im back... but now wierd fact tach issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**tkeduk8r** · 06-18-2005, 12:32 AM

well now that school is out, I finally hve a little more time to work on my truck than on my kids' grades... SO I went out to hook up my fact tach cluster (74 c70 tach/vac) and using the fact harness, plugges everything up and the tach reads backwards... to explain, it idles at a beautiful 4200 RPM and revs counterclockwise perfect!!! i know... weird... so i took a tach out of one of my other clusters (79 standard tach) and plugged into same harness and that one works fine.. idles 800, yadayadayada... since i have yet to find another 73-75 tach with the fuel gauge (which works by the way) i almost hate to switch any coil, 12V, or ground connections for fear of REALLY toasting the tach... any electronic gurus out there, i know i saw a post ott one time about someone changing the tach calibration to read 0-8000 RPM, and i wonder if that person or people understands the electronic stuff of the tach.. I did have two semesters of electonics in undergrad, but i dont seem to have any old notes pertaining to tach operation... any suggestions???? crossy or jeff???
==Mike

Russell · 06-18-2005, 01:08 AM

Interesting...

Nemocramncer (or however it is spelt, hehe

) recalibrated my tach to a 7k unit, so he may possibly be able to help you out. However, I have never heard of a tach working backwards...

If you want a new tach, I've got a spare one sitting around collecting dust that I'd be very willing to get rid of

You can have it for 75 dollars shipped to your door

**tkeduk8r** · 06-18-2005, 08:06 AM

Russ,
is that a tach w/fuel gauge on the bottom and does it work??? I do have a few standard tachs around, likei said my 79 does work, but i would like to keep the 73-75 tach w/fuel gauge... any other suggestions???
--Mike

Russell · 06-18-2005, 01:59 PM

There was a tach with the fuel gauge built into it?

Cool, never seen one before! This one is just a standard tach

NeCrOmAnCeR · 06-19-2005, 10:33 AM

Here you go, the jumper is the orange plug.

jermasaurus · 06-19-2005, 11:22 AM

There was a thread about the tach/fuel gauge a while back. From what I remember, they could bring in a couple hunedred for a working one. I think whoever had it got it from a one ton truck.

**tkeduk8r** · 06-20-2005, 12:49 AM

allright... got out to the truck this evening and snapped some pix... looks like i have a different circuit board than you do. both on my 73/74 fuel/tach and the 79 i have ripped apart... some pix are below...the 74 fuelie has the circuit mounted on a ceramic board and the tach has four terminals that attatch to it... the 79 has circuit board simialr to yours but no jumper.... i tried the 79 board on the 74 tach, but there is no provision for the lower tach terminal screw, so therefore will not fit.. the 74 board would however fit the 79 tach... but here are the pix...

**tkeduk8r** · 06-20-2005, 01:14 AM

ill try to get the pix tomorrow... gf's computer doesnt have any software to resize the 1024x768 pix, I can get the pix compressed but nothing to readjust the size.. wheres my photoshop when i need it...

NeCrOmAnCeR · 06-20-2005, 09:45 AM

Don't bother trying to switch the board from the three post tach to the four post tach, two totally different style "motors" It won't work and you might end up with two inoperative tachs.

**tkeduk8r** · 06-22-2005, 02:27 AM

update... well i d got my photoshop installed on her puter and now i have pix... however i did decide to do with a traditional 74 tach for now (since when i pulled the cluster out i saw it had been recently refurbished and calibratedby a place in the cities) and sure enuf worked like a charm... so guess its more orignal that way an when i getthe fuelie tach fixed (if possible) then i can still put it in with the vac gauge... but here are the pics i was talkin about...

Russell · 06-22-2005, 02:36 AM

Man, thats a cool little tach! I often wondered how the big trucks had a tach, and vac gauge

06-18-2005, 12:32 AM	#1
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	im back... but now weird fact tach issue well now that school is out, I finally hve a little more time to work on my truck than on my kids' grades... SO I went out to hook up my fact tach cluster (74 c70 tach/vac) and using the fact harness, plugges everything up and the tach reads backwards... to explain, it idles at a beautiful 4200 RPM and revs counterclockwise perfect!!! i know... weird... so i took a tach out of one of my other clusters (79 standard tach) and plugged into same harness and that one works fine.. idles 800, yadayadayada... since i have yet to find another 73-75 tach with the fuel gauge (which works by the way) i almost hate to switch any coil, 12V, or ground connections for fear of REALLY toasting the tach... any electronic gurus out there, i know i saw a post ott one time about someone changing the tach calibration to read 0-8000 RPM, and i wonder if that person or people understands the electronic stuff of the tach.. I did have two semesters of electonics in undergrad, but i dont seem to have any old notes pertaining to tach operation... any suggestions???? crossy or jeff??? ==Mike __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's Last edited by tkeduk8r; 06-18-2005 at 12:33 AM.

06-18-2005, 01:08 AM	#2
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	Interesting... Nemocramncer (or however it is spelt, hehe ) recalibrated my tach to a 7k unit, so he may possibly be able to help you out. However, I have never heard of a tach working backwards... If you want a new tach, I've got a spare one sitting around collecting dust that I'd be very willing to get rid of You can have it for 75 dollars shipped to your door __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)

06-18-2005, 08:06 AM	#3
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	Russ, is that a tach w/fuel gauge on the bottom and does it work??? I do have a few standard tachs around, likei said my 79 does work, but i would like to keep the 73-75 tach w/fuel gauge... any other suggestions??? --Mike __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's

06-18-2005, 01:59 PM	#4
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	There was a tach with the fuel gauge built into it? Cool, never seen one before! This one is just a standard tach __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)

06-19-2005, 10:33 AM	#5
NeCrOmAnCeR Registered User Join Date: Feb 2004 Location: Hodgenville, Ky Posts: 735	Here you go, the jumper is the orange plug. Attached Images

06-19-2005, 11:22 AM	#6
jermasaurus Registered User Join Date: Dec 2004 Location: New Lenox, IL Posts: 442	There was a thread about the tach/fuel gauge a while back. From what I remember, they could bring in a couple hunedred for a working one. I think whoever had it got it from a one ton truck.

06-20-2005, 12:49 AM	#7
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	allright... got out to the truck this evening and snapped some pix... looks like i have a different circuit board than you do. both on my 73/74 fuel/tach and the 79 i have ripped apart... some pix are below...the 74 fuelie has the circuit mounted on a ceramic board and the tach has four terminals that attatch to it... the 79 has circuit board simialr to yours but no jumper.... i tried the 79 board on the 74 tach, but there is no provision for the lower tach terminal screw, so therefore will not fit.. the 74 board would however fit the 79 tach... but here are the pix... __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's

06-20-2005, 01:14 AM	#8
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	ill try to get the pix tomorrow... gf's computer doesnt have any software to resize the 1024x768 pix, I can get the pix compressed but nothing to readjust the size.. wheres my photoshop when i need it... __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's

06-20-2005, 09:45 AM	#9
NeCrOmAnCeR Registered User Join Date: Feb 2004 Location: Hodgenville, Ky Posts: 735	Don't bother trying to switch the board from the three post tach to the four post tach, two totally different style "motors" It won't work and you might end up with two inoperative tachs.

06-22-2005, 02:27 AM	#10
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	update... well i d got my photoshop installed on her puter and now i have pix... however i did decide to do with a traditional 74 tach for now (since when i pulled the cluster out i saw it had been recently refurbished and calibratedby a place in the cities) and sure enuf worked like a charm... so guess its more orignal that way an when i getthe fuelie tach fixed (if possible) then i can still put it in with the vac gauge... but here are the pics i was talkin about... Attached Images __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's

06-22-2005, 02:36 AM	#11
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	Man, thats a cool little tach! I often wondered how the big trucks had a tach, and vac gauge __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)



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