thrashingcows

I have been noticing a difference in the RPM my tach is showing when I'm cruising on the Hwy. In the cool of the morning...about 5am...heading to work, I am seeing 2000rpm at approx 100Km/h. But in the heat of the day, late afternoon heading home, I am seeing 1850-1900 going the same speed.

I never saw any fluctuations before when I was running the magnetic tach signal pick up on the harmonic balancer, and this very same Isspro tach.

Only thing I can think of is maybe the CPS is going bad, or there is a weak link in the wiring? And the heat of the day is causing a deterioration of the CPS signal?

Thoughts, ideas?

Alec

With 4.10's if you have the .74 OD in your 4500, you should be running about 2,080 rpm at 60mph. Do you happen to have a multi-meter that can measure frequency (Hz)? That would be the best test, to see what the signal into the tach is, but my guess would be a flaky sensor, or maybe just a flaky wiring connection.

Angry Johnny

It's been my experience that no tach is deadly accurate, maybe +/- 100 rpm. As for the difference in driving morning to evening I'm thinking the air is much denser in the morning than in the evening and maybe it's just easier to punch that barn door through the air at that time.

Or maybe you are just heavier in the morning.

j_martin

With lever select gears, it shouldn't make any difference, unless you're driving a 1948 Dodge 4 door with a fluid clutch.

Angry Johnny

I know it shouldn't but after nearly 40 years of playing with drag car gear ratios and tire sizes I can tell you it does. I don't know why.

j_martin

It's a fairly simple circuit that shapes pulses from the input, then counts them by charging a capacitor. Semiconductor parameters vary with temperature, and if the circuit is too simple to be properly compensated for that, there'll be variation.

Judging from other Detroit electronics of this era, I'm surprised they're as accurate as they are.

thrashingcows

Not sure if my multi meter does Hz or not...I will check. What would I be looking for?

I like you analogy...punching a barn door through the air.... That about sums it up the aerodynamics of these trucks.


Very interesting.




Thank you everyone for your thoughts and opinions.

BearKiller

Way back when I was a lad and Shep was still a pup, my father drove a 1948 Dodge COUPE (two-door) that had "fluid-drive" ; black as a crow's wing she was and sharp as a razor's uncle, Port-a-walls and all.

It had a normal-looking three-on-the-column shifter and normal-looking clutch-pedal; however, you could leave it in third and drive almost as if it were an automatic.

Alas, my father never had much appreciation for the finer things in life, and was too stingy to fix the park-brakes when they quit working; thus someone always had to hop out and poke the chock-block --- always carried in the back floor-board --- under whichever side of the back tire that she was apt to head when you let off the brake.

I clearly remember, as a young shaver of five winters, sitting under the steering-wheel on a steep Kentucky hill-side, fingers clenched underneath the steering-wheel and my weak little five-year-old leg trembling, pushing that brake-pedal down with all my strength, while my father spent seemingly hours with his foot propped on a track, having a lengthy discussion with the "dozer man" ; "You set here and hold them brakes and if you let this car roll off I'll skin your hide" ; and he would.

Lost in his visit with the dozer man, he had completely forgotten either I or the 1948 Dodge coupe ever existed; that is until the brakes leaked off from some wheel-cylinder or other that had been causing him to have to add DOT-3 a couple times a week; me being five, I did not yet know of such things as pumping the pedal or cutting the wheels uphill.

First she just sort of rolled an inch, just enough to renew my fading strength and cause me to near bend the pedal and squeeze water out of that steering-wheel; by the way, that steering-wheel was bigger than some you will find in Kenworths and Peterbilts.

Then, she went a couple yards; my father had his back to the coupe, King Edward clenched in his teeth, and mind somewhere oceans away from that steep Kentucky hillside.

Then she really let go, riding down big Sumacs and black-berry briars, jumping and lurching across big soap-stone rocks and tearing through the saw-briars --- if you ever get one wrapped around your ankle, you will know why they call them saw-briars --- ; me dreading the cussing and hiding I was going to get and him bringing up my failing at every chance for the next fifty years, I renewed my death-grip on that wheel, straining to put every last ounce of strength onto that stubborn brake-pedal.

Love-vines were flipping by the side-mirrors like the telephone-pole picket-fence in Hot Rod Lincoln and then she went airborn into empty space for what seemed like an eternity, then slammed into the root-wad of a big beech, red clay clods and rocks avalanching over the wind-shield and a cloud of dust that didn't clear 'til bed-time.

My father was long-legged and nervous-minded; before the last clod hit the hood, the driver-door was yanked open, by the arm I was yanked out, and I took the hiding of my life --- right there in front of the dozer man.

The impact-absorbing cars of today would not have faired quite so well as the old Dodge; she emerged from the wreckage with nary a dent in her hide.

A few years later, the battery went dead one night and she sat right where she failed for ten years, then she got log-chained under the tractor-shed where she sat another twenty years, then a wealthy antique car restorer guy laid $5000 cash across the hood and hauled her away.

A couple years later, she was in better shape than the day she rolled off "the line" in 1948.

j_martin

Yep. It had fluid drive, which meant you didn't have to shift if you weren't in a hurry. A neckin' ****, and seats like living room couches. It was a high school boy's dream car.

maybe368

Hey BK, have you considered writing a book or short story? You have the knack my friend...Mark

thrashingcows

Here is a pic of one of my other cars....

rrgrassi

Very nice DeSoto, there TC!

Ok, back on track. Heat make metal expand, and could be moving just enough to throw the tach sensor either a bit too close or a bit too far from the magnet.

BearKiller

Thanks for the compliment.

Would I be paid money for such an endeavor ??

Where would I start ??




And YES, that is a sharp De Soto, and it is the right color too.

maybe368

Yes, you certainly could be paid, there are many people that self publish, something made possible by the interweb. I would suggest just penning some of those true stories that you have and see what happens. If it flows out of you easily, you could crank out a few. I have been meaning to write a book, but life keeps getting in the way...Mark

Alec

The idea behind a frequency function on the multimeter is that you could measure the input signal to the tach, and see if what the tach read coincided with the signal, thus determining if the signal (pickup or ecu) was suspect, or the tach itself flaky.

I'm pretty sure the Isspro tack uses a CMOS IC and actually counts the pulses. But I could be wrong, and haven't had time to look it up.

Sweet coupe, by the way!