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 Post subject: Speedo reads wrong
PostPosted: Tue May 27, 2008 12:15 pm 
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Ok this a long post so strap yourself in.
I'll post an introduction very soon too as I'm new to this forum.

I have a 97 T/A m6 Euro Spec.
It has the primary scale on the speedo in kph.
I have a GPS radar detector and also an LT1 Scanmaster connected up.
Last year I had the rear gears changed from 2.72 to 3.73. Before I had this done, the speedometer and GPS and scanmaster all agreed exactly on the speed. I knew the rear gear ratio was going to affect the speedometer and on the way home only the GPS showed the correct speed, with both the speedometer and scanmaster agreeing with each other but reading fast.
I wasn't concerned as I had a Hypertech PPIII I used to modify the fan switch on temps and thought I could correct it with this.
Unfortunately, this wasn't the case. As soon as I modified the PCM programming with the correct gears the speedometer and scanmaster read low.
I figured their was some incompatibility between the Euro Spec programming and the US spec hypertech but as I planned to have a new PCM program done by PCM4Less and was on the verge on changing wheels and tyres too I left it.
Fast forward a couple of months and some nice new shiny wheels found their way on to the car. So I contacted Byran at PCM4Less and gave him the details of all the bolt-on mods, new wheel/tyre size and rear end ratio. I decided to pay the additional to have a new (to me) PCM sent to me so I could keep my original PCM as well.
This arrived within two weeks and as far as performance goes I couldn't be happier, but the speedometer problem is still there.
The PCM also through up one other issue - my car is the 6speed version and doesn't have CAGS. The solenoid isn't present on the transmission. But with the new PCM in place I was getting a light on the gauges and when I checked for codes I was also getting a code set. (This was end of last year and I don't remember which code but it was to do with CAGS). I bought a little plug-in doohickey from ebay and that solved that issue.
This was the end of last year, and my car is definitely not driven over the winter, so I at this point the car was garaged whilst I cleaned and detailed everywhere.
Fast Forward to this year and I'm reminded of the speedometer issue when I start driving the car again.
A couple of emails to Bryan to see if he has any suggestions are left unanswered.
I spent a while researching around (how I found this forum!) and decided that there could be something in the Euro spec speedometer that the US spec PCM program doesn't like. Of course gauge clusters are very year specific so finding a US spec gauge cluster for a 1997 wasn't as easy as it sounds (there are none when you are looking!). But eventually I did find one and had it shipped over. Of course you already know it didn't solve the issue as if it had I wouldn't be posting!!
So how low does it read? - 30mph on the GPS is an indicated 20kph on the kph primary scale speedometer and 20mph on the scanmaster. I'm struck by the con-incidence here - 30kph would be 20mph so its almost as if there is something else doing a conversion in reverse that the original EuroSpec PCM needed but is confusing the US spec programming in the PCM (and of course was also the case with the Hypertech).
Now the Scanmaster is tapped into the under-dash ODBII port and it reads more than just speed, so it is receiving a stream of data from the PCM. Now if this data is wrong it explains why the speed reading on the scanmaster is wrong and why the speedometer is wrong. (Anyone wanna buy a US spec gauge cluster?? :) ) So assuming that Bryan entered the correct variables on the program, (a fair assumption as the Hypertech was also an issue) then the speed signal the PCM receives is wrong. I understand the VSS is a standard device and I can't imagine them changing the VSS for Euro-spec cars, then did they somehow modify the signal in between the VSS and the PCM? I did read on the 'net where someone with a EuroSpec Holden found a similar device behind their speedometer, but this is slightly different as it looked like it was modifying the signal after the PCM to change his speedo from its default KPH to MPH. In my case it appears to be modifying the signal before the PCM. So if the engineers did indeed fit some device, where would they put it? It would need both power and ground and be somewhere along the wire run from the VSS to the PCM?
Feel free to shoot me down in flames with my diagnosis here - I have to admit I'm at a loss and I'm really hoping that all my explanation is all un-needed as someone is going to say "its here" or "you need to do this".
I can test with both EuroSpec and US spec speedometers, I can test with EuroSpec and US spec programming. I have since sold the Hypertech as I needed to pay for the US spec gauges!!
I really don't want to go back to 2.72 rear gears and my stock programming - can anyone help?


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 Post subject: Re: Speedo reads wrong
PostPosted: Tue May 27, 2008 12:50 pm 
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Joined: Sat May 26, 2001 9:59 pm
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Location: Bad Hersfeld, Germany
After changing from 3.42 to 3.73, I used my EFILive to correct the Speedo on my car - even using the correct gear ratio I ended up wrong (10% over), no telling why - luckily, I had the chance of correcting the impulse count directly, after another try where I was 10% low (drove it another 40mi again to correct for the over-miles I drove before). On the next attempt, I was something like 1-2% under what the GPS said, so I just left it there ...
No telling what you can do with the LT1, especially not with the crappy Hypertech ...

_________________
Garry Glendown * '99 Firehawk Convertible

"Horsepower is how fast you hit the wall and torque is how far you take the wall with you."
"Speed doesn't kill - suddenly becoming stationary on the other hand ..."


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 Post subject: Re: Speedo reads wrong
PostPosted: Tue May 27, 2008 3:37 pm 
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Garry, Is your car a EuroSpec too?
If so I think its more than co-incidence that even after using the correct gear ratio your speedometer was still off, and oh I sold the Hypertech.
EFI-live is something I will look at.


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 Post subject: Re: Speedo reads wrong
PostPosted: Tue May 27, 2008 3:38 pm 
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EFILive ist only for LS1 and newer ... no LT1 I'm afraid ...
No, the car is US spec ... there are no SLP-modified Euro-cars ... no SS, no Firehawk, etc ...

_________________
Garry Glendown * '99 Firehawk Convertible

"Horsepower is how fast you hit the wall and torque is how far you take the wall with you."
"Speed doesn't kill - suddenly becoming stationary on the other hand ..."


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 Post subject: Re: Speedo reads wrong
PostPosted: Tue May 27, 2008 7:31 pm 
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Joined: Tue May 27, 2008 11:28 am
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Nope very true, there are no SLP modified cars, but I didn't notice your sig before.
I could likely achieve something similar with LT1 edit if i wanted to pay the 450USD.


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