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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter · #1 ·
A number of years ago I swapped a 150 MPH SHO speedo head into the cluster. Worked fine for about a year, then it started acting weird. I replaced it with a spare head I had and the problem went away, but it's now back again after about 3 months of trouble-free operation.

The easiest way I can describe the problem is to say it looks like there's a loose wire somewhere and the signal drops, causing the needle to "bounce" around. Sometimes I'll be going 30 or 40 and the needle just sits there at 0 for 10 or 20 seconds at a time. It seems to be most pronounced at lower speeds (40-50 or under, seldom does it at 60 or 70) and only for the first, say, 20 minutes of driving. Both the odo and speedo are affected (when it sits at 0 the odometer doesn't move either, and I've lost 9 miles off the trip odo on the last tank's worth of travel alone).

I thought for sure it was the head gone bad after the problem went away after I swapped it out and put in my spare. Now I'm not sure that's the issue.

I'm wondering if it's a connection somewhere, although it doesn't seem like the speedo bounces are in any way related to road bumps/vibrations.

I am pretty sure the PCM is starting to do weird things (have to wait for the SES light to flicker after turning the key on before starting otherwise it cranks without firing for a few seconds) so I'm wondering if the PCM could be the cause.

The other possibility I can think of is the head has indeed gone bad (again), and I need another replacement. I can always swap the stock 120 in without pulling the cluster out (leaving connections as they are) to try and determine if it's the head or not.

It's not trans/VSS related. The speed signal is good and clean since I've never seen it do anything strange while datalogging. I also have my ScanGauge set to read out speed, and it also is normal.

What do you guys think I should try first? :confused:
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter · #3 ·
Thanks for the thoughts, but for me this cannot be a problem with the speedo gear in the transmission because the signal from the speed sensor being received by the PCM has been verified as accurate and clean.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter · #5 ·
The speedometer itself is not gear driven, only the odometer is. My problem is what can accurately be described as "signal dropping" when both the speedo and odo stop. If there was a gear problem, it would only affect the odometer, not the speedometer, since the only relation the two have to one another is a pair of wires that come off the PCB for the speedo that power the odometer motor. They are not mechanically linked.

The more I think about it the more I am inclined to swap the stock 120 back in without disturbing anything connection-related behind the dash. If it behaves erratically then I'll know there's a problem elsewhere and the speedo is likely fine. Otherwise I'll know I need another new speedo.
 

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Yeah, see if the 120 works correctly, and take it from there.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter · #9 ·
Finally got around to digging in today.

Put a stock 120 into the car without removing the cluster and it was doing the same silly business. So I yanked the cluster and started examining connection points, then sure enough... all the contact tabs were heavily oxidized. Cleaned on left, dirty on right:



So I cleaned them off with some 800 grit and rubbing alcohol and did the same to the metal spring contact clips. After that I stuck it back in, and voila, no more dancing speedometer. :D
 

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theterminator93 said:
I yanked the cluster and started examining connection points... all the contact tabs were heavily oxidized. Cleaned on left, dirty on right:

I cleaned them off with some 800 grit and rubbing alcohol and did the same to the metal spring contact clips. After that I stuck it back in, and voila, no more dancing speedometer. :D
Is that your original cluster?..Those contacts are reallllly oxidized..

It looks like a cluster you'd find in the junkyard..That's been exposed to the elements for a long time..


Glad you got it fixed..
















For now.. :D



Rayo..
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter · #12 ·
Yep, it was a donor cluster I bought as a spare from an ebay junkyard dealer. When I did the rewiring for the idiot lights so I could use the SHO tach face/gauge I didn't want to chop up my original cluster, so I bought that one as a spare. Ironic I missed that they were that dirty when I installed it considering I scrutinized the rest of it when I was doing the wiring. :zdunno:
 

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I'm starting to, every time I unplug/replug anything, DeoxIt the sucker out of them ...

RwP
 
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