Tachometer help

[swizman]

Polaris daily driver resto is coming along nicely, but now the tachometer is not working.  I'm pretty sure it was working before I set off on this project...pretty sure because it's been sitting for about six years...  The car starts and runs nicely.  Battery and subsequent areas of the car are well grounded/checked and cleaned.  Instrument panel lights, gas gauge, coolant temp all work well.  The fuse box contacts have been well cleaned and all fuses have been replaced with new ones.  Photos show blue wire sheathing separated from keepers for clarity.

 

Here's what I've tried:

Checking and cleaning contacts at the coil end (photos below...I've got the Crane XR700 set up, that I'm very happy with, incidentally).

Pulling the instrument cluster and cleaning/checking contacts there.

Pulling the tach and looking for any obvious gremlins/damage.

Setting a good ground wire from the back of instrument cluster to bare metal on the car.

Pulling another tach out of my stash and trying that one.

Tracing the black tach wire to see if any critters (or my body work guy) may have damaged it.

Going inside, crying, praying, pledging fealty to any deity who might help fix the damn thing.

 

And thus endith my repertoire of troubleshooting skills.  Any help/wisdom is, as always, much appreciated....

 

['76mintgrün'02]

This doesn't look good

Is that the clear coated resistor wire, with what looks like a little chunk of clay prior to a burned section?

If you have the stock resistor wire, you will not need/want the ceramic resistor inline as well.

 

[HBChris]

Be sure tach has switched power as well as the ground.  The black wire goes to the distributor and then the coil generally although it can be run separately.  I suspect a bad black wire.

['76mintgrün'02]

The resistor wire is crimped onto a green wire, inside the blue sheathe, behind the distributor.

You can measure its resistance from that crimp to the end with an ohm meter and see if it is still okay.

If it still has the original resistance, you can eliminate the ceramic resistor.

If not, you could remove it and crimp a 'regular' wire in there and run that to/through the ceramic resistor.

Tom

 

 

[Dudeland]

The resistor wire is crimped onto a green wire, inside the blue sheathe, behind the distributor.

You can measure its resistance from that crimp to the end with an ohm meter and see if it is still okay.
If it still has the original resistance, you can eliminate the ceramic resistor.
If not, you could remove it and crimp a 'regular' wire in there and run that to/through the ceramic resistor.
Tom
 
 

It looks like you are running a double whammy of resistors. You could just get a blue coil and cut all that stuff out.


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[swizman]

Thanks guys!

 

RE:  The resistance wire...yeah I noticed the double whammy of resistors when I looked at it with fresh eyes.  I'll take the comments into account, but I suspect that Mintgun's comment about the burnt area on the resistance wire and that "this doesn't look good" prolly means the resistance wire is fried anyway....which would necessitate the need for the additional ballast.

The car runs great and the motor rips...its easily the best of my '02s, so whatever voodoo is going on there, I'm hesitant to "fix what ain't broke".  The tach was working with this set up when I was using Polaris as my daily driver several years ago.  I just didn't remember if it was still working when I drove it to the body shop last Fall.

 

The simple solution seems to just make another black wire and run it to the coil as HBChris suggested.  That's a good idea, and I was prolly so frustrated at the end of my quest, I overlooked this simple test.  I'll give this a go and post a follow-up as soon as I'm able.

 

As an aside, my day yesterday was occupied acquiring a Grey Market 1979 320i with the sport package.  So it looks like Polaris will finally get Recaros AND an LSD.  F--k Yeah!! 

 

Yup, the Recaros need a bit of attention...

 

Thanks again for the help...and if anyone else has any ideas about why that tach isn't cooperating, I'm all ears...

 

[PatAllen]

did it ever worked before with the fireball ?

[swizman]

Hey PatAllen, in answer to your question, yes, the tach was working with the Fireball.  That's the set up its had since I bought the car in 2004. 

 

im suspecting that's something happened at the body shop or that a critter found its way into the wiring.  Still have not tried running a separate black wire...busy pulling and crating the "new"Recaros and getting them down to Aardvark for a refresh.  Then new front pads on Malaga.  It'll be this weekend before I can touch Polaris again.

[AustrianVespaGuy]

My advice would be to get a good blue coil (not terribly expensive, or just borrow one temporarily) and then bypass all the resistors and run a fresh new wire from the negative of the coil to the back of the tach, and then see if everything works.  Assuming it does with all of those variables (resistors and wiring) cut out, you can then start piecing things back together until you find the issue.

And if it doesn't then you KNOW the problem is at the tach itself, though I highly doubt this will be the case.

Edited August 15, 2018 by AustrianVespaGuy

[PatAllen]

i reversed the whole circuit of the 02 tacho some time ago

 

the 02 tacho needs to see an inductive positive spike of around 150v minimum to "read" the pulse itself.

 

is the fireball an CDI unit ?

 

[AustrianVespaGuy]

7 minutes ago, PatAllen said:

i reversed the whole circuit of the 02 tacho some time ago

 

the 02 tacho needs to see an inductive positive spike of around 150v minimum to "read" the pulse itself.

 

is the fireball an CDI unit ?

 

While all true, and I think the Crane is CDI, I'm pretty sure that voltage spike comes from the flyback voltage of the coil field collapsing, so it shouldn't matter at all how you 'trigger' the coil, as long as it fires (and it clearly does in this situation as the engine runs) and no one intentionally put a flyback diode in the circuit to suppress it, then that pulse MUST be there. . .

[PatAllen]

 

the spike comes indeed from the coil field collapsing in normal operation but it must be referenced to ground/positive of the car, while most CDI are not ie the coil is literaly grounded all the time and the positive of the coil receives the cap pulse. it may have destroyed the tacho then.

 

in many cases the CDI instructions shows that you should not ever connect anything to the coil low side with CDI units, especialy a tacho. this is/was true with Crane, mallory, MARK TEN and MSD. a seperate circuit could have been provided t odo so but it was clearly and electronically really not the way to do.

There are seperate wires on the unit to provide tach pulses outputs.

fwiw i reversed many CDI units in the past and made mines too. this is soo long ago gawd....

[AustrianVespaGuy]

22 minutes ago, PatAllen said:

most CDI are not ie the coil is literaly grounded all the time and the positive of the coil receives the cap pulse

Thanks, I didn't realize that CDIs ran coils 'backwards' like this, so yeah now I see how that could be dangerous, good to know, thank you!

[Simeon]

Rather than being an inductor, the coil acts as a pure transformer. The primary voltage is circa 400V depending upon the type of ignition. 

[PatAllen]

1 hour ago, Simeon said:

Rather than being an inductor, the coil acts as a pure transformer. The primary voltage is circa 400V depending upon the type of ignition. 

 

the comment was in regards to what the tachometer sees in normal usage, ie an inductive pulse. if you forget that there is a secondary winding, it becomes a simple inductor.

i have 30+ years worth of electronic engineering for cars behind me. sometimes i say things that seems too obvisous to explain. sorry.

Edited August 16, 2018 by PatAllen