Hi,
I have a 1990 F250 4x4 7.3 idi with an intermittent tachometer. At idle it reads near zero RPM but as soon as I step on the accelerator it will jump to around 1k RPM. While driving it reads normally. It is only when I'm stopped when it drops to near zero. Transmission seems to shift normally.

Any thoughts?

Thanks!

 

its the tach sensor. it looks like a nut with 2 wires coming out of the top. its on the right side of your oil filler.

 

Mine has the same issue.

 

could you give some more detail how to fix this? I have the same issue on my 2wd E40d trans. It doesn't cause a problem but id like to know where im idling.

 

its arround $70 and a dealer only part, and finding a good used one isnt gonna happen

 

the fix is go get a new one then replace the old one nothing special

 

Might play with the wires around it a little, mine will act up and i finnagle em a little and it goes back to normal.

 

The phenomenon of the tach reading going to zero at idle, but reading properly at higher RPM is typically due to a small buildup of magnetic wear particles on the face of the sensor. Remove the sensor, clean it off and reinstall. That will often do the trick.

Some contributors say the wires coming out of the sensor may be shorting together or against the body of the sensor. That seems generally inconsistent with the tach operating properly when off idle. I used silicone sealant to reinsulate the ~1/2” bare length of both leads on mine before reinstalling anyway, though, just in case. It appeared to me that the base of the leads might have originally been encapsulated in a mound of epoxy atop the sensor when new, but that likely was broken away the 1st time the sensor was removed, due to tight clearance to the vacuum pump body above it, as referenced in a previous post. I was able to reinstall mine w/o damaging the reinsulated leads, without removing the pump.

The reason magnetic wear particulate on the magnetic sensor face causes this problem is that this buildup dampens the inductive influence of the passing timing gear teeth on the sensor coil, as each one passes by the sensor face while the engine is running. At higher RPM the deformation of the sensor’s magnetic field is more pronounced due to the higher speed of the gear teeth as they pass by the sensor face, and so it detects them properly, allowing the tach to work properly.

Hope my analysis is accurate and helpful, at least for the situation most of you are facing🤠!

Incidentally, I’m told that a buildup of wear particulates on the face of the speed sensor in the differential case can cause it to similarly malfunction, causing speedometer and ABS brake system trouble on some vehicles.

 

why the NecroPosts ??

A necropost is a post on an "old, abandoned thread that has been considered 'dead' for a while...that no longer serves any purpose but is bumped back up to the top of the forums by someone posting in it." Necroposting is seen as a form of spam and clogs up the forums with old and unneeded topics.


Welcome to FTE BTW and yes getting Iron on the Pickups is like installing a Keeper on a Magnet, this has been mentioned many times before.

The first test you should do is a Resistance test to see IF the Coil is within specs.. the Tach sensor and the VSS have different Values.
IF resistance is out of range it will cause the same issue as it will not produce the proper output voltage at slow speeds.

 

You never know. My sensor, and a few others failed when these pickups were new at very low mileage. In fact my truck was still under warranty at the time and the sensors were right around $60 retail if I remember right.