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SOLVED. 79 Tachometer erratic voltage regulator output question
So long story short..read tons of threads..pm folks active from 18 years ago..lol..
got car tach didn’t work..willcox new tach board..fixed yay!
driving bang smoke dash panic freak out pictured car burning down.stunk...anyway willcox saves me again says voltage spike board..suggests new coil..i get new coil, cap etc..no module replacement though..replaced tach filter no difference with or without filter
been working great for months..except..
when starting up tach will jump up as its cranking..or down..depending on moon...not always same..severity changes too..
and when warms up idle off around 400 rpm low but all other rpm ok...all good..
been checking grounds wires etc..nothing..
i read one post where a guy says replaced alternator and fixed wacky tach as alternator was overcharging..i plug my pp3 in cig lighter and get 12.5 v off
at alternator car off
big red 12.5
little red 12.5
brown 0 off
ignition on not running
same as above but BROWN wire 2.5 volts
start car high idle
big red 15.5
little red 12.1-3
brown 15.5
low idle warm about same...trends down to about 15..but no lower..
nothing on ..never had issue of battery dead or anything else funky..starts runs great...maybe once in a while i notice at idle slight tach drop and hear engine drop for a second...but never dies runs 600 -650ac on 900 w/o ac..
after test voltage 12.8 drops to 12.7 and stays...
guidance appreciated.. Big red Little red Brown
Last edited by interpon; Apr 15, 2021 at 08:12 PM.
Reason: Solved
I thought maybe my power probe plugged into lighter was giving me weird reading..
fluke at battery 14.7...pp3 at battery 14.7..
so the lighter and alternator are in fact same as small red wire 15 plus..
here is video of starting tach...of course it doesn’t swing as wildly while filming...but you get the idea..
Your aternator appears to have a bad diode trio. To verify if an alternator is influencing an issue with the electical system, disconnect it and see if the problem is resolved. Rule of thumb for excessive A/C voltage test readings is to (engine running @1500 rpm) have no more than .5 a/c volts at the battery terminal on the alternator with all accesories switched on.
Your aternator appears to have a bad diode trio. To verify if an alternator is influencing an issue with the electical system, disconnect it and see if the problem is resolved. Rule of thumb for excessive A/C voltage test readings is to (engine running @1500 rpm) have no more than .5 a/c volts at the battery terminal on the alternator with all accesories switched on.
Your aternator appears to have a bad diode trio. To verify if an alternator is influencing an issue with the electical system, disconnect it and see if the problem is resolved. Rule of thumb for excessive A/C voltage test readings is to (engine running @1500 rpm) have no more than .5 a/c volts at the battery terminal on the alternator with all accesories switched on.
so disconnect the 3 wires? All 3 and crank again?
sorry 0.5 volts difference you mean from where i measured at big red to battery? As little red is about 0.5 and big red about 0.8 difference in voltage readings
normal to have brown 2.5 volts ignition on and not running?
i get 20 mv between battery posts AC when running..does that mean diode ok?
Last edited by interpon; Jun 13, 2020 at 10:31 PM.
What you could try is start the car, verify the tach is nutz, and then unplug the 2 wire connector. This kills the field and stops the output. See if the tach settles down.
Last edited by Big2Bird; Jun 13, 2020 at 08:44 PM.
What you could try is start the car, verify the tach is nutz, and then unplug the 2 wire connector. This kills the field and stops the output. See if the tach settles down.
no issue when running..just cranking...
i did disconnect all 3 wires..
and ignition on moved lever through off ac high fan and vent..each time it jumps the needle..lights too..doesn’t do it with lower loads like power antenna or radio..electric fan also on all the time..
here is that
Last edited by interpon; Jun 13, 2020 at 10:32 PM.
Battery, terminals,cables,main ground,starter. What is their condition?
Most Delco starters suck 30-40 amperes on SBC. If the battery is large enough, no biggie.
I suspect the tach does not like voltage dips.
Battery, terminals,cables,main ground,starter. What is their condition?
Most Delco starters suck 30-40 amperes on SBC. If the battery is large enough, no biggie.
I suspect the tach does not like voltage dips.
walmart nov 2017..seems rock solid..cca last i checked exceeded oem..it has never stranded me..needle moving on amp draw nt even the starter
your point is accepted..i shall remove ground under frame and reclean..everything really looks good though...
i shall in recheck...
thank you
Last edited by interpon; Jun 13, 2020 at 10:42 PM.
Lets test the grounds. Your power probe may be long enough if not you will have to come up with an extension like a jumper cable or similar. Engine running all accessories on Attach your negative lead to the battery neg directly inside the car and touch your probe to the case of your alternator (ground). Document the number. Next With the engine off disconnect power to your distributor. Leaving your lead attached at the battery now probe a clean metal spot on your intake manifold and have someone crank your engine. Is nice to have a min-max feature for this test. Document the number. The Next ground test point will be behind the left kick panel. Leaving your meter lead attached to the neg battery cable, probe the exposed metal and crank the engine and document the number. These tests should bear fruit if you have a general ground issue.
Lets test the grounds. Your power probe may be long enough if not you will have to come up with an extension like a jumper cable or similar. Engine running all accessories on Attach your negative lead to the battery neg directly inside the car and touch your probe to the case of your alternator (ground). Document the number. Next With the engine off disconnect power to your distributor. Leaving your lead attached at the battery now probe a clean metal spot on your intake manifold and have someone crank your engine. Is nice to have a min-max feature for this test. Document the number. The Next ground test point will be behind the left kick panel. Leaving your meter lead attached to the neg battery cable, probe the exposed metal and crank the engine and document the number. These tests should bear fruit if you have a general ground issue.
thank you..so to be clear..if i use pp i am hooking it up to battery pos neg..and using tip to measure at alt etc? Ac?
or do i use multimeter from neg battery to locations? I have plent pp cord
DC Volts. My first recommend is see if a diode is bad producing AC volts. You will be measuring Neg to Neg To identify a difference. In a perfect world there would be non or 0 volts.
Ok will assume vdc..
power probe shows zero..switch to fluke mm....
neg cable battery to alternator lights fans defogger on car high idle cold..
0.061 alternator..recheck 0.043
0.02 to manifold recheck 0.012
Cranking distributor power pulled fans on lights off cranking...
manifold 0.28
left kick panel scratch 0.13
Not sure why extra zero on first test? Vs cranking?
thank you
Last edited by interpon; Jun 14, 2020 at 03:13 PM.
DC Volts. My first recommend is see if a diode is bad producing AC volts. You will be measuring Neg to Neg To identify a difference. In a perfect world there would be non or 0 volts.
SOLVED. 79 Tachometer erratic voltage regulator output question
