Hello all,

I recently rewired my truck from bumper to bumper. Pretty much everything works now with some bugs to iron out. However, I am having an issue with my factory tach reading ~400 rpm low. I read through the guide to make sure it's set up as an 8 cylinder motor but as far as I can tell everything is connected up correctly. I cannot see a second ground that needs to be connected. Does anyone have any advice or experience getting the tach to read normally without resorting to an aftermarket one? I have attached pictures of the new wiring.

 

The terminal you are looking for is on the tach itself. If it's wired correctly, make sure it has a good ground there. Here's a link. https://www.garysgaragemahal.com/tachometers.html

 

I will have to look at that when I get home. I have to take the cluster out anyway to replace some bulbs. Is it just the speed nuts that have to be attached or is there a wire inside the housing that I'm looking for?

 

I was going to ask if it was the factory tach in the dash and I see it is.
With that said how you know it is reading 400 RPM low?
Have you used an after market one along side to see if both read the same?
Dave ----

 

As I was using a timing light to make sure everything was set correctly it was running at ~800 RPM but when I looked at the tach in the truck it showed just under 500 RPM. It's always been this way but as I was planning on putting a whole new harness in the truck I ignored it thinking it would be fixed with the new wiring.

 

When you get time you will have to explain what harness you put in the truck and how it went and what was the method of doing it. A lot of people want new harnesses in their trucks, but the common universal harnesses can be difficult to install and do not have any of the Ford connectors in them, so bunch of splicing. I would be curious how you did it and how much trouble it was.

 

I will do that. It was an American Auto Wire harness I ordered through Jegs. It was built specifically for 80-86 F series trucks. There were some quirks but it went pretty smoothly. I'll create a post detailing how it went.

 

If it was me I would just live with it reading low.

But I have to ask you "know" it is low from the timing light tach.
What happens when you bring the RPM up is it still 400 RPM to low or ????
Dave ----

 

I'm sure it is. When going down the road in my last gear at 55mph (using a GPS to make sure my speed is accurate) my tach reads around 2000 when in reality with my tires, gear ratio and final gear in the transmission should be around 2400 RPM. It's a known thing that the factory tach can be configured for either a 6 cylinder or an 8 cylinder engine. I just wasn't sure where the ground was that I needed to check as I verified the ground from the coil was good. I thought there was another ground from the ICM that I needed to check.

 

So you know it is 400 RPM low across the board.
Did that cluster come with the truck and did the truck always have the v8?
If yes and yes then it was always low from the factory and I cant see the ground being wrong from the factory.
But that is just me.

Think you said you had to pull the cluster for something else so check the ground.
If you find that is good then what? Get another and hope it reads right?
I cant remember and to lazy to go look at mine, is there a pin the needle rest on when not running?
If not and you feel brave you might pull the needle and move it up 400 RPM and hope it is right.
Dave ----

 

The cluster did not come with the truck. I got it from another member on this forum who had painted it.

I'm going to verify that all points are connected with the speed nuts on the back of the tach. If they are I might go your route and check the needle to see if that may be the problem. I've done worse on this truck so far and it seems to have worked out one way or another.

 

See pages 27 and 98 here:

https://www.garysgaragemahal.com/1985-evtm.html

The way I understand it, all gasoline trucks with the gauge package used the same harness to the instrument panel, whether 6 or 8 cylinder. The harness include wires #11 and #60.

Wire #11 connects the coil to terminal C on the back of the tach, whether 6 or 8 cylinders. This is the variable input for engine RPM.

Wire #60 provides a ground to terminal 8 on the back of the tach, via the engine harness. This is a fixed signal for the tach to operate as either 6 or 8 cylinder mode. This wire is installed on all gas models, but only grounded on 8 cylinder models. On the 8 cylinder engine harness, the wire is grounded via the distributor housing. On the 6 cylinder engine harness, the circuit is not grounded.

Pretty ingenious, if you think about it. As a truck came down the assembly line, when the vehicle harness was connected to the engine harness, the correct tach mode was automatically enabled.

For your situation, you should be able to simply run a separate ground wire to terminal 8 on the tach. You could ground the other end at any convenient point nearby. You don't have to run the ground all the way to the distributor like the factory arrangement. The ground routing via the engine harness was only for convenience on the assembly line. Your aftermarket harness may not have this wire, or it may not be properly tied into the engine harness.

Pictures here:

https://www.garysgaragemahal.com/tachometers.html

 

I think I see what you mean but when you say Terminal 8, do you mean Terminal B on the tach assembly? At least it looks like a B to me.

 

I added a tach to 1 of the 2 clusters I had.
I got it from a member along with the right printed board.
I did not have to do any wiring swaps and it worked.
Now as for it being dead nuts on that is a different story.
I feel it is close enough that I dont worry about it.

BTW my truck has a 300 six
Dave ----

 

Does it look like this one?:




That's from the link in my earlier message.

At the 1:00 position: Label G - fixed ground #57 on page 87 of the wiring diagram, via the flex printed circuit board on the gauge cluster.

At the 5:00 position: Label B - battery power #640 on page 98. The stamped AA appears to be the part number suffix, not a terminal label.

At the 7:00 position: Label 8 (stamped) - 8 cyl reference #60 pages 98 and 27. Note how this stamp looks like a B.

At the 11:00 position: Label C (stamped) - Coil signal #11 pages 98 and 27.