Where does the tach get it's "signal" from?

[Blackducati750]

I have a 2002 Ranger with the 2.3L . The tach needle sometimes "gets stuck" even though the engine revs are getting higher. Also, sometimes the tach needle jumps +/- a few hundred rpm when crusing at a steady rpm. The engine DOES NOT run any different when the tach starts "going crazy."

Where does the tach get it's signal from? The coil or the crank/camshaft sensor?
The coil only has about 20,000 miles on it.
The truck just hit 240,000 miles and runs like a top except for this tach problem and a small misfire.

Any help would be great.
Thanks!

 

[fishon91]

i htink the sensor is located on the bottom side of the serpentine belt. it reads a magnet or something located in the belt. i may be wrong but that looks like what that sensor does.

 

[Blackducati750]

Right, that's the crank sensor. I was about 2 seconds away from ordering a new one online, but a local (parts store guy) said, "Crank sensors either work, or they don't work. I could see the crank sensor being the problem if the engine ran weird when the tach was acting weird."

Made sense to me.

 

[Rifleman]

Everything I know about the 2000+ Fords, the PCM is what feeds the data direct to the dash panel. It uses the current ignition timing (which the PCM monitors and adjusts throughout the RPM range) signal to tell the tach what to display.

2002 has a slower processing PCM than 2003+ vehicles. Processes are prioritized I know, but if it would have outward indications such as that, no clue. I guess it is not unlike other computers that will retry a command a few times before reporting an error. If the PCM is glitching, causing retries on commands but not having a high enough fail count to throw a code, it could in theory cause less important functions like the tach to have lag and hangs in operation. I'm not sure if this could be something to do with it or not, but it is a thought. Keep in mind if you discover it is the PCM glitching or failing, it may well be covered by the dealer under the EPA mandated warranty on all emissions related items, PCM is covered.

I'm more inclined to believe the tach in the dash itself is at fault. Ford PCMs are quite hearty. The tach gauge in the dash is the only part of the system there is both mechanical and electronic "action" occurring.

 

[Blackducati750]

EPA mandated warranty on all emissions related items?
Could you explain that more? You almost make it sound as if the EPA makes Ford pay the bill on any emissions related repair....

 

[Rifleman]

Not just Ford, all manufacturers who produce consumer vehicles for sale in the US. The warranty length is 8 years and 80,000 miles.

It's all on the EPA website.

http://www.epa.gov/oms/consumer/warr95fs.txt
http://www.epa.gov/obd/warranties.htm

Nobody wanted emission control systems on their engines. The fight started back in the 70s. Up until recently, the American people had enough backbone to stand up to the government and tell them "no" on certain things. EPA would have been closed and buried if they screwed the public over by making them pay for all the repairs. Catalytic converters, adjusted for inflation, used to be three times the price they are today. That is the main reason people used to just knock the core out when they would become clogged. When they decided to raise the emissions standards and tighten enforcement, to keep the public from complaining about the cost of repairs on emission related items they forced the auto manufacturers to start providing higher quality emissions components, to do so they mandated a warranty on them.