I have a 1993 F-250 w/ 7.3 liter factory turbo. I just purchased the truck this week and I've been trying to clean out some electrical gremlins.

My current project is the fix the erratic Tachometer, the tach "works" in the sense that it moves around when I rev the engine, but its pretty erratic. I'm very sure its not reading the right RPM and it bounces around a lot while driving.

I'm going to pull the dash cluster out this weekend and check those connections, but where in the engine does the tachometer hook up on this diesel? I've never messed with a tach on a diesel, so I'm a little confused from where the reading is taken from. I have a feeling once i find the connection in the engine bay it will be covered with oil or be a gross connector.

Any suggestions on what I should check will be great, thanks.

 

First check the sender in the oil filler tower....it's the large 1"nut with wires. Could be just wires touching where they exit the sender or a bad sensor.

The dead tach sender should/would send the automatic trans into a "failure management mode" (limp mode) with harsh engagements and firm shift feel, and an abnormal shift schedule. It has no effect on a manual trans only the Tach readings.

Replace the tach sender on the oil fill housing a big 1" nut with 2 wires. Ford only part Engine RPM sensor E5TZ-17B384-A about $50. You can remove and clean it but usually changed later too. You can test it too, hold it in the air away from ferrous metal, using an ohm meter lead on each wire you need DC resistance between 2000-3000 ohms.

Put up a signature with all the truck info......including trans type.

 

Sweet, thanks. I'll go try this after work. Yeah I figured the sensor is probably just old, but I had no idea where to look. Thanks for all the direction.

Also my truck is a 5 speed so I wouldn't notice any harsh shifting unless my foot slips...

 

I have a 1994 F-350 w/ 7.3 liter factory turbo. I am working this very day on the same problem. My tach works as long as the rpms are above 1700-1800. If they drop below that, then the tach drops to zero. When i am driving, when the tach drops, my transmission goes crazy falling in and out of the lower gear. When checking all the wiring trying to figure where the tach ties into the engine, I took the breather assembly off. When putting it back on, I noticed I was pushing hard against the wire/plug coming out of the right side of the fuel filter tower. I checked it and it was indeed broken. Could this be my problem?

 

Tach sending unit location and the wiring connector.
The zip tie is not factory, so it may be down by the IP somewhere.

 

I took the sender out and cleaned it. some of the insulation on the wiring was bad. I taped that, put it back together, still the same. I got my ohms meter and checked it and got no reading. So I ordered a new sensor. Will I have to take the battery cables loose for a while to reset the computer in order to get it out of limp mode?

 

Not necessary.......the re stategy is only required after batteries are disconnected.

 

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I took RPM sensor out and cleaned the outside, the wires and the plug contacts. The 1st ~1/2” of both wires were bare of insulation, out from the sensor, and had indications on the base of the remaining insulation as though an epoxy encapsulation had broken away, leaving the wire exposed. I coated them with silicone sealant to reinsulate them and let them cure before reinstalling. There is a very tight clearance between the sensor and the vacuum pump, which would explain how the base of the wires became exposed the 1st time this sensor was ever removed.

Incidentally (or perhaps more importantly?), while inspecting the sensor before cleaning it, I noticed a (~1/16” thick) buildup of what looked to be fine metallic/magnetic powder mixed with engine oil on the center of the face of the sensor. Obviously this was iron wear particulates from the engine oil that had adhered to the magnetic sensor over the 125k miles on this truck. I wiped that completely clean.

Upon reinstalling, my tachometer went from consistently reading ZERO below ~1000 rpm, to working consistently and seemingly accurately, throughout the operating range.

MY THEORY:
I believe this sensor utilizes the “Hall Effect” principal. As the teeth on a gear/sprocket pass by, in close proximity to the face of the sensor, a voltage pulse is generated within a coil surrounding a magnet housed within the sensor. The two wires are connected to this coil and transmit these voltage pulses to the tachometer assembly where they are processed into movement of the tach needle and a signal to my eo4d transmission control module. I suspect that the tiny blob of magnetic material on the sensor face was distorting the magnetic field just enough to reduce the voltage pulse below detectable levels at low rpms. It’s been a while since my high school physics class, but I believe the faster the tooth passes through the field produced by the magnet embedded within the sensor, the higher the induced voltage in the coil; thereby explaining why the tachometer worked at higher engine RPMs.

I have had problems with a my transmission overdrive “off” light flashing within moments of touching the brake, whenever I was decelerating with the engine at idle after pulling a hill. I had codes 23,67, and 14 (which was in “continuous memory”) prior to this work.

I will post further if my tach acts up again anytime soon, or if this doesn’t resolve the transmission trouble codes.