factory 73 ss tach does not work

by clanceman427 Tue Jul 19, 2016 8:02 pm

Tach just rebuilt by tachman, car has hei ignition. I have continuity between the factory underhood Brown wire to the lowest post connection at the tach. The tach blade attached to the yellow wire in the hei coil sends out a pulse that varies with engine speed that I verified with my multimeter. How come no worky?

by clanceman427 Wed Jul 20, 2016 1:36 pm

more digging: I verified that the hole in the dash that supplies 12V to the 12V peg on the tach is in fact providing 12V with the key in the on position, both with engine off and engine running. I also checked the ground hole in the dash that goes to the ground peg on the tach for continuity between it and the metal emergency brake pedal, and I got continuity, and I also checked at the sill plate and had continuity too, so that tells me the ground hole is good.

I will check for Hall effect signal at the hole in the dash that goes to the coil peg on the tach this afternoon while the engine is running to verify that the sine wave is coming thru. Beyond that, I'm gonne be scratching my head....

by Hawk03 Wed Jul 20, 2016 7:40 pm

If you pull the tach out of the cluster and hook it up to the distributor, does it work? If not, then it is probably a dead tach. If it does work then you will probably need to check the wires to see if which one is causing the issue.

by clanceman427 Wed Jul 20, 2016 8:53 pm

I got it working! I ended up wiring the stock brown wire that originally went to negative post on the points coil to the filter that I already had bolted to the firewall back before I had the tach rebuilt. Other end of the filter went to the tach output on the HEI distributor. I started the engine and then plugged the tach in and I noticed that the tach would be touchy how it was aligned vs. the alignment of the mounting screws . So I just made sure it continued to work as I put in the 2 screws and the mounting bolt. I revvd it while I tried headlights, high beams, turn signals, etc. and it functioned fine. I probably did engine damage revving it to the sky testing out the new tach! Hehe. factory 73 ss tach does not work Imag1512

factory 73 ss tach does not work Imag1512

by Hs1973 Thu Jul 21, 2016 4:36 am

I dont know why you need a filter though? I converter my square dash with idiot lights to a 73 ss with gauges. And i just hoked everything up. No filter and the tach works fine with my hei. No problems with it ever. I did buy a new wiring loom that was made to fit 73 with gauges. I bought it from elcamino store. Pricey but Nice to know all wiring is new.

by clanceman427 Thu Jul 21, 2016 6:18 am

It might not need the filter, yesterday I went with what was currently the most convenient setup as it stood with my car (already had a blade connector on one end of the filter to plug into the hei tach port) and then made a temporary setup with a ring terminal on the other end of the filter and used a bolt, nut, and washer to secure the factory brown wire connector to that end. this preserves the end of the factory vrown wire for later when the car gets restored back to factory points setup.

by Joe73 Thu Jul 21, 2016 7:19 am

Just a thought.
But could these issues have something to do with the distributor supply wire? Points cars have a resistance supply wire feeding the distributor. When you put an electronic distributor in a points car, you have to run a new power wire direct from the fuse block and "hot" when the key is on. Otherwise you dont get full power to the distributor.

by clanceman427 Thu Jul 21, 2016 1:28 pm

Good thought, but whoever converted my car to HEI made sure to cut off the resistor in the wire, and they combined the 2 wires that would have gone to the positive turminal on the points coil, and connected them to one pink wire with the HEI connector (my hand is holding the 2 factory wires before they go into the aftermarket connector. Also, the tach power was reading full battery voltage, so that would presume the HEI distributor is getting fed full battery voltage.

factory 73 ss tach does not work Imag0712

factory 73 ss tach does not work Imag0712

by SSTOOLMAN Fri Jul 22, 2016 3:13 pm

I have been off line for the last couple of days or I would have chimed in... with my rebuilt tachman tach I went straight from the brown wire to the tach port on the HEI. No filter needed on mine.

by clanceman427 Fri Jul 22, 2016 4:21 pm

Thanks SSTOOLMAN. I kinda figured that would be the case. So I am chalking it up to just a quirky alignment issue with those pins on the back of the tach and how they interface with the holes in the dash for the electrical connection. My tach has been working fine.

Have you figured out why your tach started acting funky with the high beams and turn signals and such?

by SSTOOLMAN Fri Jul 22, 2016 4:44 pm

Nope I haven't pulled it apart... one of the pin holders must be touching the circuit board ribbon
on and off. Its too hot in my garage to even think about it!

by 73ss Fri Jul 22, 2016 5:14 pm

actually it looks like you do still have the resistor. The whole wire is the resistor. I think it is the black one. Pull some of it out of the loom and you should see "resistor wire" marked on the insulation. The whole length of wire is 3 ohms or so. The other wire goes to the starter to bypass the resistor during crank. You will have to cut it off at the bulkhead and replace it with straight wire to eliminate it, or use it to trigger a relay to provide full voltage to the coil. If you are getting a full 14 volts at the coil when it is running then it should be fine.

by clanceman427 Fri Jul 22, 2016 5:25 pm

Hmm, I'll have to unplug that connector that supplies the power to the HEI distributor and see what the voltage is. Thanks for the info on that wire.

by clanceman427 Fri Jul 22, 2016 9:50 pm

I measured voltage at battery, 12.2V, then measured voltage at plug that provides power to HEI, 11.9V. I consider that .3 drop just because of the wiring, and not a factory resistor drop. Wouldn't you agree?

by Joe73 Sat Jul 23, 2016 5:56 am

I think you need to measure the voltage with the car running.

by 73ss Sat Jul 23, 2016 9:33 am

If you are getting .3 drop or so with the motor running you should be good. The resistor wire would drop it down to 9 V or so. Someone may have already replaced that wire out..

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factory 73 ss tach does not work

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