Odd Speedometer Behavior


Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Even with extensive and exhaustive use of the forum search function :lol I have not found this issue discussed.

Recently, when driving from a cold start at under about 40 mph, the speedometer needle bounces from zero to 40. Over about 40 mph it is normal. After a few miles of driving it's completely normal.

I thought I might have imminent gauge failure, so I installed my ScanGauge, which displays almost everything reported to the OBDII port. I figured if the speedometer failed, I could read speed on the ScanGauge.

Well, the ScanGauge digital readout behaves exactly as the speedometer needle does.

This tells me the issue is not in the gauge.

What's going on? Since this falls in the "Prince of Darkness weird electrics" area, I guess I'll check the battery ground. Any other ideas?
 
bonehead

bonehead

GT Owner
Mark II Lifetime
Aug 18, 2012
2,827
Houston, TX
Same issue?

http://www.fordgtforum.com/forums/showthread.php/9314-Speedometer-not-working
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
I don't think so, Ed, because the speedometer always works. Meaning, the needle always moves, although erratically as I described. I have not had the needle sit at zero while moving.

It's always on a battery tender at home, and the ScanGauge shows charging voltage >14 volts.
 
N

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
4,281
Check the VSS at the rearmost driver's side of the transaxle. Disconnect and make sure contacts are clean. Use a dab of electrical grease before re-assembling if you have some handy. It's good news that the Scangauge has the same odd behavior. That pretty much exonerates the speedometer as the culprit (whew!).
 
FlagstaffGT

FlagstaffGT

GT Owner
Mark II Lifetime
Aug 15, 2006
669
Sedona but was Flagstaff, AZ.
$15 dollar sender unit (same as F150) in the rear under air filter. I've replaced two of them. Heat can crack and warp 'em and they act funny.
 
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S

shelbyelite

PERMANENTLY BANNED
May 10, 2007
1
Speed sensor that's bolted in the side of the tranny. Its gone bad. We have replaced a few because of this and it fixed it every time. Be willing to bet that's what's going on with yours.
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Thank you, gentlemen. I will give it a try. :)

GTVSS.jpg
 
Indy GT

Indy GT

Yea, I got one...too
Mark IV Lifetime
Jan 14, 2006
2,545
Greenwood, IN
Wow, Gary, that was better than a search function!

That was a new one on me as well, but luckily we have some great experienced owners (and dealers) here on our Forum.

Uh, er the Prince of Darkness comment is usually reserved for English cars with Lucas electrical systems. At least that is the context in which I have heard it used.:biggrin
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
The part is up to $31... still a bargain repair.

So far, this, the clamshell struts, and the clamshell lift springs are my only parts failures.
 
otis

otis

GT Owner
Mark II Lifetime
Nov 13, 2011
349
Stillwater, MN
I have the same behavior. Looks like I need to order one asap.

Thanks again guys. I was considering a new speedo for major money.

Otis
 
otis

otis

GT Owner
Mark II Lifetime
Nov 13, 2011
349
Stillwater, MN
$19.20 at fordparts.com and you can pick it up at the dealer closest to you. Even better no shipping! Sure hope this fixes mine.

Otis
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Boom! Problem solved.

Time to R&R the speed sensor: 1 minute (plus 30 min to R&R diffuser pan).

IMAG1296.jpg

Round of drinks on me in LV.
 
FlagstaffGT

FlagstaffGT

GT Owner
Mark II Lifetime
Aug 15, 2006
669
Sedona but was Flagstaff, AZ.
Next time approach it from the top side! (remove air filter box-a couple of accessible bolts and a hose clamp).
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Also doable, but a long reach. I'm less likely to do damage from below. :)
 
Howard

Howard

GT Owner
Mark IV Lifetime
Apr 26, 2007
1,150
Florida/North Jersey
If you've met Gary you'll see immediately why he went after this from the bottom.

Howard
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Howard said:
If you've met Gary you'll see immediately why he went after this from the bottom.

Howard
Click to expand...


^^^ This!
 
Sinovac

Sinovac

GT Owner
Mark II Lifetime
Jul 18, 2006
5,862
Largo, Florida
Howard said:
If you've met Gary you'll see immediately why he went after this from the bottom.

Howard
Click to expand...

:lol

My speedometer exhibits some odd behavior too. Sometimes it stays at 0 mph even though the tach is registering 5-6k rpm.
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Sinovac said:
:lol

My speedometer exhibits some odd behavior too. Sometimes it stays at 0 mph even though the tach is registering 5-6k rpm.
Click to expand...

Welcome back! It has been quiet around here without regular injections of sarcasm.
 
Sinovac

Sinovac

GT Owner
Mark II Lifetime
Jul 18, 2006
5,862
Largo, Florida
Xcentric said:
Welcome back! It has been quiet around here without regular injections of sarcasm.
Click to expand...

I hoped that you would pick up the slack. I see you were a chatty Cathy in my absence and surpassed my post count.
 
Xcentric

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,213
Myakka City, Florida
Sinovac said:
I hoped that you would pick up the slack. I see you were a chatty Cathy in my absence and surpassed my post count.
Click to expand...

Along with surpassing your mileage.
 
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