# Stabilizing External (Auto Gauge) RPM Meter reading



## Shift_Xtreme (Sep 13, 2005)

Hie, Any Electronics Engineer here?
Need help with my Sentra B11. 
I am having this 2" Auto Gauge RPM meter (I think it's 2")..and it's reading is erratic.The reading goes higher than it should when I am accelerating...when I lift off leg off the gas pedal,it goes down and shows a more correct reading (though still erratic) 
The distributor system is still the conventional contact breaker system.

Any idea what electronic component I can put in between the sensor's wiring and the breaker system's electrical point? Thanks.

5:19pm GMT+8 - Could a 12 volt diodes do the trick? Any recommendation?


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## bob89sentra (Sep 15, 2005)

Check your ground. Was it spliced into another wire, or terminated with a lug to the chasis? Also check the voltage on the +12v wire (usually red) while reving the car up in park. Mabey there is an incosistancy in the voltage. It should stay between 12.5v - 13.9v. Could the tacometer be hooked to the rong wire? In some cars there is a "sense lead" that gives the coil a different signol depending on throttle posistion, so the more gas you give it, the spark voltage will adjust according to the cylander pressure.(I'm not shure about the B11/B12 though)
If all the wiring seems to be in order, I would suggest using a 14v-20v 30-50uF capasitor between the tach and the +12v lead. The - side hooks to the tach wire, and the + side to the +12v wire. This might help filter out some noise from the alternator.


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## bob89sentra (Sep 15, 2005)

BTW did you just recently have it installed, or was it working properly then started reading improper?


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## bob89sentra (Sep 15, 2005)

Oops I got that wrong I was thinking of a diode :fluffy: anyway The Capacitor should be wired between the ground - wire and the positive + wire in paralell.


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## Shift_Xtreme (Sep 13, 2005)

bob89sentra said:


> BTW did you just recently have it installed, or was it working properly then started reading improper?


Well it was like that since day 1 the technicians installed it...I asked them why is it like that and he replied "Well because this is a carb..and it's normal for carb cars..unlike EFI which feeds the sensor from (forget where he mentioned it)"
Kinda annoying when it swings e.g. instead of a steady 2000RPM, it shows 2500RPM..2300RPM..up and down.

And I have no either where is the ground wire, wired up to. 
There are 3 wires I remember..1 I know is hooked up to the negative of the coil (after the resistor)..but previously I hooked it up to where the condensor connection is..no difference

I'll try your way and see how it goes..now I am off to the electronics shop..


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## Shift_Xtreme (Sep 13, 2005)

Capacitor: *16V 33uf * (smallest the electronic shop they have)
Green wire goes to the Tachometer.I think this is the ground wire, as it goes to the negative of the coil.Tried that..still swings abit (no improvement)..
in fact..if I put it the other way..the reading seems to be slightly higher when idling and still swings. 
Tried a 100V 1amp IN4002 tiny DIODE..worse; the meter doesn't run at all.


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## Shift_Xtreme (Sep 13, 2005)

_Sample of large capacitor. I had this one's leg's busted_ 

Okay,
I've changed the capacitor to a large 16V 2200uf...I salvaged it from a faulty Pentium 4 motherboard (near the processor socket) and resoldered it. 

Meter still erratic but at least for now (a short drive around the neighbourhood) does show signs of the meter being "cushion";
What I meant was previously, when I accelerate..it will usually show 500+RPM more than actual (and jumping up and down) e.g. 2500RPM instead of 2000RPM.
..letting it go (the gas pedal) would drop 500RPM to a more steady reading.

Would have to try a longer period of time to conclude..
Oh yes, it does make some difference if I put the capacitor connections the wrong way.


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## astreamk1 (Apr 16, 2004)

This is probably a dumb question, but I assume you have verifed that the selector switch on the tach is set for a 4 cyl engine ? 

My tach came with a filter, but I didn't need it. I don't think it was a capacitor, it looked more like a potentiometer. I will try to find it when I get home tonight and post a picture of it for you.


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## bob89sentra (Sep 15, 2005)

For the standard wiring of an aftermarket Tach the red wire is usually +12v, the black is usually the ground, and the green is the tach signol lead that should hook to the positive side of the coil. If there is also a 4th wire it is usually white, and is for the back light, It should hook to the dash light circuit. Try checking the switch on the back also, some models can do 4,6,8 cylander cars.


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## Shift_Xtreme (Sep 13, 2005)

astreamk1 said:


> This is probably a dumb question, but I assume you have verifed that the selector switch on the tach is set for a 4 cyl engine ?
> 
> My tach came with a filter, but I didn't need it. I don't think it was a capacitor, it looked more like a potentiometer. I will try to find it when I get home tonight and post a picture of it for you.


Yes, this tach has lights also..when I switch on the small lights..it lights up too. 

It's always left at 4 cylinders..tried 6 cylinders before..and the reading is alot lower.. 
Well this is one of those cheapo Taiwan-made tachs. 
Oddly..this is what the tech done..minus the junction box in the picture which i once cut it and rejoined it back for a diagnosis reason.

I've tried taking the wire and putting it on the postive..no go..Is there a posibility the wiring technicians kinda wired it up "the other way" and still work?


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## Shift_Xtreme (Sep 13, 2005)

bob89sentra said:


> Check your ground. Was it spliced into another wire, or terminated with a lug to the chasis? Also check the voltage on the +12v wire (usually red) while reving the car up in park. Mabey there is an incosistancy in the voltage. It should stay between 12.5v - 13.9v. Could the tacometer be hooked to the rong wire? In some cars there is a "sense lead" that gives the coil a different signol depending on throttle posistion, so the more gas you give it, the spark voltage will adjust according to the cylander pressure.(I'm not shure about the B11/B12 though)
> If all the wiring seems to be in order, I would suggest using a 14v-20v 30-50uF capasitor between the tach and the +12v lead. The - side hooks to the tach wire, and the + side to the +12v wire. This might help filter out some noise from the alternator.


Bob89sentra,
I owe you one for your capacitor suggestion..though I need a bigger capacitor. :cheers: 
I found the positive wire today..it's underneath the dash..hooked up to the fusebox..to a 20amp wiper (seems it's wired that way..I may rewire it to a 10amp fuse for safety sakes)
When i first tried with the tiny 16V 33uF Capacitor..the reading was very steady..BUT..the reading was Godly LOW..like as though "not enough juice"; instead of 800RPM, it read like '200RPM'.
So temporary I wire up the large 2200uf 16V (that was previously hooked up to the ignition coil)..seems stable..though during idling can see abit of jump..like 800-850RPM.May get a smaller cap..somewhere more than 100uf or so..something which looks bigger than the 33uf I bought.

Does it matter if I hook it up any way? I find both ways work...but now I hook the negative side to the tacho..and the positive side to the fusebox.


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## Shift_Xtreme (Sep 13, 2005)

Here's one article talking about big and small capacitors..advantage and disadvantage :cheers: 

http://www.overclockers.com/tips469/


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## bob89sentra (Sep 15, 2005)

Yes, Unless its a Bi-Polar CAP you should hook the negitive to the Tach wire and the possitive to the fuse box. A Bi-Polar CAP is basically two CAPs hooked with the negitives togather. They are used for AC (alternating Current) applications.


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## Shift_Xtreme (Sep 13, 2005)

Today tried a 16V 200uf...no luck...then 32V 1000uF also little luck (slight increase in reading during idling, but still in accurate)..tried parralel connection...200uF+1000uF to make up '1200uF'...still NO luck..
So was a sore man..and fixed back the earlier 2200uF capacitor..

I think I must be close enough..


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## bob89sentra (Sep 15, 2005)

A capacitor acts like a filter. The smaller the cap is in uF, the higher the frequency it will filter out. To a point the bigger the cap is, the lower the frequency filtering will be. If a 2200uF cap doesn't seem enough, maybe you need more of a low pass filter. On the back of my car radio there is a little box in line with the power, this is to filter out ignition, and alternator noise. (I bought the radio at Wal-Mart, so it is a cheep one) Maybe you could see if the transformer from a broken radio would work. (I wouldn't suggest buying a new radio just to use the filter from) I have actually bought things new, just to butcher them for parts though


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## Shift_Xtreme (Sep 13, 2005)

bob89sentra said:


> A capacitor acts like a filter. The smaller the cap is in uF, the higher the frequency it will filter out. To a point the bigger the cap is, the lower the frequency filtering will be. If a 2200uF cap doesn't seem enough, maybe you need more of a low pass filter. On the back of my car radio there is a little box in line with the power, this is to filter out ignition, and alternator noise. (I bought the radio at Wal-Mart, so it is a cheep one) Maybe you could see if the transformer from a broken radio would work. (I wouldn't suggest buying a new radio just to use the filter from) I have actually bought things new, just to butcher them for parts though


Yeah I remember seeing a 'transformer-like' under the dash of my car, when my CD player had intermittent 'cut offs'..I saw that transformer when the technician was re-doing the wiring..
But I believe your theory as it works now for my CD player in filtering out the interference.










Is this what u are talking about..about the "Low pass filter"? I have seen this on a faulty computer motherboard that I have


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## bob89sentra (Sep 15, 2005)

I'm not sure if that is a low pass filter, but it sort of resembles one. The coils of wire look small for one, and i don't see a transformer ,but there is more than one way to make a low pass filter. I also have seen them inside speaker boxes. The ones that are a little nicer sometimes have them. I have some sansui speakers that have them. If you try the low pass filter you might have better luck hooking it like the radio would, In series with the red wire from the tach to the ignition.


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## Shift_Xtreme (Sep 13, 2005)

Shift_Xtreme said:


> Today tried a 16V 200uf...no luck...then 32V 1000uF also little luck (slight increase in reading during idling, but still in accurate)..tried parralel connection...200uF+1000uF to make up '1200uF'...still NO luck..
> So was a sore man..and fixed back the earlier 2200uF capacitor..
> 
> I think I must be close enough..


I should had apologised earlier. I got mixed up with "parralel and series" terms. <<<---:loser: . I was all this while connecting in 'series', after consulting my father, which is wrong for capacitor usage. No wonder my reading was super LOW; the capacitor cuts out excesss voltage. 




bob89sentra said:


> I'm not sure if that is a low pass filter, but it sort of resembles one. The coils of wire look small for one, and i don't see a transformer ,but there is more than one way to make a low pass filter. I also have seen them inside speaker boxes. The ones that are a little nicer sometimes have them. I have some sansui speakers that have them. If you try the low pass filter you might have better luck hooking it like the radio would, In series with the red wire from the tach to the ignition.


Previously I connected one tiny low-pass filter on series, and no effect. 

Okay, I've just went to ENEOS yesterday and got myself a 20AMP Choke (shaped of a transformer) for US$2.39,connected in series (this time I am on the right track) and so far it does smooth out the revs alot and accurately..will try another few days more. Stupid me, I connected the capacitor in "series" and not "parralel"..


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## bob89sentra (Sep 15, 2005)

Cool, I'm glad you got it sorted out. I'm getting ready to put an aftermarket Tach, Coolant Temp gauge, and Oil pressure gauge on my B12 soon here, and I know It will be fun!


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## SuperSentra4203 (Dec 17, 2002)

bob89sentra said:


> For the standard wiring of an aftermarket Tach the red wire is usually +12v, the black is usually the ground, and the green is the tach signol lead that should hook to the positive side of the coil. If there is also a 4th wire it is usually white, and is for the back light, It should hook to the dash light circuit. Try checking the switch on the back also, some models can do 4,6,8 cylander cars.


Never hook the tach signal lead to the positive side!! You should not need a capacitor to correct the problem, that MAY act as a band-aid so to speak. All the capacitor is doing is modifying the tach signal. It's smoothining and dampening the on/off pulses to the tach. One of two things are actually your problem, the tach is just faulty (Yes, even new ones can be junk outta the box) or you are having an erratic timing issue. More than likely caused by bad/burned points


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