Tach is reading twice what it should.

[warpspeed]

Can anyone tell me how to fix a tach that is reading high.  Car is 68 Charger with Tic tac clock.  At idle, it is reading way too high.

[A383Wing]

did you change anything? New tach board? New ignition system? New tach?

You ain't telling us much here..kinda like asking why yer tire goes flat.

[warpspeed]

Brother, I erased the whole explanation that you asked for somehow.  Now, I have to start over.  It will be much shorter this time.  Sorry.

No, changa the electronia in the tachemoter.  Nope, just redid the dials and plugged it in.  Reads around 2,200 rpm at idle.  Car was 318 car, now 440 that has been  rebuilt from ground rust up.  Took ten years so far.  This cluster was original.  114,000 plus/minus miles on the odometer.

Thinking!!!!  The oil pressure gauge hit max after startup today.  Ground it out, it hits max. Take wire off sending unit and no contact on ground, it is a zero. 

Thinking some more!!!!  Water temperature sending unit must be working.  Temperature gauge was starting to move up for the first time.  BUT TOO QUICKLY!!!  SOMETHING WRONG!

Methinks, that voltage regulator in dash not working!!  Mulitply 5 divided by 12 into the three different readings, and bang:  everything at proper reading, no? 

Next question, does the tachometer get its information through the 5v regulator or need to?  Searching for more information.

I put a new electronic voltage regulator 5v unit in the circuit board of the cluster before putting it back together.  It must not be working!! Maybe that the problem.

[A383Wing]

the regulator in the dash has no effect on tachometer reading...just the fuel, oil, & water gauge

what kind of ignition are you running?

[warpspeed]

whoa, I am having a seizure, wow!

Uh, electronic.  Mopar orange box. ballast resistor, tach wire coming straight off the negative on the coil, rebuilt 67 alternator, with single wire electronic voltage regulator.  Anything else?

[charger Downunder]

Do you have the gauge cluster earthed.

[warpspeed]

if you mean grounded, the standard answer would be of course.  But, really, I can't be sure unless it check it, right.  I will do that right now.  I think my fenders have too much paint on them because they don't seem to be showing ground.  Not 100% sure but maybe why the side lights aren't working. 

[moparnation74]

You could have a grounding issue.  If this is paint buildup through restoration of parts/body.  The cluster is grounded to the frame by the attaching screws. Back a few in and out many times.  The frame is grounded to the body by the bolts on the side behind the kick panels, steering column mount area, and bolts underneath the windshield.  Back a bolt out on one side by the kick panels and ensure it has a good clean ground.

Simple checks and may not be the issue. 

Is this a new/reconditioned tach?

[charger Downunder]

if you mean grounded, the standard answer would be of course.  But, really, I can't be sure unless it check it, right.  I will do that right now.  I think my fenders have too much paint on them because they don't seem to be showing ground.  Not 100% sure but maybe why the side lights aren't working. 

yep grounded

[A383Wing]

I remember reading about this the other day....can't remember where though....I thought it had something to do with reversing the wires at the dist connection to the ECU.....

I'm still looking

[warpspeed]

Reversing wires at the distributor?  Interesting.  Wouldn't that screw up the way the distributor functions?

[A383Wing]

dist will function the same, it's the signal to ECU that gets changed somehow....again, I'm still looking for where I read that

[warpspeed]

Cool, hope you find it.

[A383Wing]

Found it....

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My tach is reading correct at 3000 RPM but it reads about 1500 RPM's at idle, what is wrong?

Glad you asked, this one was a tough one! It turns out that if you wire in a mopar distributor backwards, it will double fire at low RPM, but it will properly single fire at higher RPM. You need to swap the two wires coming from the distributor to the electronic ignition box. Here is the www.moparts.com tech archive article showing the correct way that the mopar ignition should be connected: http://www.moparts.org/Tech/Archive/elec/3.html

[warpspeed]

Everything is wired correctly.   But, I will reverse the wires to the distributor.  Hope that works.

[warpspeed]

You could have a grounding issue.  If this is paint buildup through restoration of parts/body.  The cluster is grounded to the frame by the attaching screws. Back a few in and out many times.  The frame is grounded to the body by the bolts on the side behind the kick panels, steering column mount area, and bolts underneath the windshield.  Back a bolt out on one side by the kick panels and ensure it has a good clean ground.

Simple checks and may not be the issue. 

I have power to the sidelights now.  Grounding was the issue.  They are nice and bright.  One more issue down.

[warpspeed]

dist will function the same, it's the signal to ECU that gets changed somehow....again, I'm still looking for where I read that

I changed the wiring to the distributor today.  Its true!  The tach shot up to 3000rpm!  Changed back and the tach is back to 2000!  Do I need a resistor in line from the tach to the coil?

[A383Wing]

I have never needed a resistor...something else is wrong

back to thinking now

[chargd72]

You could have a grounding issue.  If this is paint buildup through restoration of parts/body.  The cluster is grounded to the frame by the attaching screws. Back a few in and out many times.  The frame is grounded to the body by the bolts on the side behind the kick panels, steering column mount area, and bolts underneath the windshield.  Back a bolt out on one side by the kick panels and ensure it has a good clean ground.

Simple checks and may not be the issue. 

I have power to the sidelights now.  Grounding was the issue.  They are nice and bright.  One more issue down.

Sorry to sidetrack you but where was the grounding issue for you side markers?  I'm having a similar issue.

[A383Wing]

dist will function the same, it's the signal to ECU that gets changed somehow....again, I'm still looking for where I read that

I changed the wiring to the distributor today.  Its true!  The tach shot up to 3000rpm!  Changed back and the tach is back to 2000!  Do I need a resistor in line from the tach to the coil?

A little more thinking on this tach reading being off, or high.....IF nothing was changed, IF nothing was added, I then would assume that the tach board inside the tach has gone bad and needs replacing. Your comment about adding a resistor prompted this post, meaning maybe one of the resistors inside the board have gone bad and not allowing the needle to read right. The boards inside the tach have many resistors, some in parallel, some in series.

I would see about hoking up another tach (known good one) to the car and see what it reads compared to the dash tach in there now. Maybe find a digital timing light and read the RPM off that compared to OE tach

Other than that, I'm about out of options here

[Dino]

Isn't the orange box the culprit here?  I've been hearing about tachs being off with these for years.

I would try another ecu if you can or maybe remove the tach and hook it up to another car to see if it needs to be rebuilt.

[warpspeed]

A383, I thought about what you are saying.  Maybe the tach is bad. But, dino may have a point too.  I need to check my voltage at the coil.  Not sure it is reading correct.  I am sure now that my dash cluster vr isn't working right.  So I am beginning to think there is a mess about. But a383, wouldn't it just be easier to add a resistor in line thane have to tear the whole dash cluster back apart? Then if Dino is correct, maybe it is just the cpu.  I was using the cpu in the challenge r that this cpu came out of but it wasn't feeding a 46 year old tach.

[A383Wing]

make sure you also don't have reversed col polarity...meaning the 12v wire in on the "+" side of coil, not the negative.

How would you know which resistor to use and insert inline?

Instrument regulator has nothing to do with ignition or tach readings...that only controls the fuel, oil, & temp gauges, nothing else

make sure your cluster is properly grounded...try swapping ECU's, try seeing if another aftermarket tach works correctly, find a digital timmng light and see what it reads.....

[warpspeed]

First item, check. Negative to negative, positive to positive

Second item, absolutely don't know.  seems to me, warm up the engine to where it is showing good idle, check the tach in the dash for it's reading and calculate the percentage difference that it should be showing and what it is actually showing and try resistors until it brings it down to about the correct reading.  Um,  I am not an electrical engineer.  Is that making a big mistage playing with fire?  Suggestions, even if I should just let it go.

Third item. I realize the VR in the dash has nothing to do with the tach.  It is just another problem that doesn't make sense.  I put a new electronic circuit board unit in the dash as I have with the other two/three cars I have worked on and this is the first time this has happened.  It is odd to see the needles go straight up to max when I hook them up and also have the problem with the tach reading too.  That's all.

Four, I will have to pull the dash cluster now anyway and I will make sure it is grounded.  It's an old car with issues.  I will solve them with the help of my brothers in arm.

Thank you all for your suggestions.  They don't go on deaf ears.  I just need time to work on the issues.  I finally have the parking lights fixed as of last night.  As always, if in doubt, check the ground.  That did it!  Now to item four and the dash grounding.  I will keep ya'll updated.

[warpspeed]

You could have a grounding issue.  If this is paint buildup through restoration of parts/body.  The cluster is grounded to the frame by the attaching screws. Back a few in and out many times.  The frame is grounded to the body by the bolts on the side behind the kick panels, steering column mount area, and bolts underneath the windshield.  Back a bolt out on one side by the kick panels and ensure it has a good clean ground.

Simple checks and may not be the issue. 

I have power to the sidelights now.  Grounding was the issue.  They are nice and bright.  One more issue down.

Sorry to sidetrack you but where was the grounding issue for you side markers?  I'm having a similar issue.

Ok, first thing make sure of there are bulbs in the sockets, urgh!  They were new and for some stupid reason, I thought the bulbs came with the new sidelights.  Ok, I can make a stupid mistake.  But, once the sidelights were attached, there is a screw on type of nut that holds the light on in it's hole that wasn't getting ground through the new paint.  The fender had been completely dip stripped of paint, so it wasn't rust.  But, the new primer/paint on it was keeping the circuit from grounding which is why it still wouldn't work even after adding the new light bulbs.  Once that was scratched through, the circuit was complete and bingo, lights.  I was thinking earlier that maybe the fender wasn't grounded to the rest of the car becasue of the paint buildup,  but, that wasn't the case after all.  My newly painted engine block was proving that ground is important as the bolt on the negative cable to the battery was over painted and not getting contact.  No engine start.  Solved with the paint scrapped off.