Tach dash questions. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**too much stuff** · 03-11-2012, 04:27 PM

I made a harness to hook up tach dashes to test. The test rig is a 71 with hei. I hooked up a 73 dash and the tach went to zero and stayed there. I also hooked up an 87 dash and it seemed like it was 2,ooo rpms high but it was working. I tried some mid-late 70s ones and all worked fine. Does that mean i have 2 bad tach heads? or is there something different about certain years using the same tach harness? Any insight? Thanks.

motornut · 03-11-2012, 10:31 PM

i was reading about this repair
http://67-72chevytrucks.com/vboard/s...=503733&page=2
maybe some are for 6 cyl?

**hatzie** · 03-12-2012, 02:38 PM

A 6 cylinder tach should read 25% high on an 8. It may be a I6 tach or it may have a bad resistor in the DIP resistor pack.

**too much stuff** · 03-12-2012, 03:51 PM

Thanks for the info. I was wondering if the 73 dashes were better off a coil rather than hei.

**hatzie** · 03-13-2012, 10:28 AM

They both run off the coil - lead. I've looked at the tach signal from HEI and Points with a scope. They were similar. Points make more noise but are the same otherwise.

03-11-2012, 04:27 PM	#1
too much stuff Roadtrip! Join Date: Feb 2003 Location: Grand Rapids MN Posts: 13,787	Tach dash questions. I made a harness to hook up tach dashes to test. The test rig is a 71 with hei. I hooked up a 73 dash and the tach went to zero and stayed there. I also hooked up an 87 dash and it seemed like it was 2,ooo rpms high but it was working. I tried some mid-late 70s ones and all worked fine. Does that mean i have 2 bad tach heads? or is there something different about certain years using the same tach harness? Any insight? Thanks. __________________ 67 gmc custom lwb dakota truck. Sold...And bought back 69 swb 4x4 project (in progress) 70 c-10 lwb texas truck 70 4x4/BB/bkts/posi/tach(retired) 70 swb (wifes) 70 jimmy camper 71 K-5 factory 2wd 71 cheyenne 4x4/tach/posi/ plow rig 71 Cheyenne20 BB, buckets, tach, tilt, 4 speed 72 jimmy CST wyoming rig 72 jimmy Colorado rig 72 lifted texas 4x4 3/4 ton burb 65 fury conv. 383/auto 65 chrysler 300 "L" 413 auto

03-11-2012, 10:31 PM	#2
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: Tach dash questions. i was reading about this repair http://67-72chevytrucks.com/vboard/s...=503733&page=2 maybe some are for 6 cyl? __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350

03-12-2012, 02:38 PM	#3
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Tach dash questions. A 6 cylinder tach should read 25% high on an 8. It may be a I6 tach or it may have a bad resistor in the DIP resistor pack.

03-12-2012, 03:51 PM	#4
too much stuff Roadtrip! Join Date: Feb 2003 Location: Grand Rapids MN Posts: 13,787	Re: Tach dash questions. Thanks for the info. I was wondering if the 73 dashes were better off a coil rather than hei. __________________ 67 gmc custom lwb dakota truck. Sold...And bought back 69 swb 4x4 project (in progress) 70 c-10 lwb texas truck 70 4x4/BB/bkts/posi/tach(retired) 70 swb (wifes) 70 jimmy camper 71 K-5 factory 2wd 71 cheyenne 4x4/tach/posi/ plow rig 71 Cheyenne20 BB, buckets, tach, tilt, 4 speed 72 jimmy CST wyoming rig 72 jimmy Colorado rig 72 lifted texas 4x4 3/4 ton burb 65 fury conv. 383/auto 65 chrysler 300 "L" 413 auto

03-13-2012, 10:28 AM	#5
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Tach dash questions. They both run off the coil - lead. I've looked at the tach signal from HEI and Points with a scope. They were similar. Points make more noise but are the same otherwise.



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