Hello all, I've been scanning this forum for awhile looking for help on all sorts of stuff and it's been really helpful, so here I am making my first post.

Vehicle specific until I get a signature: 1997 Ford F150 4.2L V6 XLT (Styleside)

To the problems!

1) Not so recently, my tachometer has stopped working. It hasn't been a huge problem since I can tell what RPM the vehicle is at when I shift, but I'd like it to work anyways. I purchased a new instrument panel from RockAuto (specific company is Dorman) and the tachometer still is not working. What little I can read about the electrical wiring in the Haynes manual I have, the tachometer is connected via a white/pink wire that goes directly to the PCM. A friend of mine suggested that I replace the distributor cap, but I don't know how that makes any sense since, from what I know, it has more to do with the spark plugs than anything else. My mechanical expertise, or lack thereof, ends at electrical wiring and in-depth engine troubles, so I'm at a loss. I'd really rather not send it to a mechanic, since most of them around here couldn't figure out that the spark plug wires were mixed up when a family friend replaced them (Ford dealership did though, but they cost a ton.)

2) Unrelated to the tachometer, the instrument panel I received from RockAuto (this being the second one, since the first one did the same thing) has the speedometer needle flicker 1 MPH. Ex: With needle at 66, it'll flicker to 67 and go back to 66. As far as I know, it does not do this under 45MPH, but I'm paying more attention to the traffic than I am to the speedometer. It also will flicker from 0 to the 10MPH mark as soon as you turn the key and the engine turns over. The original panel that was factory installed in the truck does not do this, but a tech at Dorman suggested that it might be the VSS (can't remember if it was that or something else) that might be sending bad signals. Physically the vehicle does not jolt, so it's just a bad signal or a second bad panel.

I appreciate any and all help.

 

To comment, you will look a long long time before you find a distributer cap because there isn't any.
I would put the old dash panel back in since either replacement did you no good.
The speedometer is a separate function from the Tach.
As you saw in your Haynes manual page 12-30, the tach is driven by pulses from the PCM to a sold state device that in turn drives the Tach readout. Ignition system is not involved.
Question is, is there pulses coming to the dash on the wht/pk lead from the PCM?
This calls for in-depth diagnostics your not equipped to do.
(I) would put a scope on the lead to take a look for the pulses as the first thing to do because I have one.
It may be as simple as the connection on the PCM plug.
How your going to handle this is up to you but just wanted to help you understand a bit more about the issues.
Good luck.

 

Hello Bluegrass 7,

Thank you for your comment. To start off, I'm unsure what you mean by there isn't a distributor cap. Minus the Hayne's book, everywhere else says that I can purchase one for my vehicle and I thought that was what the black plastic piece was that sits on top of the engine and looks like it has the opposite end of the spark plug wire boots on it? I could just be confusing it for something else, but either way I doubt it's the problem.

Unfortunately, I'm having trouble finding where or what device actually sends the Tach readout to the PCM. Would it possibly have to do with the cam/crankshaft and any sensor attached to that? As long as I understand where the signal originates, I can at least make sure the connections are secure.

As for the scope you were talking about, how much do they usually go for? If they aren't $1000 like one I saw and if it may be something I can use on other vehicles, then I would see no reason to not have it around, especially going the "Mechanics cost too much" route.

 

You specify a 97 v6.
They are fuel injection, with one coil with 6 terminals that are fired from the computer coil drivers.
There is no distributor cap when using computer controlled coils.
Follow the spark plug wires to the coil and take a closer look.
See Haynes page 2A-1.
What you see where the distributor used to be, on this motor, is a cam position sensor used to control fuel injection timing.
The reason why an old time Tach cannot be used is the ignition is what is called a 'wasted spark' system in that two cylinders get spark at the same time.
One cylinder is on compression to fire that cylinder, the other is on exhaust cycle where it make no difference. They pair them up that way so one coil pak has 3 coils to handle 6 cylinders. How it does it is best left to deeper research if you want to know.
To be technically correct on V6 with this ignition, three spark plugs of one type is use and three of a different spec are used for a total of six.
You can use all the same plugs but three of them will have a shorter life for erosion wear.
No 4.2, 4.6, 5.4 or V10 truck has a distributor after 1997..
The speedo is driven from a sensor drive in the drive line.
The Tach is driven by pulses from the PCM. See page 12-30 of the Haynes.
The day of the ignition driven Tach are long gone for these motors unless it is some custom aftermarket unit..
If your going to tech school, your should at least get this manual and begin to learn how to consult it to solve many issues on your truck as a good start.
Once you do this and use a good Scanner, you will become an ACE after awhile..

Good luck.

 

I wanted to give a quick update to this. It seems that there is still a reading being given by the vehicle for the RPM's. A buddy of mine lent me his OBDII wireless scanner and I used the Torque app to get some readings and it's showing the RPM's moving, but the tach inside the instrument panel does not respond. Since the instrument panel is not to blame (I've tried two others and neither of them showed anything), would this inevitably leave the wiring between the panel and the PCM to be at fault?

As for the speedometer issue I was having, I have not tested this yet since I need to reinstall the panel that actually has this problem (RockAuto seems really determined to not just let us refund before getting the problem solved?), but I'll update over the weekend when I get that tested.