Factory tachs
Elder User
Joined: Jan 2002
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From: Where they take the census by counting the appliances on the front porch and multiplying by five
Factory tachs
Hey, anyone have any ideas on my factory tach problem. My truck was not initially equipped with one, and me liking options, decided to purchase one from the local automotive recycling facility. Hook up was straight-forward. Tach does fine at low rpm's(0-1000) but as you rev higher it seems to jump on up. After doing axle gearing and tire circumference calculations my tach is way off. And it gets worse the higher it revs. 65 mph should be around 2250 rpm yet my tach reads 3000 rpm while speedo indicates 50 mph. I know speedo is correct. I have a sneaky suspicion that I have a tach out of a 6 cylinder truck. Anyone?
Elder User
Joined: Jan 2002
Posts: 703
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From: Where they take the census by counting the appliances on the front porch and multiplying by five
Factory tachs
Yeah, it's got a c-6. It never occured to me to try the hand-held tach. But would the auto slip that much? It's only got approx.3k miles since rebuild.I'll try the tach idea. Thanks!
Posting Guru
Joined: Jan 2007
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From: shawnee KS
I am glad I stumbled across this- my tach in my bronco is lazy when it is cold, soetimes it'll go up to about 2700 and stay there till it warms up. Plans were to pick one up next time at pick n pull, but I did not know there was a different tach for different motors. Guess I never thought about it. Basically wanted to say thanxs for the good info! 
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Postmaster
Joined: Feb 2008
Posts: 2,658
Likes: 5
From: Out of here
The tachs are all the same, they are just factory wired differently.
Most likely, your's is just going bad. It's common for them to do what you are experiencing.
Most likely, your's is just going bad. It's common for them to do what you are experiencing.
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Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
Well, there are two different tachs.
The 3500 RPM tach, found in diesels and later circa 1984-1986 4.9L's and the 6000 RPM tach that was used 1980-1986.
The 6000 RPM tach is the most common, and was used in both six and eight cylinder engines. The "8" lug on the tach is grounded for 8 Cylinder engines, and left ungrounded for six cylinder engines. In this way the tach will read correct for the engine you have.
If your truck does not have a tach, but you have factory ammeter, and oil pressure gauges, you can swap a factory tach cluster in your truck very easily. Plug and play. Unplug the old cluster, and plug in the new cluster.
If you have idiot lights, this swap will NOT work without major rewiring.
1980 trucks also have a one year off cluster that is not compatible with later years due to circut and wire changes made for the 1981 model year.
Factory Tach Cluster printed circut compatibility.
EOTF-10C956 / Fits 1980 F-series and bronco ONLY.
E1TF-10C956-B / Fits 1981-1986 F-series and Bronco ONLY.
hope this helps.
The 3500 RPM tach, found in diesels and later circa 1984-1986 4.9L's and the 6000 RPM tach that was used 1980-1986.
The 6000 RPM tach is the most common, and was used in both six and eight cylinder engines. The "8" lug on the tach is grounded for 8 Cylinder engines, and left ungrounded for six cylinder engines. In this way the tach will read correct for the engine you have.
If your truck does not have a tach, but you have factory ammeter, and oil pressure gauges, you can swap a factory tach cluster in your truck very easily. Plug and play. Unplug the old cluster, and plug in the new cluster.
If you have idiot lights, this swap will NOT work without major rewiring.
1980 trucks also have a one year off cluster that is not compatible with later years due to circut and wire changes made for the 1981 model year.
Factory Tach Cluster printed circut compatibility.
EOTF-10C956 / Fits 1980 F-series and bronco ONLY.
E1TF-10C956-B / Fits 1981-1986 F-series and Bronco ONLY.
hope this helps.
Posting Guru
Joined: Jan 2007
Posts: 1,233
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From: shawnee KS
Another reason I like this site so much- the info is great, and you learn something new even if you weren't even looking.
Does this mean that if I find a cluster from a diesel it'll probably only be for 3500 rpm? I've been wanting to put a tach cluster in my 83 diesel for awhile, but I do know that there is a lot more involved, including some wiring.
Does this mean that if I find a cluster from a diesel it'll probably only be for 3500 rpm? I've been wanting to put a tach cluster in my 83 diesel for awhile, but I do know that there is a lot more involved, including some wiring.
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
I've been wanting to put a tach cluster in my 83 diesel for awhile, but I do know that there is a lot more involved, including some wiring.
Posting Guru
Joined: Jan 2007
Posts: 1,233
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From: shawnee KS
No, it's not a plug n play. There is a sensor on the cover of the injector pump gear that you have to get, the ones without the factory tach only have a plug there from the factory. There isn't even a harness there (on mine anyway) to plug into. I checked into doing this once before, I just didn't ever realize (notice) a different tach on the diesels.
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
You need the sensor and the harness extention that goes from the sensor to the main harness. I mentioned this vaguely in the last post, "related stuff under the hood".
Simular to how the engine compartment light is plug and play, but you need the harness extention that runs the length of the hood, etc...
All the basic wiring is there for every option these trucks had in the main harness. Though you may need the harness extention from the main harness to the component.