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factory tach issue - wiring all good
#2524253
07/19/18 09:48 PM
07/19/18 09:48 PM
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Joined: Apr 2016
Posts: 356 Greenwood Lake, NY
fuelishnsilly
OP
enthusiast
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OP
enthusiast
Joined: Apr 2016
Posts: 356
Greenwood Lake, NY
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Today I was helping a buddy of mine work on his 69 GTX with a factory tach. He got all new wiring from M&H which was ok and checked to the old factory stuff and everything was good. He hooked it up and the engine would then crank but not fire up. Disconnecting the tach, the engine would fire up. Is there a known issue with when tach's go bad, they can cause the engine to not fire? Seems like a really weird issue and we checked that the gray wire is on the neg side of the ignition coil.
inherited a 69 roadrunner from my grand pop.
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Re: factory tach issue - wiring all good
[Re: fuelishnsilly]
#2524802
07/20/18 11:44 PM
07/20/18 11:44 PM
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Joined: Jan 2003
Posts: 21,499 N.E. OHIO, USA
A12
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 21,499
N.E. OHIO, USA
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If you do a web search for "No spark with tachometer connected" you'll find it is a more common problem than one would think....one example: https://www.mgexp.com/phorum/read.php?1,3595849 ( might even try adding in Mopar to the search) MikeR no apark with tach connected link
Last edited by A12; 07/20/18 11:48 PM.
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Re: factory tach issue - wiring all good
[Re: fuelishnsilly]
#2526173
07/24/18 12:18 AM
07/24/18 12:18 AM
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Joined: Apr 2016
Posts: 356 Greenwood Lake, NY
fuelishnsilly
OP
enthusiast
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OP
enthusiast
Joined: Apr 2016
Posts: 356
Greenwood Lake, NY
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Turns out the tach is shot. We pulled the wiring harness that went through the firewall and there was a repair with a solderless connector but still had good continuity through it. We ended up also installing the dash side of the repro tach harness and still had the issue with no spark. So, the only thing left for him to do is to send the tach out over the winter and have it rebuilt.
Weird how that a bad tach would cause no spark.
inherited a 69 roadrunner from my grand pop.
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Re: factory tach issue - wiring all good
[Re: fuelishnsilly]
#2527421
07/26/18 03:36 PM
07/26/18 03:36 PM
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Joined: Jun 2008
Posts: 1,822 Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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A shorted tach line (like the wiring you already checked and is good) or a shorted tach will pull the negative part of the coil to ground continuously and not allow the coil to fire. Mopar tachs are pretty simple with a couple of transistors and a few resistors. A shorted transistor (base to emitter short, ie) will shunt the points or electronic ignition signal to the coil to ground and not allow it to discharge to the secondary and allow a spark. Craig
2014 Ram 1500 Laramie, 73 Cuda Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango Only previous Non-Mopar: Schwinn Tornado
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Re: factory tach issue - wiring all good
[Re: fuelishnsilly]
#2527546
07/26/18 07:48 PM
07/26/18 07:48 PM
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Joined: Aug 2008
Posts: 637 West Central Wi.
1969 HEMI R-T
mopar
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mopar
Joined: Aug 2008
Posts: 637
West Central Wi.
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had this happen to my old 69 RR.
A shorted tach line (like the wiring you already checked and is good) or a shorted tach will pull the negative part of the coil to ground continuously and not allow the coil to fire.
HEMI,S ARE FOREVER!!!
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