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Discuss Tachometer reading four times as normal in the Auto Electrician Forum area at ElectriciansForums.net

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Hello everyone. I installed a new instruments panel with a tachometer for my old car (Fiat Uno) the board is for a Fiat Palio but the connections are the same and the panel fits perfectly. The only problem is the tachometer, it displays 4000 rpm when the car is turned on but idle, and goes all the way to 8000 rpm when i accelerate too much just before changing gears. The Fiat Palio uses an injection system, and I believe the tachometers feeds on that, while the Fiat Uno has carburation and I fed the tachometer on the negative (-) of the power coil. Is there any way to fix this?

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Mentor
Arms
Esteemed
Hi,it would seem you are 4 times ahead,with those readings,but not knowing exactly how the original gained it's signal,is a problem.

A single injector pulse,from direct injection,gives 2 cycles per pulse.
A common,manifold injector,gives 1/2 cycle per pulse,the same as the pulses on the standard ignition,both low and high tension.

This gives you the 1:4 discrepancy...but that is a wild guess...

You got me reminiscing,with this post,as a great many years ago,a friend of mine bought a car leasing business,and asked for some help...he had 46 Uno's...talk about spinning plates...they were not the hardiest vehicles ever made:)
 
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  • #3
I know exactly how the original gained the signal as I have the electronics schematics, which you can see here if you follow the lead cable, it goes to an item numbered "136" which is the Injection Module. Yeah, it's not a good car but its been in the family for 25 years so there's that
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Mentor
Arms
Esteemed
Hi,it is probably a bit of luck,that the tacho is reading the signal,from the coil terminal....it would depend on where in the ecu the original took it's input.
If you are prepared to do a bit of trial and error,try the signal wire,on an individual injector,pulse wire...

I would hazard a bet,that a cheap,timing light pick-up,clamped to a plug lead,with a 12V feed,may even liven the clock up....or a retro-fitted sensor on the cam wheel...:)
 
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  • #5
Thing is, the car is carburated which means there is no injection or injectors to try what you are saying. Should i try to put the cable into one of the spark plugs somehow...?
 
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Deleted member 26818

I think the only way you might get this to work, is if you can get something that will allow you to pick up from one of the HT leads.
Used to be able to get after market Tachos, that you just wound a wire around one of the HT leads.
 
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  • #7
Yeah I could try that... I have no idea how though. HT Leads are so tightly plugged you cant even fit a small cable in there
 
D

Deleted member 26818

If you could slip a wire in and connect it to an HT lead, the HighVoltage would fry your Tacho.
You need something that can pick up the signal from outside the HT lead.
 
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  • #9
I was thinking maybe the problem is the noise from the HT leads contaminating the signal cable.. should I install some sort of filter or maybe use a filtered cable?
 
Thing is, the car is carburated which means there is no injection or injectors to try what you are saying. Should i try to put the cable into one of the spark plugs somehow...?
the schematic shows the path for an injector system but this scenario will not work on a carburated system and by hooking up to the neg terminal on your coil you are getting 4 signals per revolution of the engine.
in this instance if the crank shaft pulley has a balancing weight mounting a hall effect sensor and wiring its normally open output to the tachometer input will give you the proper rpm reading ( the sensor needs to be mounted close enough to detect the balance weight when under the sensor but not so close that the sensor stays on all the time).

this is a generic way to do it but the best way is to get the proper hardware and the proper instructions for that vehicle.

on an added note a carburated engine when adjusted properly at cold idle should range 800 to 1100 rpm until the choke releases and warm idle should range 650 to 750 rpm but may vary slightly from manufacturers.
yeah Im an old fart whose seen a lot of carbs before fuel injection was common.
 
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  • #11
Im pretty sure it can be adapted. Its not a complicated thing.. just a regular tachometer
 
Im pretty sure it can be adapted. Its not a complicated thing.. just a regular tachometer
you might have to find a magnetic sensor and a way to connect it unless there is special electronics package you can use.
a pick up disc is usually aluminium with a steel slug or small magnet mounted in one spot to flag the sensor each rotation.
using a pick up coil (induction) on a plug wire may work but the concern here is possibly overloading .the tachometer.
knowing what the induced voltage is prior to connecting the tachometer would tell you if you would need to install a resistor or not
 
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Deleted member 26818

With a 4 cylinder engine, unless you have a wasted spark system, the coil only fires twice every revolution.
Individual injectors fire every second revolution.
 
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