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Discussion Starter #1
Hey everyone,

So I've got a 1989 Mustang LX that I just dropped a 5.0 HO into. AOD transmission and all of that. Just your basic 5.0. It still has the factory instrument cluster in it. The thing runs and drives fine, however my speedometer and tachometer are way out of wack. Plus my temperature gauge doesn't work (though that may be just us missing plugging in a certain plug. Not a big deal unlike the speedo and the tach).

My speedometer is off by a few kilometers, but it changes depending on the speed I'm going. I've checked my speed and when it says 40kmh, I'm actually going 30 kmh. So I'm out about 10 kilometres. However that's only in the low speeds. When I hit over 100 kms it goes further out. When I'm driving at 110 kmh my speedometer is reading 140kmh. Uh, far as I'm aware, when the speed sensor is bad, or the gear on the speed sensor is wrong, it will stay at the same difference. (I could be wrong, this is just what I've found so far).

My tachometer is also way out. When driving 110 kmh, I'm pretty much redlining on the tach at 6000 rpm, yet listening to the engine itself, it is in no way spinning that fast. And yet when it shifts, it's shifting in the regular 3000 to 4000 rpm range.

Any ideas? Is my instrument cluster itself screwed? Or is that typical with a 2.3l instrument cluster in a car with a 5.0 engine.

Big thanks!
 

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I'm doing a conversion also, but I am going to a T5. from the research I have done there may be a few different things it could be. have you changed the drive gear that inserts into the trans? how about the driven gear in the tail shaft of the trans? did you go to a 140 mph speedo? use the correct computer? did you change the gearing in the rear end? if you did you will need to get the correct drive and driven gears. just a few things to look at. hope this helps,
 

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You need to get a instrument cluster with tach out of a 5.0 Mustang in order for your conversion to work correctly.If you leave the 4 cylinder cluster in place,the tach will read double the rpms with the 5.0 engine in place.If you want,you can just mount a 5.0 tach onto the 4 cylinder cluster,instead of replacing the whole cluster,but youll still be stuck with a 85mph speedo.If you buy a 89 5.0 cluster,it'll have the right tach plus it'll have a 140mph speedo too.90-93 clusters will require slight modifications,that you can read about at the following link.
http://www.allfordmustangs.com/forums/5-0l-tech/30404-replacing-85mph-140mph-speedo-question.html#/topics/30404?_k=qjgisg

As far as the temp gauge goes,the gauge could be bad,open in the wiring,bad sender or a bad ground.
The first thing to check is the secondary ground.This is a ground strap that runs between the cylinder head(rear) & driver side firewall.Make sure this ground is tight & corrosion free.Its the ground for the interior/exterior lights,alternator,ac clutch & the gauges.You should always check this ground first if youre lights are dimming or if your gauges are erratic.If youre running efi with the computer & efi harnesses in place,make sure you check pin #8 on the white 10 pin connector.The 10 pin connectors are the white & black connecters behind the upper intake that connects the computer harness with the fuel injector harness.Their diagram is shown at the bottom of this post.The temp gauge wire runs through the white connecter & these connecters are known for developing loose contact between the male & female pinouts.Use a small pick to squeeze the female pinout tighter & spread the male pinout a little so both of them will fit tighter together.The same should be done to all of the pinouts then dab some dielectric grease into the female pinouts before you reconnect them.To make sure the wiring is ok,between the sender & gauge,turn the key on/engine off,remove the wire from the temp sender(at the intake)& touch the metal cap portion of the wire end to a good ground on the engine.The gauge should peg.If it does,that indicates the wire & gauge are ok,which means the sender is bad.If the gauge doesnt peg,theres an open in the sender wire or the gauge is bad.

Did you install an 8.8 rear end in the car or does it still have the 7.5 rear end??
The 83-89 5.0 aod had a 7 tooth drive gear & the
90-98 aod had a 8 tooth drive gear.The drive gear is inside the transmission.
The speedo cable has a speedo gear on it & its tooth count depends on which rear end gear was installed on the car,that you got the aod from.You need to match the drive gear with the correct speedo gear,depending on which rear gear ratio youve got.If theyre not matched,your speedo will be off.
Find out what year car your aod came from then match that drive gear to the correct speedo gear,using the guide below.*Example* If your aod is out of a 87 model(7 tooth drive gear) & youve got a 2:73 rear gear installed, youll need a blue 16 tooth speedo gear,in order for your speedo to read correctly.

The speed sensor thats part of the speedo cable is called the vss(vehicle speed sensor) It tells the computer what speed your going.Its also used for the following reasons::
Info source Joel [email protected] sbftech.com


The VSS signal was added to all Mass Air EEC-IV system EEC's and its input does have an effect for:

1) Engine idle control. The EEC uses the 6 MPH value threshold to activate idle dashpot strategy and prevent engine stalls coming to a stop and tip in when shifting.
2) VSS input is also an engine load determiner along with other inputs (TPS, MAF, etc).
3) Having a code 29 logged in the system can cause engine stalls in configurations with a manual transmission.
4) Its cruise control use has not changed.
5) Since VSS input is used to activate the dashpot function, it is normal to have a higher idle when transmission is shifted to neutral and the vehicle is above the 6 MPH threshold value.This is NOT an idle control problem (as it is erroneously tagged)... the system was designed to work as explained.



AOD trans

7-tooth drive 1983-1989
2.73 Gear Ratio / Blue 16 Speedo Gear
3.08 Gear Ratio / Gray 18 Speedo Gear
3.27 Gear Ratio / Tan 19 Speedo Gear
3.55 Gear Ratio / Orange 20 Speedo Gear
3.73 Gear Ratio / Purple 21 Speedo Gear
4.10 Gear Ratio / White 23 Speedo Gear

8-tooth drive 1990-1998
2.73 Gear Ratio / Gray 18 Speedo Gear
3.08 Gear Ratio / Orange 20 Speedo Gear
3.27 Gear Ratio / Purple 21 Speedo Gear
3.55 Gear Ratio / White 23 Speedo Gear
3.73 Gear Ratio / White 23 Speedo Gear
4.10 Gear Ratio / White 23 Speedo Gear
 

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Discussion Starter #4
Thank you so much, this answers a lot of questions!

I'll see if I can track down a proper instrument cluster in my area, so that'll fix the tachometer thing. I'll also take a look at diagnosing what is going on with the temperature gauge. I just had no idea where to start, so your descriptions provide a major help.

The speedometer stuff actually makes a lot of sense. I was looking into the VSS but I had no idea how to figure out which one was the right one or anything. I believe the engine-tranny combo came out of a 1991 Thunderbird, so it's likely running an 8 tooth drive gear. Which I think is likely where my issue is, since the speed sensor that was put in was from an older vehicle that had a dark green gear. (I can't remember how many teeth it was. The original had gotten busted apart when the dual exhaust got installed. And I didn't know that there are so man different gears for it so it got tossed).

The original factory rear end is still installed. All that has been changed was the engine, transmission combo, as well as the wiring to hook it up. Though I never realized it plays a part with the engine idle control. That could probably explain why the thing has a higher idle in park then in drive, as well as why on random occasions the idle will go out of wack. That probably also explains a number of the codes causing my check engine light.

This definitely gives me some ideas on what to look into and check. Thank you very much! :smile:
 
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