Dodge truck, interesting VSS problem

[E4ODnut]

I have an interesting VSS problem with my Dodge truck.

According to the factory service manual the VSS module outputs a signal of 118.8Hz @ 88.6 KpH. This works out to 4829 pulses per kilometre. I checked it with my hand held scope before I started the MS conversion and it is a nice square wave at ~ 5 VDC, at the published frequency.

I have the input to MS3X pin 28 tapped into the stock harness which inputs into the stock speedometer and used to input into the stock PCM. The stock speedometer works just fine.

These are my settings:

VSS1 input type = digital
VSS Position/Type = Pulses per Km
Digital input = Table switch (MS3X pin 28)
Pulses = 4844 (initial setting)
VSS3 = off
VSS2 = off
VSS4 = off
Smoothing interval = 200 (default)
Smoothing lag factor = 50 (default)
VSS sample interval = 50.00 (default)

When I first started tuning I had other things to concern myself so I didn’t pay any attention to the speedometer read out, but now that I’m getting things dialled in pretty good I’ve turned my attention to VSS. I was getting a read out, but it was very low. With a bit of driving around I found that it was low by a factor of about 12.5. As an experiment, I bumped my pulses per kilometre up by that factor to 60,550. Bingo. That did it, I’m not sure why I had that problem, but the fix seemed to work. I had a functioning speedometer, but my gear indicator was off. I found I had an error in my ratios, so I corrected that and went for another drive. The speedometer was back to its old tricks. I checked and re-checked my settings, but they were good, so now I was back to square one.

I checked my wiring, no obvious errors. I put the truck up on blocks and checked the signal again. Nice square wave, 4.65 volts to ground according to my hand held, and frequency just what he factory stated.

I’m kind of stuck on this one. Any suggestions?

[billr]

Are you checking that VSS signal with the sensor connected to MS, with a load on it? Do you know what the freq limit is for "table switch"? I just spent many minutes in the manuals and couldn't find it...

[E4ODnut]

Hi Bill,
The frequency was checked connected to MS, and I have no idea what the frequency limit is.

[E4ODnut]

Still no luck getting VSS to work. Any suggestions would be appreciated.

[billr]

Try using PT4 for the input?

[E4ODnut]

Yup, guess I'll just have to try experimenting.

[billr]

I wish somebody would join in who knows what the max freq for "table-switch" is. I have about 12,000 pulses/mile, using PT4.

[E4ODnut]

I would think that if frequency limit is a problem, then the chosen input would work well up to the point where the frequency limit is the limiting factor. I'm not experiencing this. I just get a very low read out from square one.

I'll do some experimenting and report back. Hope I don't break anything.

[billr]

Agreed. I'm just trying to help you "grasp at straws". If we keep talking long enough, maybe the problem will finally pop to mind. Or, maybe somebody that knows more will take pity on us and join in...

[E4ODnut]

Thanks for the moral support. Be assured, I don't give up easily.

[E4ODnut]

Hi Bill,
OK. Here is what I know and what I plan to do. Wish me luck.

Tuner Studio, Speed and Gear Sensors, Digital Input Options:

Tableswitch
PE0/JS7
PE1
JS10
JS11
JS5
JS4
Nitrous In
Launch In
Datalog In
PT4 Spare In
PT2 Cam In
PE2 Flex
CAN VSS
Use CAN PWM In

-----------------------------------------

Tuner Studio Help:

VSS Input Type:
- Digital- Any pulse input or capture from an expansion board
- Digital Input – A pulse input directly connected

---------------------------------------------------------

MS3X Hardware 3.4.16 Speed Sensor Inputs:

0-5V pulsed signal
Low frequency (whatever that is?) Datalog In, Tableswitch In, Launch In
High Frequency (whatever that is?) PT4
Spare inputs on main board can be used with DIY with suitable interface circuit shown on schematic (What circuit? What schematic?)

----------------------------------------------

MS3X Hardware Page 20/240

Pin 11 Launch In, Input, Ground Switching
Pin 28 Tableswitch In, Input, Ground Switching
Pin 30 Datalog In, Input, Ground Switching
Pin PT4, In/Out, 0/5V Input or Output

-------------------------------------

Conclusion:

Maximum frequency of 5V square wave signal under normal circumstances = ~166 Hz
Maximum frequency of 5V square wave signal under extreme circumstances = ~222 Hz

Try inputs: Tableswitch In, Datalog In, Launch In and PT4 at 222 Hz and see what happens.

[billr]

Is that VSS signal a true 0-5VDC, not "zero crossing" (-2.5 to +2.5)?

[E4ODnut]

True square wave, as pretty as it gets. 4.65 volts to ground according to my Velleman hand held scope.

I had hoped to be able to report back with tests to the other input channels by now but life has a habit of getting in the way of fun stuff. I've been swamped with work and house guests lately so it may be a week or so before I can get to it.

[E4ODnut]

Resolved - sort of.

I brought MS3X pins 11 (Launch In), 30 (Datalog In) and 31 (PT4 out so I could test my VSS1 inputs on them. Then I put the truck up on blocks and tested my original connection to pin 28 (Tablewsitch In). As before, I got a low reading by a factor of 12.5.

I then tried Pin 11 (Launch In) and got nothing at all. The procedure was to change the physical connection with the engine off, go into TS and make the change, save the tune file connect to MS and accept the TS change.

I then tried pin 30 (Datalog In) and got nothing again. I thought that rather suspicious so this time I shut TS down as well. When I did a restart on TS and connected to the controller TS did not report any differences. When I fired the engine and tested the VSS1 input I had a signal, a good one, and my gear indicator was functioning correctly as well. Several more engine off, re-start and test cycles showed that things were working as expected.

Now, it is possible that Pin 11 (Launch In) might work as well if I has closed TS and started again but I didn’t bother to test it as pin 30 (Datalog In) appears to be working for me. I did not test pin 34 (PT4).

There is something wrong with pin 28 (Tableswitch In) though, as no matter what I do it still just gives me a very low signal.

Another weird thing that may or may not be related is that in my pin usage summary “Tacho Out” and “Inject LED” both say they are “HEI Bypass”. I don’t have HEI so I have no idea where that comes from.

One more strange thing that has nothing to do with MS is the VSS signal itself.

According to the factory service manual the VSS sensor outputs a signal at
41.1Hz @ 20 MpH +- 3.0. (calculated 4594 pulse per KM)
118.8 Hz @ 55 MpH +- 2.5 (calculated 4829 pulse per KM)
163.2 Hz @ 75 MpH +- 2.5 (calculated 4894 pulse per KM)

The VSS sensor itself is on the tail shaft of the transfer case and the manual states that it generates 8 pulses per revolution. This is a direct geared connection from the rear wheels through the differential, drive shaft and tail shaft to the driven gear on the VSS sensor. I am at a loss to explain how there could be different pulses per KM at different speeds. I confirmed this difference because at my set speed of 80 KpH TS agrees with my speedometer, as the speed drops I get an error that gets worse as the speed lowers. I would expect this same type of error as the speed increases above 80 KpH. Interesting!

[billr]

My guess was going to be growth of the tire OD at higher speeds, but that would me the counts/mile would be going the other way... Maybe the wind resistance pushes the truck back on the rear tires and squishes them down???

[E4ODnut]

Houston. We have a problem.

My speedometer and gear indicator using VSS1 on Datalog in has ceased to function. It worked just fine for one or two tuning sessions, but when I went to analyze a log yesterday, it wasn't there. I checked my pin assignments and it is correct. I went for a drive with my personal scope on the conductors right beside the DB37 to MS3X and have a lovely square wave at ~ 4.65 volts.

Something is weird here and I am going to need some professional help to troubleshoot this one.

[billr]

How about moving the VSS signal to the CMP input temporarily, so you can log that signal? What input is it on now, one of those "switch" inputs or PT4? Have you tried moving it to one of the other "switch" inputs?

[E4ODnut]

Hi Bill,
I appreciate your suggestions but I have a bit of a problem with them. CMP is in use as CMP. I could try PT4, but why should I have to? I have tried 3 switch inputs already. Tableswitch gave me an output low by a factor of 12.5. I corrected for the factor and it worked, but only for a short time. I tried Launch in and got nothing. Then I tried Datalog in and everything worked, but only for a short time. I can accept that I can make mistakes in wiring or set up, but this has gone a bit beyond that.

As I said, something is weird here, and there has to be a logical explanation. Time for the developers to offer an opinion.

[billr]

I assume this is that V10, and you must have a CMP input for it to run?

[E4ODnut]

correct