tachometer running off of fuel pump

[1994fordranger]

hey just a quick question, i posted awhile ago about my truck running terribly and i think i found out that it is the fuel pump as sometimes while it is still running the tach will say it is at 0rpm's, obviously meaning it would be off so does the tachometer run off of the fuel pump?

 

[adsm08]

I feel like you need to go consult a professional automotive teacher in your local area.

Anyway, no. The tach is not run off the fuel pump. However, the computer runs both the tach and the fuel pump off the crank sensor.

 

[1994fordranger]

my bad, dumbass friend told me it did

 

[adsm08]

my... dumbass friend

This confirms my feeling. I would replace your friend with a local automotive teacher.

Now, he was in the ballpark, in that the two devices are run off the same sensor. If the sensor itself is starting to send erratic signals it could affect a great many things, including the running quality and the tach.

This is one of those things that could become very hard to find if the problem is intermittent. If you can deal with it until it becomes consistent that would make it easier to find and fix.

 

[RonD]

And just to add, the Ford fuel pumps do not run all the time the engine is running.
The computer(PCM) turns the pump on and off based on RPM and vehicle speed.

The fuel pump should come on for 2 seconds each time the key is turned on, that is to "prime" the fuel system, it will not come on again until engine is started or you turn the key off and then on again.

 

[RangerNielsen]

Adsm, the tach in my diesel is run off the IP (injection pump) and so I figured it might be the same.
The truck will idle for a while, but when we went for a little drive, it started acting up. Didn't get more than a football field away from the house before it stalled. Restarted it, got turned around and died again, when around a corner to his house, stalled again, and I told him to just let the clutch out since we were moving. Then parked it and the tach read 0rpms and I asked if it was still running, he pumped the gas a couple times and it was indeed still running.

 

[adsm08]

Adsm, the tach in my diesel is run off the IP (injection pump) and so I figured it might be the same.

Totally different beast. Mechanical injection diesels, especially pre-OBDII units have nothing else that generates a rotational signal, except the spinning injection pump that is run off the cam. It's kind the same way that the 2.9L tach runs off the ignition pickup in the distributor.

But since the fuel pump in a gasser is in the tank and has no mechanical connection to the engine and a tach is a gauge that displays the mechanical rotating speed of the engine.... Well that makes my assessment of the situation unchanged.

FWIW I have worked with brilliant diesel techs who couldn't work on a gas engine to save their lives. I once watched a 5 year Ford Senior Master diesel tech who could diag, tear out, rebuild, and reinstall a 6.0 without even looking at the vehicle spend two hours to figure out that a Ranger wouldn't start because it was out of gas.

 

[notaprob]

I had an issue with my truck misfiring and the tachometer would not work while it was running rough. It turned out that a bracket wore a hole in the wiring harness and grounded out to the body. I had to take the battery out to find it. There may have been other places also but I found another wiring harness and computer for $70 at a pick n pull.