I'm confused as what you call the tach signal. Tach signal is the engine rpm.

VSS is the signal for vehicle speed which is displayed as MPH.
The PSOM takes the sine wave frequency and displays the information as a MPH
reading and odometer reading. It also takes the sine wave frequency and turns
the signal into a square wave that equals 8000 pulses per mile.

This signal is used by the PCM and Cruise Control.

Here's the schematic that was posted above. It appears it's for a 96 so
the color of the wires may not match your vehicle.

The sensor has two outputs Red/Pink and a Light Green/Black wire.

These two wires carry the signal to Pin 4 and Pin 5 of the PSOM.

The PSOM process the signal and the 8k pulse square shaped wave comes out
on pin pin 7 a Gray/Black wire. It then goes to the PCM and Cruise Control
Servo amp.

Two PCM's are shown because one PCM is for a Diesel or a Gas engine with
a GVW less than 8500 lbs. The other PCM is for a Gas engine with a GVW
greater than 8500 lbs.

The dash line parallel to the VS sensor you asked about indicates the
two output pins of the Vehicle speed sensor go to the same connector.

I'm not sure of the Code 14 but on a gas engine erratic profile ignition
pickup is an error for the engine ignition system or the code can set if
the engine runs rough. Not sure how that applies to a VSS problem.
I didn't realize you culd get that code on a Diesel.

I though you said you got a 29 which is insufficent Vehicle speed sensor
signal.

A new VSS is less than $20.00.

You should also cut the Varistor out of the circuit.

 

Ok the VSS problem is fixed, I already cut the varistor out of the PSOM but should I not replace? Why do you say cut it out?

I believe the AC speed signal travels from the sensor at 86,190 pulses/mile to the ABS Module and then to the PSOM where it is converted to 8000 pulses/mile. Then like you said out to the PCM and Cruise (which I don't have.)

Once I had the speedo problem fixed I moved onto the no tach read below 1000rpm problem. Which in turn was tellin my onboard computer that the engine was off.

I looked at the Tach Sensor and noticed it worked on the same principles as the VSS, so I took similiar action to diagnose it. I never scoped the AC speed signal at the dash after the PSOM samples it so I don't know what a sampled signal looks like, but now I'm thinking that if a speedometer gauge requires a sampled signal, so might a tach gauge.

I cannot find the input wires from the speed sensor at the dash but if it indeed does need to be sampled to be diplayed it must come in at the PSOM with the VSS signal and then travel to the RPM Gauge, where I was probing it.

I have been doing a ton of research today and I haven't found a single way to test for certain within limits wether or not it is the sensor or the gauge. But everyone else who reported almost the exact same tach problem solved it with a new rpm sensor

 

Code 14 for a diesel with the E4OD indicates a problem
with the rpm sensor.

You could disconnect the sensor plug under the hood and
with your scope monitor the signal directly at the rpm sensor.

Should be a sine wave and the frequency will vary with engine
rpm.

You can verify if the signal is there at idle. If the signal is there
the problem must be further back in the circuit. Maybe someone
will post some information on how the signal is processed.

I said to cut the Varistor out of the circuit because it has failed.
The purpose of the Varistor was to protect the circuit from a
voltage spike.

No telling how that failed component would now affect the circuit it
was protecting. I'm guessing it failed when the vehicle was jump started.

Here's a picture someone posted which shows the location of
the rpm sensor plug.

It's been mentioned the wire insulation on the sensor becomes brittle
and the wires become exposed so you might want to check that out.

 

I assume the sensor is shot becuase their is no continuity or resistance. But it still throws out a fine looking wave at idle and throughout acceleration with no spikes. At both the sensor and the first connector on the passengers side inner front fender.

 

Could be the amplitude of the signal is not great enough.

Someone mentioned you can't properly read the resistance
of the sensor unless you remove the sensor so it's away from any metal.
Don't understand that reasoning but that's what was
said.

Here's the thread for that.
https://www.ford-trucks.com/forums/1...ch-issues.html


Here's the thread with the guy who had a similiar problem like yours.

http://www.dieselbombers.com/ford-83...-problems.html

 

Well went to the local ford scrapper in search of a new tach sensor. They had two in stock, the first one looked brand new but it tested at 40 Mega Ohm and didnt give me an idle reading. The second one which looked awfull with exposed wires, oil everywhere, and score marks along the magnet, tested frin at exactly 2500 Ohms and perfect rpm reading at idle. So dont go trusting any of these sensors by looks.

Anyways put it in and after some finicking with the throttle position sensor everything seems to be working perfectly, well allmost.

I can't remember all these dumber for sure, but I will edit them after further test drives.

When at the top end of second gear around 2400 rpm @ 50km/hr the E4OD will **** into 3rd but not before it does this weird thing. As soon as it's ready to shift into 3rd at the top end of second at 2400rpm the revolutions drop about 50 or 100 down to say 2300rpm but only for about 2-3 second before it fully shifts into 3rd.

The other thing being OverDrive seems to kick in around 70km/hr. I little low for my liking (would prefer 80km/hr) Is this a set standard or could it be adjusted anyway? i already have my TPS at 1.05VAC @ closed throttle, and not too inclined to lower it, not even sure it would change anything.