Speedo Correction

[michaelsgreene]

OK. Now I have the bigger tires put on and the speedo is off by about %10. I am considering changing the diff ratios as well. I went to the deal and he hooked up the computer box and said I had two choices:



Michellin 215/85 R 16 or

Michellin 215/85 R 16 M/S



no options for gear ratio change.



I have done some research on electronic boxes to correct the speedo. Here is what I have found:



http://www.sporbaugur.is/truspeed/template.php3?02_whytruspeed



http://www.ricksontruck.com/RicksonCatalog.pdf (see page 14)



http://www.azttora.com/speedometer.html



The prices range from about $80 to $200, but the cheaper one says not to use it with ABS.



Is it possible to alter the signal going to the computer and not to the ABS module?



Any ideas?



Michael

 

[fest3er]

Originally posted by michaelsgreene
OK. Now I have the bigger tires put on and the speedo is off by about %10. ... The prices range from about $80 to $200, but the cheaper one says not to use it with ABS.

Is it possible to alter the signal going to the computer and not to the ABS module? ...

Now that the site has good search capabilities, you can do a search for 'Abbott' and find the threads in which this has been discussed in the past.

The easiest way to to use the truspeed for 4WAL systems.
The next best way is to modify the Abbott so it can correct the signal between the CAB and the PCM (thus keeping the ABS functioning properly). See http://www.adelphia.net/~npmrphy/abbott-mod.pdf
for details on the modification.

FWIW, the Abbott is a rock-solid adapter. With good electrical connections, it works well from 0 MPH to well over 200 MPH.

I bench-tested one once. I fed it sine waves, square waves, triangle waves and anything else I could find. It handled them all quite nicely. I varied the input signal from 100mV up to 5V.

At some point the Abbott will drop the output when the input voltage is too low; this will only happen if you have dodgy (no pun intended) electrical connections to the rear wheel speed sensor.

However, the signal from the CAB to the PCM is well over 2. 5 V. If you perform the modification (in simple terms, you are adding a pull-up resistor to the Abbott's output signal) and put the Abbot between the CAB and the PCM, the drop-out should never occur unless you have a *really* bad connection.

I've had an Abbott on my truck for 55K miles now. Once I made good electrical connections, I have had *zero* problems with it. As near as I can tell, the odometer is dead-nuts on, which is what I wanted. The speedometer is none too accurate, but I don't care so much about that (What'sa behind me is not importanta!)

N

 

[Posm]

I was under the impression that the 12 valves used a gear change and the 24 valves were the start of the reprograming????? If it was me I would take the 235 MS program and then check it agoinst the mile posts. Not all spedos are correct to start with and that may get you close enough. If not, then you can look at the add on boxes. I tried to get my 24 valve reprogramed and they could not "get it to take" so I checked it with the mile posts several times and found it to be off 5. 3%. I just add it when I figure milage.

 

[fest3er]

Originally posted by Posm
I was under the impression that the 12 valves used a gear change and the 24 valves were the start of the reprograming????? If it was me I would take the 235 MS program and then check it agoinst the mile posts. Not all spedos are correct to start with and that may get you close enough. If not, then you can look at the add on boxes. I tried to get my 24 valve reprogramed and they could not "get it to take" so I checked it with the mile posts several times and found it to be off 5. 3%. I just add it when I figure milage.

DC moved the VSS from the trans to the rear wheel sensor starting with the '98s, I believe. (My '98 12V has only the rear wheel sensor. )

The CAB is only programmable for certain tire sizes. The 'any size' correction has never worked. Thus, the only solution to correctly fix the problem is to use a TruSpeed, Abbott or similar add-on box.

N

 

[michaelsgreene]

Here is the correct link for the mod. I think I will use this one.



http://home.adelphia.net/~npmrphy/abbott-mod.pdf



Thanks.



Please explain why a 20% change in tire height and circumference causes a 10% change in speedo reading.



Michael

 

[michaelsgreene]

Thanks for all the help. I should have done a search, but I didn't know that I could. All the information is already in the forums.



I think that I don't need to do the mod since I only have rear ABS (from the dealer sticker and actual experience).



Still don't know why a 20% change in tire size equates to only 10% speedo change. It makes my head hurt 'cause it just can't be so!



Michael