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Speedometer question for guys with TH400

7K views 7 replies 4 participants last post by  03Steve 
#1 ·
Before doing the trans swap I tried to research the speedometer issues as much as possible. My car is primarily a street car so an accurate speedo is a priority for me. I know Burntire/Steve and others have outlined the way to tie the OSS sensor wires into the front ABS sensor wires but for whatever reason I chose to get the GM speedo sensor in the TH400. Carl at Rossler walked me through all the parts included and told me the GM sensor is 12 pulses. I called and spoke with Walsh at D&D (T-56 experts) and he told me the OSS for 03-04 Cobra T-56 is also 12 pulses. Both are reading the output shaft so rpm/gear is irrelevant, it should just be plug and play.........."should".

Long story short I got it all together and the speedo is off. I drove around with GPS speed and at 25 my car says 40, at 45 my car says 80 etc. The relationship is linear so I could just play with the tune but if both are reading 12 pulses I shouldn't have to mess with anything right?

So my question is.......can someone confirm for me that either the GM sensor or the Ford sensor is in fact 12 pulses? Is there a polarity concern at all, should I try and flip the 2 wires? So if anyone chose to use the GM sensor I would like to hear how you got it to work. I know I can just tap into the ABS sensor and modify the setting in the tune but I would like to try and get this to work.

Thanks!

--Joe
 
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#4 ·
The stock tune is 12 holes.
With the C4 I ran the oss set to 6. I had a 3.27 gear set and the axle ratio was set to 2.40.
I had 26" tires and the rotation was set to 600.
 
#5 ·
I can make a long post with a bunch of tech info on this, but here is a quick summary...

Regarding the sensor, there are generally two types used for mustang OSS sensors. Variable reluctance (two wire, found on 2003-2004 Cobras) and hall effect (3-wire, found on 2011+ Mustangs). The two wire sensor is not polarized, or provided with a supply voltage. It is a magnetic pickup that generates voltage in part from the gear rotation.

Your observed speed numbers:

25/40 = 0.625
45/80 = 0.563

Assuming those are proximate relationships, you are correct...there is a somewhat linear relationship between actual (GPS) and observed (speedometer).

There are three ways to correct this.

First, make a proportionate adjustment in the number of holes for OSS (recommended).
Second, make a proportionate adjustment in the rear gear size.
Third, make a proportionate adjustment in the tire revs per mile.

I am not sure how the mechanisms are laid out with the TH400, but there is an apparent reporting difference between the T-56. That is a direct result of the output shaft change. Some numbers to consider for the next step:

12 / 0.6 = 20 (current number of holes / correction factor = new number of holes value to try)

The working principle is that the higher you adjust the number of holes value, the lower the speedo reporting value. I'm going to assume between 19-22 is going to be the ticket. Fine tune with tire revs at 70+mph.

Good luck.
 
#6 ·
Two wire reluctor sensors do not have polarity. As long as you have the non-grounded side of the sensor going to the PCM, you are ok. You would not see any speed if you hooked the grounded side to the PCM, so that is not causing your problem.

The T56 does have 12 holes. I am not familiar with the TH400, but found this thread that implies that there are 40 teeth on a TH400 reluctor wheel. I think it explains the issue pretty well: http://www.modularfords.com/threads/124623-TH400-speedometer-parts
 
#7 ·
Steve,

Thanks for the details. I follow everything you said. As for my speed numbers, I was just recalling those from memory but they should be close. I think the linear relationship of about 0.6 is very close.

--Joe
 
#8 ·
No problem Joe.

So you know, the pulses are generated from the transmission output shaft itself. Same way the 50-tooth ABS ring generates 50 pulses per revolution observed by the ABS sensor. If you know the number of teeth on the shaft pickup point, you know how many pulses any sensor will see per revolution, and you then know the value that belongs in that scalar.
 
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