC would go to the trouble to make a dummy light behave like a gauge.

all they did was take the 1500 2002 cluster with the L and H and added some numbers instead of letters. . pressure switch is much cheaper than a transducer, so they save that extra bit every truck they sell. . and they seem to follow the "ignorance is bliss" rule to have the customers live by...

 

[Dieselnerd]

The computer (ECM) has plenty of capacity to run a "Oil Pressure Program" without any extra cost. The only cost is the initial cost of writing the software. It is pretty neat when you think about it. I'm sure the average consumer isn't aware that it's a computer generated pressure reading. Just us TDR folks that need to know exactly how it all works. I bet the reading when warmed up is probably close to actual if measured with a mechanical gauge. My '05 gauge mimics my '96 gauge pretty close under similar conditions.

 

[kcnizdodge]

Now THAT is funny!!

That's OK. I've beat them at their own game... I use fake oil

Now, if we can just pay 'em with fake money. . .



:-laf

 

[cyborg]

As I said in another post, isn't that fraud?

How many do you need for a class action?

 

[DaveHess]

Ok, guys. Let's take this to the next step for us newbies. Where can I hook up an aftermarket mechanical or electrical oil pressure gauge to get real readings off the Cummins? I prefer mechanical but the location to place the tube or sensor should be the same so where do you guys recommend???? I want to know what that baby is really doing!

 

[Matt400]

Now, if we can just pay 'em with fake money. . .

:-laf :-laf Reminds me of my Grandma being upset over all the TV re-runs, I told her to send in her canceled check and tell them to re-run it!