Topic: Tachometer reading high...

[lilbuddy]

Hey everyone 
I just put a stock 1980 tach into my 81 sr5 cluster (other tach broke). When i put the tach in, and turn it on and it says my truck is idleing at 2,000 RPM. I know for a fact it isnt at 2000 RPM. So my question is...is there any way to adjust the tach so it actually says like 800 RPM when its idling or make it actually say what the engine is at? Its almsot like its getting too much power to it. I dont really want to have to tear into the whole gauge and take the whole gauge it self apart because i dont want to destroy it. Also when i press on the gas and it says 4,000 RPM (im guessing its about 2800) it jumps up and down...is the gauge broken? Any help is appreciated.

Thanks everyone,
Chris

[Marlin]

Way back in '84 when I put a factory tach in my '80, I recall having the same problem.
I was told that the factory tach was non serviceable, and it took 3 tachs from the wrecking yard before I finally got a good one.

It does sound like you made have a weak or dirty ground connection. The tach could be searching for a ground and having to go thur other circuts to find it. This would give a false reading.
Begin by removing the instrument cluster and trace the ground lead of the tach on the printed circuit board and then use a jumper wire to a good ground on the metal dash support. Then run the truck.
Also look carefully for a cracked circut board or poor connections.

Marlin

[lilbuddy [OP]]

I was looking on the circuit board and there is underneath the 3 screws that are lined up (of the 5 total) there is a "+", a "P" and a "3" (I think is what they were, a "+" for sure though)...I was thinking maybe if you backed off the "+" a little bit it might bring it down.

Marlin, I will defenatly try that tomorrow. I was looking at the back of the curcuit board and the copper around where the screws go in were also kind of dirty. Ill get something and clean them off. It could as well be just as simple of a bad ground.

Thanks a lot,
Chris

[lilbuddy [OP]]

I went out today and tried grounding it better, it only went down by about 400 RPM and still read somewhere about 1500-1700RPM. I think that it just might be bad. I can just assume its about 1200 more RPM then the engine is really at, so if im up at 5500 RPM where it redlines, its probably 4300.

Chris

[BigMike]

Hey Chris,

If you can not get it to work correctly and don't have time/money/energy to acquire another tach brain, then try this simple trick.

Run down to Radio Shack and buy a POT (Potentiometer) and install it between the red and black leads that route from the tach brain and to the tach needle assembly.
Try to find a POT that has a very wide range of adjustment, like a 20 or 30 turn POT. I am using a small blue 25 turn POT on my MR2. I think it is 0 - 1k ohms but I am not sure.

What this will do is put resistance between the wires that run to the tach which cuts down on the amount of current that actually makes it to the tach itself. It is completely harmless. All a POT is, is a variable resistor, and since we are wiring it in a series manner, the equivalent resistance of the circuit is increased so less potential voltage is placed across the tach itself.

The reason you want a good number of turns is so that it will be more precise. If the range is 0 - 1,000 ohms and only a 1 turn, then for every 90 degrees you'll gonna get a resistance of 250 ohms, whereas if its a good number of turn POT, like a 20 turn that ranges from 0 - 1000 ohms, then every 90 degrees will just be 12.5 ohms -- which means you will have a great deal of precision to get your tach adjusted just where you need it.

I had to do this on my MR2 when I converted it to a Supercharger from a JDM Levin. The Levin's tach is a full sweeping 180 degree tach, whereas my MR2 is just like a 110 degree tach, so when I would hit redline at 7,600 rpm, the tach would go off it's scale and read like 13,000 rpm

So by placing resistance between the red and black wires that go from my tach brain to my tach sweeper, I can reduce the movement of the needle and effectively "dial in" my tach reading.

Hope this helps,
BigMike

[lilbuddy [OP]]

BigMike,
That is a very good idea. That is what i was thinking that it was getting too much power but didnt know how that would work. I tried Marlins trick and it brought it down a little bit, like 400rpm but i just screwd it all back in and called it a day. Im going to try to go to Radio Shack tomorrow and try it out. I just bought this gauge cluster for a good price ($20) so Im going to keep it. Everything works on it and is in great condition. Its kinda funny though shifting at where it redlines. It does bounce up and down when im keeping it at a certain RPM so itll be nice to get rid of that too. Thanks alot for the good explanation. Id really like to get that part for it.

Thanks,
Chris

[BigMike]

Is it bouncing above where you are at or bouncing down?

I had a bad connection on my ignition wire that leads to my tach, purple wire I believe, and since my computer in my MR2 is way in the back trunk, its a long path to go to the tach. My tach would sometimes bounce down to almost zero and then come back up. So I found out that a plug I was using had corrosion on it, so I just replaced the plug with a different one and that took care of the problem.

But if yours is bouncing UP, then it sounds more like you have another voltage source that is somehow bleeding accross your tach sending wire.

What happened to your old tach? or are you installing a tach? Maybe the wiring harness got hot sometime ago and part of the insulation is wasted away and its picking up voltage from another circuit... if so then

[lilbuddy [OP]]

When you are idling (in my case at "2000RPM") the needle just moves up and down, not very much but it doesnt stay steady. Its not that big of a deal but its noticeable.

My old tach's needle snapped off and it stayed at 0 and when i got it, the whole guts of the tach got bent or something. The spring on the inside was underneath that part that spins (not very good explaination) but the guy that owned the truck before me must have went inside the tach to try to fix it or something. I have no idea how it got all bent. Im thinking that the connection out of the coil for the tach just needs a little cleaning maybe. Im going to go get a Potentiometer and I really hope that will bring it down from 2000RPM back to about 800RPM.

Thanks,
Chris

[kyle_22r]

the tach in my '79 occasionally goes nuts and reads an exponentially higher number as the revs go up. shows idle at about 2000, then all the way to around 5000 when i tap the gas.  a good slap on the dash usually fixes it

[pimped4Runner]

make it easy and buy an auto meter monster tach!!!!

[lilbuddy [OP]]

That costs more money, other wise i probably would

[Uncle Jesse]

If you can find a good working factory tach, or fix yours, stick with that.  I ran just a 3 3/4" autometer tach on my dash.  That size and the 5" are fun to mount in an inconspicuous place.  Mine was stolen right off the dash.