Hi everyone! I have a little problem with my factory tach installation. My problem is within the wiring. First off I need to know if it normal to have a positive current through the coil negative that goes to the tach. Second, should the ground wire to the 8 on the tach have a strong ground or a weak ground. Strong ground being I get 12 volts when I touch it to positive or weak being 3 to 5 volts when conneted to positive. I have a swapped a 3.8 V6 with a 302 with a HEI coil top distributor. The only wires I reused from the factory to the engine is the coil power wire, the green tach wire and the electric choke wire. Is there anything I need to do with the factory plug that had hooked up to the stock 3.8 distributor? I feel I am missing something there that needs to be delt with for my tach to work correctly. What happens with the tach is that when I turn the key to on, without starting the truck, the tach needle jumps over beyond 6000rpms and stays in this position. When I start my truck and rev it to somewhere around 2500 to 3000 rpms I notice the needle tries to bounce a little. I am stumped with this one. Thank you.

 

Is this a factory tach that came with the truck and worked with the 3.8, or is this a factory tach you just swapped in while also changing the engine?

 

Is it a 302 in the truck or the V-6? I thought these only came with either a straight 6 or a V-8.

If it is a V-6 in there, the "8" connector should be left disconnected, it is what tells the tach how to interpret the rpm's, being that the V6 would only fire three times in a revolution vice the V8 firing four times per revolution.

My thought process here might be flawed because in theory a V6 hooked to a tach that thought it was a V8 would read a low RPM and not a high one...

with a 302 in there the 8 should be connected to a hard ground, I have a factory 302 (1980 without tach) so I had to wire it up with out the trace wires on the instrument panel, on mine the G and the 8 are connected to the same ground wire and it works great....

 

The factory tach was put in after I had swapped out the 3.8 for the 302. The factory instrument cluster had actual gauges not the "idiot" lights but without the tach. I purchased the entire replacement cluster from ebay and just used it in place of the other one. I was informed that the cluster was removed from a F150 with a 351W.

To 6fifty: My grandfather bought this truck off of the show room floor with the 3.8 V6 in it. I spent a long time, after it was passed down to me, trying to convince auto parts stores that it did have that in it. The decal under the hood even shows the proof. After a while, a friend of mine told me that Ford tried this for a few years but he was even amazed that such an early model had it. After the engine retired 7 years ago with only 109000 miles on it, it was just a good ole farm truck that spent most of its life idling, I started the V8 swap project that has taken me til now to get it going on the road.

Ok, so the 8 connecter should have full ground. I will try this and see if it helps. I am still curious as to why the needle of the tach jumps over to 6000 rpms just at the turn of the key without the engine running? Thank you Franklin and 6fifty for your replies. I will post later to let you know how it goes.

 

IIRC 80-82 wiring is different than 83-86. I believe you will need an 80-82 cluster for it to work.

You may also have a bad tach from whoever you bought it from.

 

You cannot swap in a factory guage cluster in place of one that originally had idiot lights. I know the charging system will not work. This also may be the problem with the tach. Sorry your senario is not plug and play.

 

He already had a gauge package.

 

When i put the factory tach in my 80 i didnt work. I had to ground the cab to the frame

 

Missed that, sorry.

 

1980 models is the off year.

1981-1986 instument clusters will interchange, but 1980 is one year only.

1981-1986 instrument cluster printed circut starts with the number E1TF-10C956

1980 only instrument Cluster printed circut starts with the Number EOTF-10C956


You can't put a 1981-1986 instrument cluster in a 1980, although it will plug in, it won't work right, or vise versa. Main difference is the Brake, fasten belt, and turn signal warning lamps are reversed on the 1980.

 

Really? cuz nothing in my 80 is 80 other than the harness

 

btw - it no current is flowing, BOTH sides of the coil will show up as hot, if one side is energized.

btw I added a 1984 v8 tach to my 1981 I6 truck and I verified the wiring thru a diagram that 'subford' posted - search back about 9 months or so to find it.

More correctly, I interchanged panels, cuz I had the gauge package, it was doable. without a gauge package, the conenctor plug is different.

 

What I typed above is correct.

1980 Cluster printed circuts begins with "EOTF"

1981-1986 Cluster printed circuts begins with "E1TF"

As I said before, they will plug in and work in a fashion, but they wont work correctly if interchanged. The brake, and fasten belt lamps are swapped with the turn signal lamps.

Instrument cluster bezels are also 1980 only. This is because as I said above the turn signal and brake and fasten belt lamps are swapped. The lamp lenses in the bezel are also swapped.

If you mix and match either of these components for later years, your turn signals warning lamps will light for fasten belts, and/or Brake failure, and your Brake warning and/or Fasten Belt lamp will flash if you use the turn signals.

 

O i must have used a later cluster w a later trim panel

 

Thanks to everyone for all of your replies. I have looked at both instrument clusters and matched the numbers on them. I am going to work on it more this weekend to see where I will end up. For some reason I do believe I may have a faulty tach. I will post later and let everyone know how it goes. Thanks.