Harbor Freight Tachometer

So, has anyone found a way to make this work?

It has a switch on the back to select the number of cylinders in your vehicle, but amazingly it doesn't have 1 as an option ;)

http://www.harborfreight.com/2-inch-led-lit-tachometer-98480.html

Re: Harbor Freight Tachometer

it only goes to 8000. but it you set it to 2 cyl mode, would 8k become 16k? it might be cool if it worked like that.

Re: Harbor Freight Tachometer

So um, I guess since a four cylinder four stroke will fire four times per revolution, it will actually register 2000 rpms on a moped engine as 500 rpm... or and I tarding that up?

Re: Harbor Freight Tachometer

nah that would make sense. does the sensor just wrap around the plug wire?

Re: Harbor Freight Tachometer

Looks like it wants to take a signal off the actual wire before the HT coil. I'm not sure if it needs as 12v source to work of if that's just for the backlight to work.

Re: Harbor Freight Tachometer

Mars™   Wrote:

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> Looks like it wants to take a signal off the

> actual wire before the HT coil. I'm not sure if it

> needs as 12v source to work of if that's just for

> the backlight to work.

Does it have a 2 cylinder setting? that would be 1 cylinder for a 2 stroke AND read Accurately. BUT 8,000 RPMs is not enough it needs 12,000

Re: Harbor Freight Tachometer

Unfortunately it does not

Re: Harbor Freight Tachometer

4 stroke sparks once but goes around twice per combustion cycle. That tach will read double revs on a 2 stroke.

Re: Harbor Freight Tachometer

What ross means, is that at 8000rpm and two cylinders selected, it's going to read 8000rpm. Try four cylinders. Then you can just double whatever your tach says.

Re: Harbor Freight Tachometer

Ross Maser Wrote:

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> 4 stroke sparks once but goes around twice per

> combustion cycle. That tach will read double revs

> on a 2 stroke.

Don't most cars fire a wasted spark on the off cycle?

Re: Harbor Freight Tachometer

Not on an electronically fired system with a distributor. Maybe two cylinders sharing a single breaker point - certainly not anything that's been in production for a while.

Re: Harbor Freight Tachometer

As described, the unit seems to be designed for a single coil with distributor 4 stroke engine. If so, when set to 2 cylinder configuration it would display 4 times engine speed if hooked to a 2 stroke single.

Re: Harbor Freight Tachometer

Just looked at the product manual. Coil with distributor, 4/6/8 cylinder 4 stroke.

Re: Harbor Freight Tachometer

Jimbo Grand Daddy /

A distributor is typically geared to rotate once for every two rotations of the crankshaft.

Re: Harbor Freight Tachometer

yea, that's true but not always, alot of motorcycles still fire wasted spark, it'd be weird. well play around with it, see what it dO!

or get a trailtech or a "tiny tach"

Re: Harbor Freight Tachometer

arite word:

the controllers in those takes a pulse reading based off resistance, selected by the switch for different pulse settings and assuming 4 stroke with no wasted spark (i.e. a 4 cylinder throws 2 sparks per revolution, 6 -> 3 etc.)

so you'd just need to switch the resistor for any one of the positions (or use a variable resistor) to vary the resistance to make it read properly at 1 spark per revolution.

probably a 1/x relation?

I feel like you will be the one to figure this out. but you could measure resistances for the different settings and get a trend of resistances vs pulse/revolution and extrapolate down to 1 pulse per revolution

Re: Harbor Freight Tachometer

or really simply just relabel it at half and run it at the 4 cylinder option.

Re: Harbor Freight Tachometer

Awesome, Will! Well, there is an adjustable pot in there already. I'll take a closer look at the circuit. Should be easy enough to figure out what resistor the replace the switch with. I know my lathe runs at 1250 RPM, so I'll just throw my test unit on there and see if I can get the numbers to align.

Thanks, Will!

Re: Harbor Freight Tachometer

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

Deadped Ryan Wrote:

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> What ross means, is that at 8000rpm and two

> cylinders selected, it's going to read 8000rpm.

> Try four cylinders. Then you can just double

> whatever your tach says.

you would need 12v dc to power this tach. I ran one on my 60mph lawnmower. Briggs motors have a waste spark. One pulse per revolution, just like a moped. On the 4cyl setting the tach will only read half, which would give you a 16000rpm range.

Re: Harbor Freight Tachometer

Did anyone ever actually hook this up and see what it did? Waste spark or not?

I have two bikes with 12v dc regulated and rectified systems I wired myself... I've charged phones off of them before.

Re: Harbor Freight Tachometer

Yep, and as expected it reads double, which for an 8k rpm tach becomes rather useless on a two stroke. I've been lazy but I am going to see what I can do to hack it to work in a way that is easily reproducible.

Re: Harbor Freight Tachometer

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

Mars™   Wrote:

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> Yep, and as expected it reads double, which for

> an 8k rpm tach becomes rather useless on a two

> stroke. I've been lazy but I am going to see what

> I can do to hack it to work in a way that is

> easily reproducible.

I don't know who or why you would expect it to read double.

The tach in 4cyl mode is expecting 2 pulses per revolution. a moped only has one ignition pulse per revolution, therefore only reading half, not double.

Like i said, run it on 4cyl and double the reading. 16K tach if you have 12vdc to power it.

Ryan, i am interested in your 12v dc rig? is there a thread on it?

do you have a schematic?

Re: Harbor Freight Tachometer

Re: Harbor Freight Tachometer

Oh, please, help this ASE MasterTech that's been working on cars for 45 years.

What working 4 stroke coil over distributor system fires twice per revolution?

Re: Harbor Freight Tachometer

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

Ross Maser Wrote:

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> Oh, please, help this ASE MasterTech that's been

> working on cars for 45 years.

>

> What working 4 stroke coil over distributor system

> fires twice per revolution?

Ah, any 4 cylinder 4 stoke engine with a distributor.

Cylinders number 1 and 3 fire on one revolution of the

crankshaft. Cylinders number 2 and 4 fire on the second

revolution of the crankshaft. Two pulses to the coil per

one revolution of the crankshaft. Got it?

Re: Harbor Freight Tachometer

ASE techs are so quick to throw out their certification in arguments....

right along with their credibility. ASE certified means nothing to me.

From wikipedia:

This system has been widely used, including such engines as the Buick V6 engine 3800 LN3 and newer, Harley Davidson V-Twin, air-cooled BMW Motorcycles, 1948 Citroën 2CV, Mazda B engine, Chrysler V10, PSA TU Engines, Volkswagen Mark 3 2.8 VR6 (other than 2.0 ), Saturn Corporation 4 cylinders, and Chrysler 1.8, 2.0 & 2.4 engines. Some Ford engines also do. Many Honda and Kawasaki motorcycle and PWC engines also follow a similar design, to allow for a smaller number of more powerful coils to replace a larger number of smaller coils in the same limited space. (edited)

Re: Harbor Freight Tachometer

Some opinions are based in fact and real world experience. Others are like yours.

Had you gone to the HF website and downloaded the manual for the tach being discussed, you may have noticed the diagram of a coil over distributor ignition system.

Re: Harbor Freight Tachometer

Ross Maser Wrote:

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> Oh, please, help this ASE MasterTech that's been

> working on cars for 45 years.

>

> What working 4 stroke coil over distributor system

> fires twice per revolution?

Any 4cyl cap/button (who the hell calls it coil over distributor) ignition fires twice per revolution actually... mr. ASE tech and so does an electronic 4cyl one with waste spark, just to different coils instead of the same one.

If you'd mentioned the manual instead of your certification (because you guys don't have to justify anything - you're ASE!) then nobody would've challenged your post.

Since you suck at life and like to say "just because dumbass, I'm an ASE tech, you wouldn't understand" is why.

Nobody would think to look on HF's website for a manual, because all their stuff is disposable and shit - all it's good for is experimenting and cutting up their tools to make special ones you're only going to use once. (edited)

Re: Harbor Freight Tachometer

So much for an old man giving you room to dig yourself out.

The distributor of a 4 stroke rotates at cam speed, which is half of crank speed.

Re: Harbor Freight Tachometer

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

I replied to this thread to clear up the

confusion about using this tach. I believe

Ryan was the only one who had it correct

as far as having to double the reading of

the tach in 4cylinder mode to get correct

rpm of a single cylinder 2stroke.

Ross, i'm not sure what you are arguing with

Ryan about. I have ASE master certs as

well as L-1 and a few others also. Big deal.

It just means that someone can use a study

guide and pass a test so your shop can

Legally display the ASE sign in there window.

I've worked with several certified idiots over the

years. It means nothing really. The only reason

I got them was the shop that i worked for paid

for them and gave raises if you passed.

Coil over distributor. What the hell kind of talk is that?

Makes me think of a gm electronic distributor with the

coil IN the cap. All distributors use a coil, somewhere.

Coil over PLUG is a term that is familiar, but coil over

distributor? Come on. How about coil aside of distributor?

Technician A(you) says: The distributor of a 4 stroke rotates at cam speed

Technician B(me) says: who cares, a moped doesnt have a distributor or a

cam and i DO know how many pulses a distributor provides to the coil per

revolution of tha crankshaft, so like ryan said and i said, put the tach on 4 cyl

mode and double the reading.

And what was your point about the HF manual for this tach?

That you looked at it and knew YOU would never be able to

figure out how to make it work? Not enough certifications, i guess!

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