This is related to my thread a couple weeks ago about problems with a variable reluctance sensor. Finally got the speedometer to work by disconnecting the cruise control (though haven't yet figured out why that works).

It did highlight my confusion about ac in a car. In a house, I mostly understand hots, neutrals and grounds. In a car, there is no 'neutral', and everything I've read about var. rel. sensors calls one wire the signal and one the ground. Some schematics show one wire as "hi' and one as 'low'. What??

I'm hopeful getting a better understanding of how this works may help me figure out where my problem now lies.

Thanks!

 

Variable reluctance sensor:

With this type of circuit the outputs from the sensor are floating (ungrounded) from the circuit power ground.

Outputs a (mostly) AC sine wave signal so normally that's connected to some type of signal conditioning circuit (inside/outside the ECM) to convert it to a signal that's suitable as a input to a digital control system. The AC signal directly from the sensor might have and 'hi' and 'lo' labels on the wires to show the normal +- signal polarity as the gear rotates in the direction of the arrow.

They could also just call it signal and signal return (ground) like in top of this diagram. On the bottom you have a digital Hall sensor that needs a supply voltage +5 and supply ground to generate the correct type of signal (digital) output.

Sometime you have to read between the lines on schematics to understand what they really mean.

 

If the "return signal" isn't a 'ground', maybe they shouldn't call it one?

This diagram is helping confuse me. Note the ground symbol at the bottom/center--PK/O from the VSS???



Thanks for your help!

Variable reluctance sensor:

With this type of circuit the outputs from the sensor are floating (ungrounded) from the circuit power ground.

Outputs a (mostly) AC sine wave signal so normally that's connected to some type of signal conditioning circuit (inside/outside the ECM) to convert it to a signal that's suitable as a input to a digital control system. The AC signal directly from the sensor might have and 'hi' and 'lo' labels on the wires to show the normal +- signal polarity as the gear rotates in the direction of the arrow.

They could also just call it signal and signal return (ground) like in top of this diagram. On the bottom you have a digital Hall sensor that needs a supply voltage +5 and supply ground to generate the correct type of signal (digital) output.

Sometime you have to read between the lines on schematics to understand what they really mean.

 

If the "return signal" isn't a 'ground', maybe they shouldn't call it one?

This diagram is helping confuse me. Note the ground symbol at the bottom/center--PK/O from the VSS???

View attachment 122039

Thanks for your help!

Your diagram looks like a power wiring (DC power input) interconnect schematic for each assy or module so the GND power symbol would be correct. Sometimes they have a separate signal (signal input/output) wiring interconnect schematic for the same devices.

 

Your diagram looks like a power wiring (DC power input) interconnect schematic for each assy or module so the GND power symbol would be correct. Sometimes they have a separate signal (signal input/output) wiring interconnect schematic for the same devices.

You lost me there. GY/B and PK/O are the signal (and only) wires from the VSS, and the diagram, as I understand it, shows their connections to the PCM, speedo, CC and GEM.

 

All those parallel connections look like power or some sort of bus. What does the schematic define for GY/B and PK/O in the wiring info block(if it has one)?

I'm just guessing but your diagram shows other page number(s) next to most modules that might show more information about signals.

 

This might be hard, but let me TRY to help. Back when cars consisted of lights, switches, starter motors and a generator,we really didn't care what part of the metal car we got our return from.

Well, the starter is special, the engine gets a big fat wire from (-).
===
Now we do care and we care when we build electronics.

It's VERY DIFFICULT to make all grounds go to a single point (the negative battery terminal). It would really be messy.

So, what we do is, we break up the grounds. Some are references, some are returns, some are shields, some high power, some low, some data, some may have big spikes on them, some do not.

We can still generate ground loops or differences in ground potentials, but we manage them. Our ECM sits somewhere with it's ground and those sensors are returned to that ECM ground (NOT THE BODY OF THE CAR). Many sensors have an output relative to whatever the 5 V supply is doing at the time. If it's 5.1 it's 100%. If it's 5.6 it's 100%.

Shields can still use the car body.

Each ground in the car is DIFFERENT in some way. It's true when designing printed circuit boards. The types of grounds are separated and eventually tied together in one place.

The home is the same way, Ground and neutral tie together at ONE PLACE. When you have multiple panels, they tie together at ONE place for the entire house. This method has issues. Hospitals and radio stations may use a different method.

Take the alternator, it does not regulate the voltage at the battery. The sense wire is back at the ignition switch,. It's a low current carrying wire.

 

All those parallel connections look like power or some sort of bus. What does the schematic define for GY/B and PK/O in the wiring info block(if it has one)?

I'm just guessing but your diagram shows other page number(s) next to most modules that might show more information about signals.

Other than the anti-lock brake system (RABS), I don't have more schematics for my '97. I do have a pin-out of the PCM which says GY/B and PK/O are both vehicle speed sensor (VSS) inputs, which maybe answers your question.

Yes, other pages I think would be helpful, especially the Grounds page, but I don't have access to them.

Thanks.

 

Other than the anti-lock brake system (RABS), I don't have more schematics for my '97. I do have a pin-out of the PCM which says GY/B and PK/O are both vehicle speed sensor (VSS) inputs, which maybe answers your question.

Yes, other pages I think would be helpful, especially the Grounds page, but I don't have access to them.

Thanks.

Seems pretty logical now. It seems the raw AC sinewave variable reluctance sensor signal output feeds all the modules in a parallel bus type of configuration. So the RETURN PK/O is connected directly to some of the modules and to a signal gnd (G103) while the SIGNAL GY/B feeds the modules that need the VSS signal input.

 

Our ECM sits somewhere with it's ground and those sensors are returned to that ECM ground (NOT THE BODY OF THE CAR).

Hmmm....maybe this is where I'm going astray? The sensor I'm using for VSS was only used for ABS on my '97. (The following year they used it for VSS.) Since its signal was sent to the ABS module and was not used for a VSS signal at the PCM or GEM, could grounds/or lack of them, between them be the problem? (He says hypothesizing wildly.)

Thanks for the extensive explanation!

 

If the "return signal" isn't a 'ground', maybe they shouldn't call it one?

Thereby lies the problem, the Earth Ground symbol was established back in the '50's (or even earlier) but no one abides by it.
According to the symbol charts, the correct one for an automobile should be the Chassis or Frame symbol.
(Also Ref: JIC 1957.)
Max.

 

Seems pretty logical now. It seems the raw AC sinewave variable reluctance sensor signal output feeds all the modules in a parallel bus type of configuration. So the RETURN PK/O is connected directly to some of the modules and to a signal gnd (G103) while the SIGNAL GY/B feeds the modules that need the VSS signal input.

Agreed. But not to be TOO dense, I still don't understand the signal ground G103, and its role.

Thanks.

 

This is related to my thread a couple weeks ago about problems with a variable reluctance sensor. Finally got the speedometer to work by disconnecting the cruise control (though haven't yet figured out why that works).

It did highlight my confusion about ac in a car. In a house, I mostly understand hots, neutrals and grounds. In a car, there is no 'neutral', and everything I've read about var. rel. sensors calls one wire the signal and one the ground. Some schematics show one wire as "hi' and one as 'low'. What??

I'm hopeful getting a better understanding of how this works may help me figure out where my problem now lies.

Thanks!

You can usually count on there being a chassis - or ground. Sensors/transducers could well have balanced outputs, and may also be via twisted pair, these are often differential with no ground reference.

Obviously the cruise control needs to monitor the speedo signal - a faulty input could be gumming up the works.

 

Agreed. But not to be TOO dense, I still don't understand the signal ground G103, and its role.

Thanks.

Two of the modules on the diagram (GEM/CLUSTER) don't have direct connection to the VSS RETURN signal. You need a signal supply and signal return to make a circuit so I would assume the signal return path for those two would loop back on a separate signal ground connection(s) not shown on this diagram.

 

Two of the modules on the diagram (GEM/CLUSTER) don't have direct connection to the VSS RETURN signal. You need a signal supply and signal return to make a circuit so I would assume the signal return path for those two would loop back on a separate signal ground connection(s) not shown on this diagram.

I've just realized neither the GEM nor the speedo/cluster physically have a wire that would correspond to a return signal wire. That is, they both have a GY/B signal wire, but neither has a PK/O, or any other wire not accounted for, other than a plain B (ground), in their wiring bundles.

FWIW, I checked/found continuity between PK/O and chassis ground, at both the cruise control and the VSS as seen on the diagram.

Another mystery, now that you've got me thinking about ac and signal returns: The ABS sensor being used as a vehicle speed sensor is sending signal to GY/B wire where original VSS was, and speedo is working. That all makes sense. What I don't know is how that ABS sensor is getting a return signal. The original wiring, still hooked up, is LG/B and went to the ABS module. It does not go the the GEM or PCM, and does not have continuity with chassis ground. Arrrrggghhhhh.........

Thanks again for the help. I feel like I'm just missing one tiny piece of the puzzle!