Engine/Transmission (1994 - 1998) Factory Tach Inaccuracies

[CTD12V]

Can anybody tell me why the factory tachometer is so inaccurate (mine was off by about 300 RPM when I dynoed my truck)? I don't want to put in an aftermarket tach, so can anything be done to bomb the factory tach to increase its accuracy?



My truck is a 12-valve, so it gets the tach signal from the "engine speed sensor" pickup mounted above the harmonic balancer--isn't a Hall-effect sensor accurate enough, or does the stupid PCM have a lousy algorithm to control the swing of the needle on the dash?



I couldn't find anything specific in a search. Thanks for any info.



CTD12V

 

[BigPapa]

Was it high or low?



Scott

 

[raychem]

Can anybody tell me why the factory tachometer is so inaccurate (mine was off by about 300 RPM when I dynoed my truck)? I don't want to put in an aftermarket tach, so can anything be done to bomb the factory tach to increase its accuracy?



My truck is a 12-valve, so it gets the tach signal from the "engine speed sensor" pickup mounted above the harmonic balancer--isn't a Hall-effect sensor accurate enough, or does the stupid PCM have a lousy algorithm to control the swing of the needle on the dash?



I couldn't find anything specific in a search. Thanks for any info.



CTD12V

Make sure that the pick up is set correctly. It should be set at . 050 off the balancer.

 

[McCRam]

Are you sure the dyno was accurate? What did the dyno use to measure rpm? I would keep track of rpm driving down the road, and see if it is the same rpm at same speed. If you get inconsistant rpm's, there could be a malfunctioning part.

 

[CTD12V]

I think it was reading low, but I don't remember for sure. The dyno was calculating off of roller speed, based on tire size and pinion factor, since the optical tach stopped functioning after it got dropped on ground after about 15 trucks on dyno day.



Granted, roller speed isn't the most accurate way to measure RPM, but the dyno owner said he sees the optical readings off from the factory tach a lot. Dodge themselves say not to use the factory tach to set the idle RPM because it is too inaccurate (the reason behind my starting this thread). So I was wondering why. I will check the clearance, but last time I checked after I had the front gear case cover off it was right. It usually seems to be fairly consistent at any given speed, although sometimes it gets "sticky" and hangs and jumps for no apparent reason. Only does it intermittently though.

 

[Woodenhead]

Should be easy enough to calculate your RPMs for a given speed. A 300 RPM change will make a big difference in your speed while cruising in 5th. If you want to be really accurate measure your tire instead of going off the size on the sidewall, this'll help account for any wear.



Mike

 

[K5IP]

The engine speed sensor's only function is to output a pulse, actually two of them each time the balancer goes around. There is no room for error here. If it works, it works. The same is also true for the PCM. I suspect the error is due to the meter movement itself. 300 rpm out of a 4000 rpm scale is an error of 7. 5%. High, but not unreasonable. I do not see any external meter calibration adjustments shown in my Dodge manual so a different dashboard meter might be your only choice. Before I got carried away, I would verify the effor via another method.

If a scope is available you can measure the ess sensor output period and convert it to a very accurate speed reading.

i. e. 1000 rpm should result in 2000 pulses per minute, 33. 33/sec, etc.

Have you noticed that the tach scale divisions between 0 and 1000 rpm and from 3000 to 4000 rpm make no sense anyhow? A sign of less than insturment quality.



rog

 

[tgordon]

Should be easy enough to calculate your RPMs for a given speed. A 300 RPM change will make a big difference in your speed while cruising in 5th. If you want to be really accurate measure your tire instead of going off the size on the sidewall, this'll help account for any wear.



Mike

I have a flyer from Toyo for their tires and they state the Revolutions Per Mile on it for differant tire sizes. There are differances within a given tire size. EX. 285/75-16 M-55 625, Open Country M/T 628.

 

[Bob V]

I would say that you tack is more accurate than figuring from the tires. The circumference changes too much with wear and inflation, and the advertized size is not always that close

 

[CTD12V]

I hear you on the somewhat questionable accuracy of calculating RPM off of tire revs (and the stupid hash marks on the tach display dividing 1000 into quarters and eighths instead of tenths--I'd like to shoot whoever came up with that). :-{}#@$%!



I guess I should do a little more testing before lambasting my tach. It's just that from everything I've heard, on TDR and in the service manual, it acts like the factory tach isn't accurate enough to use for adjusting idle speed, which is annoying because I don't have either the recommended Snap-On # whatever or the DRB scan tool that the FSM calls for in the process. And I can't check my throttle breakover measurement until the idle is supposedly verified to be in spec. And if I can't do that then I can't tell if the slop in the throttle bellcrank on the side of the pump is preventing me from achieving true WOT, etc. etc.



K5IP--you think the stepper motor moving the tach needle is where the inaccuracy lies, rather than the ESS or the PCM? Hmmm, I wonder if I could figure out a way to get a digital RPM readout to display in the unused PRNDL indicator portion of my instrument panel (since my truck is distinctly a manual :-laf) . . .



Thanks for the input, everybody. If you have any other suggestions, keep 'em coming.



CTD12V

 

[Hammer]

I run a Corsa digital system on mine and run 285's for tires. The difference between my digital tach and the factory one at 2000 rpms is 50. The Corsa Intellidash at 2000 rpms on the factory dash will show 2050 on the Corsa.



If yours is off as someone has said check the distance between the sensor and the dampner,I'd bet its off..... Andy

 

[K5IP]

CTD12V

No, I think any error involved is due to the analog meter movement itself.

Still, it is probably much closer than you think.

Rog

 

[CTD12V]

Hm, OK. Well, tomorrow I'll go get a brass feeler gauge. Anybody know why the FSM calls for a brass feeler gauge rather than a regular steel one?

 

[K5IP]

The brass feeler gauge is recommended because the hall effect device in the speed sensor has a magnet which can make adjustment harder. A plastic credit card makes a quick check.

Just for the heck of it, measured my sensor output with a Tektronic scope.

With the tach indicating 1000 Rpm, I measured 988 microseconds for two output pulses (one rotation).

So: . 000988/1=1012RPM.

Much closer than I would have thought

Rog

 

[CTD12V]

Hey that's interesting. 12RPM isn't bad at all. I'd like to know more about the scope. How does it sense the pulse--do you hook it into the factory harness? Where would I find one of those, and are they pricey?



Thanks a bunch for answering my (somewhat dumb, I'm sure) questions.

Thanks for the info on the non-magnetic feeler gauge, too.



CTD12V

 

[K5IP]

CTD12V,

Yes, a good scope is pricey. Mine was about $1600, but it is a lab grade job.

Actually, cheaper ones are quite good, but the concept I was trying to make was that an external check shouold be made. If a good scope is not available, there are optical, commercial jobs available that should confirm the accuracy of any tach.

Rog

 

[CTD12V]

Ah, yeah that's a little more than I want to spend But you're right--I do want to make an external check. So far I haven't been able to find a non-magnetic feeler gauge locally without a special order. I tried the other night with my regular set of feelers, but it looks like I can't get a decent angle without removing the fan.

 

[K5IP]

CTD12V.

Again, if the tach is working there is no reason to check the gap between the sensor and damper. If it works, it works.

Gap will not change the tach calibration.

Rog

 

[CTD12V]

Hmm. What about the occasional "stickiness" when it jumps a couple hundred RPM instead of moving smoothly? I need to try cleaning the connector to the harness.