Speedometer output question on a6.2L 6L80E ?

[Hart_Rod]

Originally Posted by Street Smart

I installed a Dakota Digital VHX Series dash made for the truck and used their bus/cans interface. I absolutely love it! You can pull up all kinds of info on the dash just like a scanner. It shows 1/4 mile times that are dead-on (mph and e.t.) Sorry I didn`t install one sooner.
Happy holidays!

Yea it's a great setup and what I put in my 68. The Bim/CanBus work perfectly, however, I'm trying to get the speedo to work when I have the BIM unplugged for data logging (the scanner won't work with it plugged in). I got the tach to work with some changes in the tune, but I can't get the the ECM (pin 57) to put out a signal so that the speedo will work. I'm sure it's something in the tune, but I haven't been able to figure it out yet....

[jorgensensc]

Rob,
We were having trouble getting the gauge to work initially also. There was a drop down box in hptuners that changed the output type. I think there were three options, one of which I think was serial. Have you messed with that?
Shawn

[Hart_Rod]

Originally Posted by jorgensensc

Rob,
We were having trouble getting the gauge to work initially also. There was a drop down box in hptuners that changed the output type. I think there were three options, one of which I think was serial. Have you messed with that?
Shawn

I have used that. I still don't get any o/p when I have the ECM (X1J1 Pin #57) wire connected directly to the VHX gauges.

[jorgensensc]

Did you use a pull up resistor? Also did you change the pulse count?

[Hart_Rod]

Originally Posted by jorgensensc

Did you use a pull up resistor? Also did you change the pulse count?

No pull up resistor. Speartech built the harness and said it wasn't necessary. What is the post count supposed to be?

[jorgensensc]

Find out what Dakota says on the resistor. They built the gauges so they would know if the signal needs to be stronger with the pull up resistor. Speedhut told me on mine that some need it and some don't. I may be thinking of the tach for the changing the number.

[dayj1]

Originally Posted by jorgensensc

Find out what Dakota says on the resistor. They built the gauges so they would know if the signal needs to be stronger with the pull up resistor. Speedhut told me on mine that some need it and some don't. I may be thinking of the tach for the changing the number.

Please don't take my comments as nit-picking, but I wanted to point out a misconception that I see a lot online regarding the function of a pull-up resistor.

The pull-up resistor doesn't really strengthen or amplify the signal. It simply pulls the signal to a logic "high" (12V) in instances when the signal would normally be undefined. A pull-down resistor does the opposite and pulls a signal to logic "low" (ground).

In the case of the tach signal, it's simply a series of ground pulses on a single wire. It looks like the top graph in the picture below. Between the ground pulses, the signal is undefined.

If you add a pull-up, the signal is defaulted to logic high between the pulses and you get a nice square wave most tachometers want to see. It didn't change the ground pulses at all. It just changed the signal from "undefined" to "high" between the pulses.

Hart-Rod,

With the key on, use a voltage meter to test between ground and the VSS signal wire from your ECM. What reading do you get? 12V or 0? 12V would indicate that there is a pull-up internal to the ECM. 0V would indicate that there is not. Please post back what you find.

[Hart_Rod]

Originally Posted by dayj1

please don't take my comments as nit-picking, but i wanted to point out a misconception that i see a lot online regarding the function of a pull-up resistor.

The pull-up resistor doesn't really strengthen or amplify the signal. It simply pulls the signal to a logic "high" (12v) in instances when the signal would normally be undefined. A pull-down resistor does the opposite and pulls a signal to logic "low" (ground).

In the case of the tach signal, it's simply a series of ground pulses on a single wire. It looks like the top graph in the picture below. Between the ground pulses, the signal is undefined.

If you add a pull-up, the signal is defaulted to logic high between the pulses and you get a nice square wave most tachometers want to see. It didn't change the ground pulses at all. It just changed the signal from "undefined" to "high" between the pulses.

Hart-rod,

with the key on, use a voltage meter to test between ground and the vss signal wire from your ecm. What reading do you get? 12v or 0? 12v would indicate that there is a pull-up internal to the ecm. 0v would indicate that there is not. Please post back what you find.

11.37v

[dayj1]

Originally Posted by Hart_Rod

11.37v

That would indicate that there is a pull-up resistor internal to the ECM, so we can mark that off the list of things to try. Do you have a way to "see" what's going on with the VSS output like a scope or a logic probe? That's what I would check next. If you don't see any activity on the VSS output, it's probably something with the tune. Unfortunately, I don't know anything about using HP Tuners so I can't help much there.

[Hart_Rod]

Originally Posted by dayj1

That would indicate that there is a pull-up resistor internal to the ECM, so we can mark that off the list of things to try. Do you have a way to "see" what's going on with the VSS output like a scope or a logic probe? That's what I would check next. If you don't see any activity on the VSS output, it's probably something with the tune. Unfortunately, I don't know anything about using HP Tuners so I can't help much there.

I will have to borrow a scope or logic probe and see what type of signal I get out.

[dayj1]

Originally Posted by Hart_Rod

I will have to borrow a scope or logic probe and see what type of signal I get out.

A scope would be best since it will literaly draw you a picture of the signal in real time.

You can pick up a logic probe off eBay for less than $20 and it will indicate if the signal is pulsing but you won't get any additional information like frequency, pulse width, etc. about the signal.

There is one more test you can try with a voltmeter. With the meter hooked between ground and the VSS signal wire, take a drive and see what the voltage reading is. You said it was 11.37 with the key on. I would expect it to go up some with the engine running. The voltage meter will display the average voltage it "sees". Once you start driving, ground pulses will show up on the VSS signal wire and the average voltage will go down (probably in the 5-6V range). If it stays at a constant level, the ECM isn't sending a signal.

[Hart_Rod]

Originally Posted by dayj1

A scope would be best since it will literaly draw you a picture of the signal in real time.

You can pick up a logic probe off eBay for less than $20 and it will indicate if the signal is pulsing but you won't get any additional information like frequency, pulse width, etc. about the signal.

There is one more test you can try with a voltmeter. With the meter hooked between ground and the VSS signal wire, take a drive and see what the voltage reading is. You said it was 11.37 with the key on. I would expect it to go up some with the engine running. The voltage meter will display the average voltage it "sees". Once you start driving, ground pulses will show up on the VSS signal wire and the average voltage will go down (probably in the 5-6V range). If it stays at a constant level, the ECM isn't sending a signal.

I'll try it!

[Hart_Rod]

Originally Posted by dayj1

A scope would be best since it will literaly draw you a picture of the signal in real time.

You can pick up a logic probe off eBay for less than $20 and it will indicate if the signal is pulsing but you won't get any additional information like frequency, pulse width, etc. about the signal.

There is one more test you can try with a voltmeter. With the meter hooked between ground and the VSS signal wire, take a drive and see what the voltage reading is. You said it was 11.37 with the key on. I would expect it to go up some with the engine running. The voltage meter will display the average voltage it "sees". Once you start driving, ground pulses will show up on the VSS signal wire and the average voltage will go down (probably in the 5-6V range). If it stays at a constant level, the ECM isn't sending a signal.

With the truck running, I get 12.4 volts. I drove it down the road with the voltmeter connected and it remained steady at 12.4 volts....

On a side note, it seems like the computer always shows about 1 volt less than the gauges do. Is that normal or should I be concerned?

[dayj1]

Originally Posted by Hart_Rod

With the truck running, I get 12.4 volts. I drove it down the road with the voltmeter connected and it remained steady at 12.4 volts....

On a side note, it seems like the computer always shows about 1 volt less than the gauges do. Is that normal or should I be concerned?

That seems to confirm your thoughts. It would appear that your ECM just isn't sending a signal on the VSS line. I'm sure you will, but please post up your fix if you figure out what to change in the tune.

On the voltage question, my 2007 LY5 does the same thing. I never really paid it any attention, though. It kind of makes me wonder if that's the "default" since I don't have the alternator feedback hooked to my ECM. Kind of like the rear o2 sensors show ".4" even though they aren't connected. Does it happen to report 13.7 V or is it a different value?

[Hart_Rod]

Originally Posted by dayj1

That seems to confirm your thoughts. It would appear that your ECM just isn't sending a signal on the VSS line. I'm sure you will, but please post up your fix if you figure out what to change in the tune.

On the voltage question, my 2007 LY5 does the same thing. I never really paid it any attention, though. It kind of makes me wonder if that's the "default" since I don't have the alternator feedback hooked to my ECM. Kind of like the rear o2 sensors show ".4" even though they aren't connected. Does it happen to report 13.7 V or is it a different value?

If I can figure it out, I'll let you know. I'm not sure what you mean by reporting 13.7?

[dayj1]

Originally Posted by Hart_Rod

I'm not sure what you mean by reporting 13.7?

I'm saying that my ECM always reports the voltage at 13.7 V. (Actual system voltage is 14.7). I was just curious if you're getting the same value when you say your ECM is reporting about 1 volt low.

[Hart_Rod]

Originally Posted by dayj1

I'm saying that my ECM always reports the voltage at 13.7 V. (Actual system voltage is 14.7). I was just curious if you're getting the same value when you say your ECM is reporting about 1 volt low.

Gotcha. My volt gauge shows 13.7 and the ECM and TCM reports 12.7.

[Street Smart]

Originally Posted by Hart_Rod

Yea it's a great setup and what I put in my 68. The Bim/CanBus work perfectly, however, I'm trying to get the speedo to work when I have the BIM unplugged for data logging (the scanner won't work with it plugged in). I got the tach to work with some changes in the tune, but I can't get the the ECM (pin 57) to put out a signal so that the speedo will work. I'm sure it's something in the tune, but I haven't been able to figure it out yet....

i hard wired mine into the harness leaving the ALDL plug useable.

[Street Smart]

Originally Posted by jorgensensc

Rob,
We were having trouble getting the gauge to work initially also. There was a drop down box in hptuners that changed the output type. I think there were three options, one of which I think was serial. Have you messed with that?
Shawn

I tried those options with an old AutoMeter speedo wired up, on one of the options it just pinned the the gauge with any forward movement of the truck.

[Street Smart]

Originally Posted by dayj1

I'm saying that my ECM always reports the voltage at 13.7 V. (Actual system voltage is 14.7). I was just curious if you're getting the same value when you say your ECM is reporting about 1 volt low.

I`m seeing the same, ECM shows 13.7 but at the battery and back of the alt shows 14.5-14.7.

[Hart_Rod]

Just to follow up on this. It turns out that there was a table in the computer operating system that HPTuners hadn't made available yet to adjust. Once it was "visible" in the software and I selected the correct option, the wire now puts out the correct signal and I was able to program the speedo. I knew it was something simple,