In this edition of Fun With Transmissions, we’ll take a look at speed sensor diagnosis. We’ll also see how and why a speed sensor delivers an AC voltage to the controller and what the AC signal looks like on a lab scope.
We’re working on a 2016 Grand Caravan with a 62TE. This vehicle came in with a check engine light on, and wrong gear starts. We checked for codes and found a P0791 transfer speed sensor (TSS) circuit code. Now, the first thing we want to do is clear the codes and see if it comes back. On this vehicle, we can watch the TSS input on the scan tool. Sure, enough, after we cleared the code, we saw that the TSS was not working (it read 0 rpm on the scan tool), and the code came back.
Whatever you do at this point, DO NOT replace the sensor yet! There are a couple of simple and cheap tests that I need to show you. Most people know how a two-wire pulse generator works, and it never hurts to get back to basics. The pulse generator is also known as a magnetic inductive sensor and/or a variable reluctance sensor. No matter what we call this two-wire sensor, it produces a small AC voltage that the computer can read as rpm.
The components of a two-wire pulse generator are basically all the same, no matter who makes them (Figure 1).
Here are the parts:
- Soft Iron Core
- Permanent Magnet
- Copper Winding
- Electrical Connector
- Mounting Flange
- O-ring
These components combined create a magnetic field. To produce an electrical pulse, we need to pass a metal object, such as a tonewheel, close to the sensor. The magnetic field changes when the tonewheel passes the sensor, creating a small AC voltage (figure 2). For our testing, we used a 10L80 pump-driven gear attached to a nut and bolt, a Fluke 123 industrial scope meter, and of course, a transfer speed sensor (TSS). We then chucked the gear and bolt arrangement into a common cordless drill (figure 3). 
The test is simple, run the drill and put the sensor close to the wheel. You should have a signal coming from the sensor. If you don’t, then replace the sensor.
TESTING
Let’s get back to our Grand Caravan. Figure four is a factory wiring diagram that is used for diagnosis. The factory lists the probable causes as short/open to ground, short to voltage, the speed sensor, and lastly, the PCM. We can test the circuits for all the issues listed using a good digital-volt-ohm meter (DVOM).
Testing for short to power is simple. We’ll test both the ground and the signal wires with our DVOM set on DC volts. We test the ground circuit and find 0 volts. Great, so far so good. Next, we check the signal wire, and we read 3.2 volts. Whoa, now wait a second. This appears to be a problem!
Reading this voltage may lead many to think that there is some sort of short to voltage, which is on the list of issues that can cause the P0791. This may lead you to replace the PCM or the wire. Because we’re pressed for time, instead of tracing the wire back to the PCM to look for the voltage source, we will pin #8 on the C1 connector and replace the wire going to the TSS. 
Rechecking the wire, we find the voltage is still present at the sensor. It’s important to point out that the voltage is there whether the ignition is on or off.
COMPUTER CHANGE
Because the voltage is still there, the computer must be the source of this mystery voltage, right? A computer gets installed and programmed, a recheck takes place, and we find the voltage is still there! What the heck?
Here is the answer to the mystery.
Many manufacturers use a pull-up voltage on sensor signal wires to verify that the circuit is complete and free of faults. 
This is considered normal even with the key off. Here’s what you need to know, this pull-up voltage has very little amperage behind it. We are talking so little that it takes a very sensitive DVOM to even read the small amount of amperage.
Definition Of Pull-up Voltage
Pull-up voltage is a voltage supplied from within an ECM through an internal resistor (typically 22K ohms). This pull-up voltage is used to monitor the state (open or shorted) of a signal circuit. Pull-up circuits are used on most sensor and switch inputs of electronic controls.
Testing the ground wire and the fix
Up to this point, we have been talking about the signal wire. The next logical step is to check the ground wire.
It’s not always a good idea to use our DVOM to test circuits for continuity, here’s why: A DVOM will read continuity or it will make a sound if it’s in the audible mode. The problem is when there may be only one or two strands of the wire that are actually connected. It will show good or make noise and still not be able to handle a load.
We’re going to test the ground circuit with the DVOM on the DC volts setting. Check the voltage at the connector with the sensor disconnected (figure 5). Next, we are going to leave the positive on the sensor connector and move the negative over to the ground wire at the computer. You should have the same voltage as before (figure 6). You can also use the battery voltage to check the ground side. Put the positive lead on the battery positive terminal and the ground lead on the sensor connector side. The readings should be the same (figure 7).
If we find the voltage is reading way less than what it is supposed to or 0 volts, we know that there must be too much resistance or an open in the circuit.
An inspection of the ground wire reveals a break in the wire near the computer. The wire is fixed, the codes cleared, and the car road-tested, and the code didn’t come back.
CONCLUSION
So many times, we get in a hurry to get the job done that we replace parts hoping for the “quick fix.” We have learned here that this is not always quick or cheap. Testing circuits for shorts to ground, shorts to power, and continuity does not take that long and will give you enough info to identify the problem area.
This will save you money and make the customer happy. We all know that a happy customer is the best way to keep having Fun With Transmissions!
