What relays engine RPM to the ECU?

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Shadow_Death

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I've noticed this issue before and never really put it down to a specific issue until today. There are some times when I'm at WOT the truck won't shift. Instead of shifting it will hold that gear until it bounces off the rev limiter.

Today I had my bluetooth scanner hooked up and Torque Pro running off to the side. I had to pass someone who decided to do dumb stuff, that happens a lot around here, and so I had it at WOT when it did it again. That's when I noticed that when I glanced at Torque Pro my RPM went from 4,100 to -65 RPM.

All other conditions the truck runs fine. It idles fine, drives fine (except some minor shifting glitches), etc. I mean if it ran like crap I'd assume CPS but it doesn't.
 

dudeman2009

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Engine RPM is derived from the crank sensor, but the cam sensor is also read by the PCM, one going bad can cause a code for the other. I've never heard of this problem with a -65rpm. Was the trucks tach working correctly? The engine rev problem sounds like the trans is slipping. What is the fluid color?
 
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Shadow_Death

Shadow_Death

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Engine RPM is derived from the crank sensor, but the cam sensor is also read by the PCM, one going bad can cause a code for the other. I've never heard of this problem with a -65rpm. Was the trucks tach working correctly? The engine rev problem sounds like the trans is slipping. What is the fluid color?

Ok, so to start off. I don't have a tach to compare it to. The truck continues to accelerate and doesn't even try to change gears. Like when you see someone in a manual just run a gear until it hits a rev limiter. It will do this one minute then if I try to recreate it again it may or may not do it again. The transmission shifts firm for something this old. Since I don't have a Tach I have to use an OBDII reader and Torque Pro for Android to get my engine RPMs if something seems off. The engine RPMs will register from 0 all the way up to about 4100 RPMs then suddenly it will drop off but the engine will continue to rev and the vehicle won't shift until I let off and it's right as the RPMs show up on Torque again. Fluid color is still redish. It's a little brown and about due for a change but not that far into being due for a change. >.>

I'm no stranger to getting the issues that people haven't encountered before. lol

Edit: I suspect I'll just have to get an aftermarket Tach and install that to keep up with the engine RPMs while I figure this out.
 
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dudeman2009

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I'd suggest getting an ignition based tach, they are very reliable. The problem with non DRB scantools is some will communicate with the CCD bus without a problem, but some do have problems trying to communicate and will often act strange by giving false readings and the such.

The ignition based tach is going to show true RPM as long as the ignition coil is firing correctly.

Holding a gear likely has nothing to do with engine RPM as down/upshifting is done based on road speed and throttle pressure. Throttle pressure being relayed to the transmission via cable.
 
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Shadow_Death

Shadow_Death

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I'd suggest getting an ignition based tach, they are very reliable. The problem with non DRB scantools is some will communicate with the CCD bus without a problem, but some do have problems trying to communicate and will often act strange by giving false readings and the such.

The ignition based tach is going to show true RPM as long as the ignition coil is firing correctly.

Holding a gear likely has nothing to do with engine RPM as down/upshifting is done based on road speed and throttle pressure. Throttle pressure being relayed to the transmission via cable.

Yup, so after thinking about the last part of your post over I remembered that the issue was way more prominent when I first got the truck. It used to do it ALL the time if I was over 1/2 throttle. Come to find out the previous owners had it set to 1 notch too tight. I reset it according to the FSM lining the lines on the connector to the spot that it snaps onto the throttle lever. So on a hunch I went back out there and loosened it by one more notch. So now the connector actually sits one notch loose. For whatever reason that seems to have fixed it. The truck doesn't lurch into next gear and it doesn't hunt for gears when I'm turning corners like it used to. Previously it couldn't make up it's mind if it wanted 1st or 2nd when I'd make turns until I applied more throttle. Tomorrow's commute to work will tell more though.
 
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