Drivetrain First fox with a couple of questions

Espo720

Member
May 4, 2016
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Hey guys new here and hoping you can all help me sort some stuff out. Picked up a 90 GT Automatic with 49k on it that has some minor issues.

1. Speedo is wayyy off, it reads 95 when the gps says 70, but thats not consistent. Car is an automatic and has cruise control. The airbag light is flashing 6 times so I’m assuming the clockspring is shot. Question is which speed sensor would be correct? I think it should be a VSS car because of the cruise control but don’t wanna order the wrong parts.

2. The car supposedly has a shift kit installed, but seems to shift inconsistently meaning at all kins of rpms. Would the TPS be the culprit or could the above issue be related to tranny shift points?

3. Last is every switch no matter what, makes the radio hum when used. I’m sure its a ground issue but does anyone have experience where they have fixed this and can pinpoint a common ground area to check?

Everything else is small issues that I can knock out quickly. Aside from the PO painting the car, it seems fairly original for a 30 yr old car. Has a bbk intake and an offroad h pipe but thats really it from what I can see.
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KRUISR

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Apr 16, 2015
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For #1... Likely you have the original speedo gear and the rear end gears have been changed requiring a different gear to get speedo correct (or close depending on gears). What is your door tag for AX? If it is M, I believe you have 3.73 gears in the rear end and the speedo gear is for the factory 2.73s.

Air bag light will flash a code to represent the issue. On my 90 GT automatic car, when I reassembled to car and started it I got a flashing air bag light as well (mine was a code 4). To solve my issue I unplugged the module, cleaned the terminals and reconnected. Light gone. Still off 3 years later. My car had 243,000 km and had been disassembled sitting in an old chicken barn for 12 years before I got it. Maybe after all these years you just have dirty connections.

Cruise control gets its speed signal from the trans end of the speedo cable.

SS11859W__ra_p.jpg

Speedo cable plugs into larger diameter at top of pic, speedo gear goes on the bottom, the clip bolts to the side of trans. Basically from o-ring down in pic is inside trans. Cruise plugs into smaller connector at top left. This is the only speed reading for cruise. Cruise will have no concept of actual speed, just want to maintain the signal reading when you engage a speed setting - it doesn't care if your speedo is correct, only that the cable is turning.

For #2... Do me a favor. Pull off the section of BBK tube between the MAF meter and throttle body and take a picture of the throttle bracket under the throttle body where the throttle cable and TV cable (kickdown cable) are connected. One common problem is the plastic mount for the TV cable breaks causing cable to not perform correctly. Misadjusted cable can cause soft shifts and is not good for long term trans health.

For #3... Your on your own for now on that one.


Nice '93 Cobra grill insert. I have one too.
 
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fox racer v2

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Jun 2, 2019
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1. For the speedo, rear axle ratio and rear tires will determine what speedo gear (in trans) you'll need. Just check out LMR's calculator to figure out which you need, and check the one you currently have to see if this needs to be changed. The VSS has no impact on the speedo gauge on the dash.

If you don't know the rear axle gear: Jack the car up, secure it on jack stands so the rear wheels are off the ground (chock the front wheels as well). Put the trans in neutral and release the parking brake. Take a piece of painters tape and put it on the inside sidewall of one of the tires - note it's position as if the tire is a clock. Also put a piece on the driveshaft. Spin the driveshaft, counting rotations, until the tire makes one full rotation - this will give you an idea of the axle ratio (One full tire turn and ~3.5 spins of the drive shaft would indicate 3.55s, for example). Stock (if AOD) would likely be 3.08 gears.

EDIT: The VSS on Speed Density cars is mainly a cruise control input. On Mass air cars, it helps with more. Your 90 is Mass Air, so it's important to have this running and functioning (Mass air likes to know vehicle speed and how it interacts with other sensors like the TPS, IAC, etc)

2. I don't know shift kits, but the TPS is not causing the trans issues. if it's a stock AOD, it's completely mechaincal - no electric inputs. Honestly, it sounds like it's slipping/other issues. It very could be a bad TV cable like the previous user said, if so do not drive it out of adjustment, you will wreck your trans very quickly.

3. Never had this issue before, but is the alternator/battery in good shape? I would do a once over of all the grounds while you're at it - but hopefully someone else will have a better answer.
 
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KRUISR

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Apr 16, 2015
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Stock (if AOD) would likely be 3.08 gears.

My 90 GT automatic came with 2.73s. AX code from door tag will nail down what his car came with. I believe you could get 2.73s, 3.08s or even 3.27s from the factory with an AOD if ordered that way. I am speculating that his car is a factory 2.73 car and he now has 3.73s. Why? The math works for the speed he is seeing right now when driving. Do the following equation:

3.73/2.73 x 70mph = 95.6 mph

The speeds he is seeing on the gauge and GPS


It doesn't work out that close with any other combination of ratios. So my speculation is he still has the 2.73 speedo gear in the trans and 3.73s in the rear. But your method of counting rotations is the best way to check/verify without taking the rear cover off and counting gear teeth (or finding stampings indicating ratio).
 
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fox racer v2

Active Member
Jun 2, 2019
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Brooklyn
My 90 GT automatic came with 2.73s. AX code from door tag will nail down what his car came with. I believe you could get 2.73s, 3.08s or even 3.27s from the factory with an AOD if ordered that way. I am speculating that his car is a factory 2.73 car and he now has 3.73s. Why? The math works for the speed he is seeing right now when driving. Do the following equation:

3.73/2.73 x 70mph = 95.6 mph

The speeds he is seeing on the gauge and GPS


It doesn't work out that close with any other combination of ratios. So my speculation is he still has the 2.73 speedo gear in the trans and 3.73s in the rear. But your method of counting rotations is the best way to check/verify without taking the rear cover off and counting gear teeth (or finding stampings indicating ratio).

Good point on the stock ratio, I misspoke.
 

Espo720

Member
May 4, 2016
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My 90 GT automatic came with 2.73s. AX code from door tag will nail down what his car came with. I believe you could get 2.73s, 3.08s or even 3.27s from the factory with an AOD if ordered that way. I am speculating that his car is a factory 2.73 car and he now has 3.73s. Why? The math works for the speed he is seeing right now when driving. Do the following equation:

3.73/2.73 x 70mph = 95.6 mph

The speeds he is seeing on the gauge and GPS


It doesn't work out that close with any other combination of ratios. So my speculation is he still has the 2.73 speedo gear in the trans and 3.73s in the rear. But your method of counting rotations is the best way to check/verify without taking the rear cover off and counting gear teeth (or finding stampings indicating ratio).

Finally getting outside to check things out today. Also forgot to mention it has 275/40r17s out back so I'm sure that would throw things off as well. Will be jacking the car up today to check the axle ratio, someone repainted the car and the door badges are gone.
 

KRUISR

Well-Known Member
Apr 16, 2015
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Actually 275/40/17s are essentially the same diameter as the stock tires. They will affect things very little if any (likely not even noticeable).

If there is a tag on the rear axle still, it will list the stock ratio as well (attached to diff. cover bolt). But these usually get tossed the first time the rear is opened up.
 

Espo720

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May 4, 2016
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Door Tag is actually there, I thought it was on the frame like on my f150 so my bad there. AX code is M as you originally suspected.
 

Espo720

Member
May 4, 2016
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Actually 275/40/17s are essentially the same diameter as the stock tires. They will affect things very little if any (likely not even noticeable).

If there is a tag on the rear axle still, it will list the stock ratio as well (attached to diff. cover bolt). But these usually get tossed the first time the rear is opened up.

Not sure how to read the axle tag at the mkment and trying to beat the rain but jacked up the shaft turns roughly 3 3/4 times per 1 rotation of the wheel.
Also snagged a couple pics if the TV adjustment cables

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