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~Can one safely tap electricty from the HEI-distributor's 12V./switched-wire to operate elect.-choke?

Vette/Berlina-coupe

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Joined
Nov 18, 2002
Messages
284
Location
southern california
Corvette
C3-coupe/MercedesBenz-540K replica! Racing-SB-V8!
a.) Wondering if i can tap the oem 12v. wire which clips-in (white plastic fitting which pulls-down to remove) to the HEI-distributor; --would like to simply run a wire to the Rochester-carb's. Elect.-choke's male-terminal, --or would that possibly interfere with distributor performance?
b.) Also, the Tach. was working perfectly prior to having done an eng.-R&R, -the eng. runs terrificly now but the Tach does not operate, -only floats up a few hundred rpm's when Ign. turned-'off', and quickly moves to '0' when Ign. is turned-'on', yet stays at '0'-rpm's when engine is running!! Cannot find any wire having that special clip that would plug-in to the Tach.-receptacle on the HEI/distributor-cap! --Or is the oem-Tach. somehow fed its timed-signal some other way?
c.) Is the Tachometer fused? --if so, what Fuse-terminal, and what amperage-rating?
Have the Vette factory-manuals and a large color/wiring-diagram laminated in plastic, --unfortunately these vital references are presently buried in storage after moving...
Please advise... ~Robair :w
 

Peer81

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Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
I run 12v from the HEI to my heated O2 sensor, no problem before or after the mod.
But if you want to do it the right way, it's difficult to get 2 wires in a new connection that will fit in the white clip.

Greetings Peter
 

TimAT

Well-known member
Joined
Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
a.) Wondering if i can tap the oem 12v. wire which clips-in (white plastic fitting which pulls-down to remove) to the HEI-distributor; --would like to simply run a wire to the Rochester-carb's. Elect.-choke's male-terminal, --or would that possibly interfere with distributor performance?
Yes. But it might be better to simply add a wire from the fuse block "IGN" terminal to a relay, and then power the choke heater with a direct relay, fused source.

b.) Also, the Tach. was working perfectly prior to having done an eng.-R&R, -the eng. runs terrificly now but the Tach does not operate, -only floats up a few hundred rpm's when Ign. turned-'off', and quickly moves to '0' when Ign. is turned-'on', yet stays at '0'-rpm's when engine is running!! Cannot find any wire having that special clip that would plug-in to the Tach.-receptacle on the HEI/distributor-cap! --Or is the oem-Tach. somehow fed its timed-signal some other way?
Mounted to the intake, usually just to the left of the distributor is a tach filter. It looks like a capacitor with a mounting tab and a wire on each end. The tach plugs into that, and the other end into the TACH terminal on the HEI cap.


c.) Is the Tachometer fused? --if so, what Fuse-terminal, and what amperage-rating?
Have the Vette factory-manuals and a large color/wiring-diagram laminated in plastic, --unfortunately these vital references are presently buried in storage after moving...
Please advise... ~Robair :w

Yes, thru the "GAUGES" fuse. But since the tach is responding when you turn the key "ON", and all the other gauges appear to be operating correctly, the fuse is good.

And there you are.
 

Vette/Berlina-coupe

Well-known member
Joined
Nov 18, 2002
Messages
284
Location
southern california
Corvette
C3-coupe/MercedesBenz-540K replica! Racing-SB-V8!
a.) Wondering if i can tap the oem 12v. wire which clips-in to the HEI-distributor (white plastic elect.-fitting which pulls-down to remove); --would like to simply run a wire to the Rochester-carb's. Elect.-choke's male-terminal, --or would that possibly interfere with distributor performance?
Yes. -but might be better to instead add a wire from the fuse-block "IGN" terminal to a Relay, thereby powering the elect./Choke-heater via an intermediate Relay, albeit thus fused source.
~Okay thanks, -keen suggestion to wire via Relay so as to relieve the Ign.-switch from having to bare the direct likely substantial amperage-load of the Choke-heater (-thereby avoiding deliterious arcing within the Ign.-switch each time Ign. is switched On). Apparently with Ign.-'on', the Choke/heater-coil element is always imposing an Amp.-load on the electrical-system, -even when the Choke-flap is fully opened? --which seems a poor design; --rather like the notion of driving with lights On in daytime, --which is work being done at the cost of greater fuel-consumption!

b.) Also, the Tach. was working perfectly prior to having done an eng.-R&R, -the eng. runs terrificly now but the Tach does not operate, -only floats up a few hundred rpm's when Ign. turned-'off', and quickly moves to '0' when Ign. is turned-'on', yet stays at '0'-rpm's when engine is running!! Cannot find any wire having that special clip that would plug-in to the Tach.-receptacle on the HEI/distributor-cap! --Or is the oem-Tach. somehow fed its timed-signal some other way?
Mounted to the intake, usually just to left of the HEI-distributor is a 'Tach-filter'; -which looks like a capacitor with a mounting-tab and a wire extending from each end thereof. The Tach plugs into one end, -while the other end plugs into the 'TACH'-terminal indicated on the HEI-cap. Once thus so wired the Tachometer should operate...
~Xlnt. explaination, shall now look amongst my spares to see if i can find that missing necessary 'Tach-filter' device, which obviously should have been more carefully left connected with the HEI-distrib. (-snafu). Wonder what color the Tach-wire is? --i see a couple of wires not yet reconnected, one is 'sky-blue' other is 'grass-green'!

c.) Is the Tachometer fused? --if so, which Fuse-terminal, and what amperage-rating?
Have the Vette factory-manuals and a large color/wiring-diagram laminated in plastic, --unfortunately these vital references are presently buried in storage after a usual disruptive moving event...

Yes, thru the 'GAUGES'-fuse-terminal. But since the Tach is presently properly moving to zero when you initially turn the Ign.key-'On', --and all the other gauges appear to be operating correctly, -that indicates the fuse is good.
~Okay, and thanks again for your great help further on preceeding items- a. & b. ... -Robair :beer
 

bpassmore

Member
Joined
Sep 11, 2009
Messages
21
Location
Alvin, Tx,
Corvette
82 Collector, 75 vert Red
I think the choke circuit on some GM vehicles was through the wiper circuit, and picked up it's power from there. Save running another wire through the firewall. (Yellow wire?)
 

Vette/Berlina-coupe

Well-known member
Joined
Nov 18, 2002
Messages
284
Location
southern california
Corvette
C3-coupe/MercedesBenz-540K replica! Racing-SB-V8!
Appreciate the three respondents to my stated Tachometer-wiring question; --but still not solved..!

~Again, -my HEI-distrib. is missing its Tach.-wire (-was advised it was connected to a condenser-like Filter-device), --yet upon close inspection, there is no unused single Wire feeding from the unmolested oem Wiring-harness proximal the HEI-distrib. and the windshield-wiper Motor on my 1980-Corvette (15K-miles documented). Today i looked at a friend's oem-'79-Vette and see there's a brown-wire extending from the Wiring-harness directly into the HEI-cap right next to the likewise plugged-in 12V./Ign.-wire (-there's NO Tach.-filter on his wiring, --the brown-wire goes straight to the HEI-distrib.). His Tach.-wire is brown, -yet on my '80Vette the brown-wire (coming from the exact same place as his Wiring-harness), -goes to an Oil/pressure-sensor at the rear of the block (immediately behind the intake-manifold). Thus i remain in total quandary as to what Color wire to look for on my '80-Vette, which prior to my eng.-R&R must have came from someplace to plug-in to the Tach.-port on the HEI-cap !?
Perhaps i should also mention that i removed the Speed-control apparatus entirely, -thus there are some wires dangling unused from that component; -and in particular, there is dangling now unused from the Wiring-harness 6-inches from the HEI-cap a square/black-female-receptacle (-having two/green-wires to one internal-contact and one/brownish-wire to the other internal-contact). Hummm, maybe my disconnecting the Speed-control system has something to do with the Tach. not now working??
Was on a quest to simplify underhood clutter, --thus removed all Smog-related plumbing stuff, removed the Pwr.Steering, and the Pwr.Brakes, ---just wanted a basic clean engine-compartment; --thus the now manual-steering & manual-brakes function perfectly; --but not the Tachometer!
--Please advise... :w
 

TimAT

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Joined
Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
Link to a 1980 Wiring Diagram:
http://www.nrjvette.com/johdotukset/Chevy Corvette 1980.pdf

The tach filter on the print is shown just above the distributor. Anyway, it gives the OEM wire color
so you at least get that in your hunt.

When I'm chasing a problem like this, I print the diagram as big as I can, mate and tape the edges so the wires correctly bridge from page to page, then take a highlighter pen and start chasing the wire I'm looking at.

IT's there, just keep digging.
 

Vette/Berlina-coupe

Well-known member
Joined
Nov 18, 2002
Messages
284
Location
southern california
Corvette
C3-coupe/MercedesBenz-540K replica! Racing-SB-V8!
~Thanks Tim, -for the legible 1980Vette/Wiring-diagram, -your linked PDF-pages printed-out nicely, and was a big help indeed! Thus now have the Tach. operating (-albeit without the Tach.-filter presently; --i presume the Tach.-filter only serves to blank-out RF/Ign.-static to the radio). Strangely, could not find the Wiring-diagram stated 'white-wire' in the main/Wiring-harness where the 12V./pink-wire emerges for connection to the HEI-cap.; --but strangely in a separate wiring-harness i did find an unconnected concealed 'ivory'-wire (-definitely not white) having the appropriate end-clip for attachment to the Tach.-port on the HEI-cap, --so with uncertaintly i turned-on the Ign. the floating Tach.-needle moved to '0', -then upon starting the cold-engine 'zingo' she went to 1,200-rpm (-initial cold-idle), and seems to be operating perfectly at various eng.-speeds!
As mentioned, a friend's oem/'79-C3Vette's wiring i looked at is quite different, -in that a brown-wire emerges from the main/Wiring-harness about 6" from the HEI-cap connecting-port; --so let this be a lesson to anyone else, -there seem to be considerable wiring differences between these two years.
Accordingly, i'd still like to hear from other 1980-C3 owners who can visually confirm my observations to be correct?; --or, do they instead actually find a pure- 'white-wire' (#121 on the Wiring-diagram) coming from the main/wiring-harness with the 12V./pink-wire. The wiring-diag. indicates the White-wire (#121) goes to a "Printed-circuit connection" in the dash apparently -whilst the Tach/Filter-wire (missing on mine) is shown going as a Tack-input to what is called a "Closed-loop Carburetor-control unit" (which location is not known), -but i've installed a simpler conventional Rochester-4B having no such pesky electronic features (-the way God originally intended it to be)! The car is nevertheless 'passing' Calif./Smog-tests, --so to me, -that is all that matters...
~Thanks again... Robair in SanDiego 92123 :beer
 

TimAT

Well-known member
Joined
Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
There is a huge difference between 79 and 80 wiring. 79 was the last year for no ECM control. 80, they went to a pretty simple (compared to today's systems) ECM control. The ECM controlled the mixture but also timing. Does your distributor have another plug coming out of it below the cap?
 

Vette/Berlina-coupe

Well-known member
Joined
Nov 18, 2002
Messages
284
Location
southern california
Corvette
C3-coupe/MercedesBenz-540K replica! Racing-SB-V8!
Hello Tim: -No, there are no other wires to be found associated with my '80-Vette/SB-V8 HEI-distributor; --however, the Tach does now seem to be operating properly thanks to your able advise. :w
 

Tepot

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Joined
Sep 13, 2005
Messages
307
Location
Moncton, NB, Canada
Corvette
1974 Metalic Blue L-48 Coupe 4 speed
I really don't recommend this. Leave your distributor alone. I had done this on my Chevy 350 on my boat and I had issues when I was at full trottle. it's seemed to miss at 3000 RPM. turned out that there was not enouph power going to distributor. I then fixed that with getting power from another feed.
 

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