Problem with my 89 foxbody's speedometer,

It will read 0km/h until a certain gear, or around 70km/h ish and then the needle quickly 'bounces' up to speed; when i decelerate to below 40km/h the needle will bounce just above 40km/h.

Any ideas? Do you think i may need a new speedometer cable or something (assuming it has a cable being an 89 i doubt it has a speed sensor)

 

I managed a speedometer shop in the mid-late 90's and we serviced a number of mustangs.

Is the odometer registering distance when the speedometer needle is not moving? If you drive the car a know distance is that distance accurately shown on the odometer? If you answer yes to these questions then the issue is the speedometer itself. See the last paragraph.

If the odometer and speedometer needle are not working at the same time then the issue is with how the speedometer is being driven. Start at the bottom and work your way up to the speedometer.

As I recall the speedometer cable housing snaps into the speed sensor at the transmission. Make sure the cable housing is seated properly in the sensor and that the sensor isn't slipping on the cable. They can go bad.

Also, make sure the speedometer cable is firmly attached to the back of the speedometer. If it is loose it could cause this but that is pretty uncommon. Usually they fall of the back of the instrument if they are not connected properly.

Cables don't typically wear out, they break. So they either work or they don't. Some times the plastic tip on the OE cable will wear out but this usually means there are other issues. Like the cable was attached properly at one time or the speedometer is failing/locking up/binding.

You can lube the cable using GREASE ONLY - no graphite - as a preventative measure but it unlikely that this will solve the issue. To lube the cable, disconnect the housing from the speedometer and put the cable out of the housing. Wipe it down and apply a light grease - Mobile One synthetic is a good choice - as you feed the cable back in. Graphite has abrasive characteristics that can and does damage a speedometer. Saw this first hand allot so DON"T USE GRAPHITE.

Next, inspect the driven speedometer gear (the one on the end of the sensor). The gear teeth should be straight when viewed from the side. If it shows wear in the center of the gears it could be slipping. If you have a 23 tooth driven gear (I believe they are white) this is likely because they didn't mesh properly.

It is also possible that the drive gear in the transmission is slipping or worn. We would do a visual inspection and then a slip test. A slip test involved grabbing the cable at the top with the rear wheels off the ground, the car in a forward gear and the engine running. If the cable could be stopped (without twisting it) the gears are slipping. We used a turbo 400 GM driven gear to test this because they have a long shank that could be pinched between two fingers.

Speedometers in these cars can fail due to the way the cable is routed from the factory. The issue was that the speedometer head frame would bend because the cable routing put a side load on the speedometer. Eventually this caused internal parts that aren't supposed to touch to come into contact. When this happens you will need a new speedometer as there is no repair for this. BTW this was not an issue on 79-86 mustangs/capris because the cable was routed better in that dash.

 

Yeah it measures distance just fine, but ****, how much do u think a new speedometer is? (Ballpark) and should i just get like an autometer Spedo?

 

A new OE speedometer would be expensive - they were back in 1988 - and rare now days. What you need is a good used one. I see them on Ebay and mustang sites all the time. Since you are in Canada you will need a metric speedo because the odometer gears are different. A speedometer shop (they still exist) can combine the odometer from your istrument with a good speedo head and that way you have the same distance indicated.

An after market speedometer is not a bolt in