2nd Gen Non-Engine/Transmission Tach not reading right?

[CTaylor]

Ok guys after having the 97 in my sig turned up it was found my tachometer is way off beyond 2000 rpms, this was verified on a dyno. It seems accurate to 2000 but when I plant my foot in it in 3rd or 4th and take it to 4K it only shows about 2500 and while the truck is pulling away hard in the midrange(the point where it accelrates the fastest) the tach acts like the engine is barley increasing speed even tough I can tell its racing down the back side of the tach quick. What is the problem? Is it the sensor or in the dash? The truck was virgin all its life untill now so its doubtfull the tach ever saw the other side of 2k rpms



Ok guys I borrowed a scan tool from a freind and it seems to be showing the actual RPM the engines turning. After 2000 rpms the scan tookl was showing about 650 700rpm faster than the tach was registering. What is you all idea on it now? Should this rule out the CPS sensor and PCM?

 

[Missouri Mule]

It could be the tach or the PCM I doubt it is the engine speed sensor.

The ESS sends a signal to the PCM which interprets the signal for running of other components (auto trans,asd relay,speed control unit,tq,) it also sends a signal to the tach, one the tach will understand.

But the ESS would be the cheapest item to replace.

From 96 FSM Tach (short version) The tach receives an egine pulse signal ffrom the PCM. "An electronic itergartated circuit contained with in the tach unit reads and anylyzes this pulse signal it then adjust the ground path signal resitancefor one of the electronic magnets in the gauge to control guage needle movement.

Tach Calibration I-6,V-6,GV-8 3000 rpm (+/- 200) =Frequency 100 Hz



Reading this the PCM mite be sending a similar signal to the tach that it is recieving from the ESS.

 

[HHhuntitall]

If it's got the original sensor on the crank, it could easily have gone bad. Hot, cold, water, freezing temps all contribute to failure over time. I've seen several that go bad just out of the blue... . losing the alternator and A/C. Personally, they're almost cheap, so I'd try that.



https://www.napaonline.com/Catalog/...635_0166843737&An=599001+101997+50022+2022010



Duralast/Crankshaft Position Sensor (SU3159) | 1997 Dodge RAM 2500 3/4 ton 4WD 6 Cylinders D 5. 9L Turbo Diesel | AutoZone.com

 

[CTaylor]

If it's got the original sensor on the crank, it could easily have gone bad. Hot, cold, water, freezing temps all contribute to failure over time. I've seen several that go bad just out of the blue... . losing the alternator and A/C. Personally, they're almost cheap, so I'd try that.



https://www.napaonline.com/Catalog/...635_0166843737&An=599001+101997+50022+2022010



Duralast/Crankshaft Position Sensor (SU3159) | 1997 Dodge RAM 2500 3/4 ton 4WD 6 Cylinders D 5. 9L Turbo Diesel | AutoZone.com

You said loosing the alternator an A/C? My A/C still works and I belive my alternator is still functioning since I have been driving it like this for 2 weeks. If it were the PCM what options do I have for that? I wouldnt think I could still get one from the dealer

 

[Missouri Mule]

Since your tach basically works,a/c works and if you have auto trans and it works your engine speed sensor is working leaving the tach and PCM.

I would lean toward the tach not doing its job. You could look in junk yard for tachs. I don't have the manual explainng how to test tach.

 

[CTaylor]

Since your tach basically works,a/c works and if you have auto trans and it works your engine speed sensor is working leaving the tach and PCM.

I would lean toward the tach not doing its job. You could look in junk yard for tachs. I don't have the manual explainng how to test tach.

Mine is 5 speed truck, I should also mention the A/C works but it only blows out the defrost vents, Im pretty sure thats due to the blend door acuator being shot but I thought I should mention that just in case it could be PCM related? Is the tach gouing to be it own separate gauge in the cluster or would I have to replace the whole thing, speedo,tach and other gauges?

 

[HEMI®Dart]

It's probably not the problem but do you have . 050 clearance between the sensor & the damper?

 

[HHhuntitall]

If the sensor is going bad, it will still send a signal to the PCM. It just doesn't send the proper signal. Any kind of signal will trigger the PCM to make the alternator charge and the A/C work, as it senses the engine is operational. IF the sensor is going bad, say the internal spring is bad, it will only work up to a certain speed. Think of it as floating the valves if you over-rev your engine. If the spring is weak, it hangs, or can only move so fast... .



I was suggesting that versus the PCM, as it would be easier to and cheaper to repair. If your tach is working normally, other than over a specific rpm, it's my thought. You haven't changed the crank harmonic dampener, have you? It's clean, without anything covering the two notches in it? The sensor isn't touching the dampener, is it?



FWIW, I've never seen an actual tach quit in a dash by itself. I've seen whole instrument clusters quit, but never just the tach... ...

 

[Missouri Mule]

CTaylor, memebers have replaced the speedometer/odometer separately I imangine the tach can replaced separately also.



Air blowing out the defrost vent is a default setting when there is a vacuum problem. You either have vacuum leak or a pinched vacuum line. The most common place for a vacuum leak is the vacuum line that goes over the engine to the passenger side of the truck and into the fire wall it usually touches the exhaust manifold and melts a whole into it (if it is not the problem reroute it so it is not near the manifold). Start at the vacuum pump and check all vacuum lines.

The control switch could be the problem also.

 

[GAmes]

I have a '97 gauge cluster in my garage. If it is worth the 200 mile drive to you, you are welcome to use the tach to troubleshoot. I'll be here all next week.

 

[CTaylor]

I have a '97 gauge cluster in my garage. If it is worth the 200 mile drive to you, you are welcome to use the tach to troubleshoot. I'll be here all next week.

Thank you very much for the offer GAmes. Im going to hook it to a guys scan tool at work if it will show RPMS and see if it is reading the same as the tach or pretty close. If so then that should narrow it down to the sensor or PCM. Im shooting for the sensor 1st.

 

[CTaylor]

update made in OPOo.

 

[The patriot]

I doubt this is it, but when I did my KDP I didnt have a non magnetic feeler gauge. I set the gap on the sensor with a business card and havent had any problems.

 

[Missouri Mule]

I would think so.