Hello. I have a 1965 Mustang Fastback, with a 6 cyl engine, and a four-speed trans. I am having some problems with the speedometer bouncing. Initially, I installed a new speedometer head, that we had laying around. This didnt help.
    Next, I lubricated the cable with lithium grese. I finally bought a new speedo cable from the local auto parts store. No change. Someone then told me that I needed more lube on the cable. I applied the rest of the packet that came with the cable and that made it better, but it has since gotten worse.
    The next step was buying some driven gears from a Mustang vendor. They were repops and happened to be smaller, and the speedometer didnt work at all. We tried ordering gears from Ford, however they didnt know which part numbers we needed. We found some PNs on the internet, however they were long shaft and the wrong direction.
    I am completely confused what to try next. The problem at this moment is intermittent. In the morning when it is cold out, it was really bad. In the afternoon, it is bad at first, however it gets better, however, it isnt right yet.
    Any suggestions would be greatly appreciated.

TIA,
Brennan  

You may want to make sure the gear in the trans isn't loose on the output shaft, or otherwise damaged.  

I am pretty sure that the gear is tight. It is worn a little. We tried the 16 tooth small repops on another car and they work, so I am guessing that the drive gear (on the tailshaft) is worn. We have a new one of those. Is that worth taking the tailshaft off to replace?  

You won't actually have to remove the tailshaft. Youwill have to remove the driveshaft, drain the oil, remove the cable, and remove the tailshaft housing. The shifter will probably have to be removed. There is a gasket between the housing and main housing. You also need to replace the rear seal. You may have to remove the trans mount. I did mine on the bench and I can't remember exactly how my mount is configured.  

Oh, after removing those parts, the gear is held on by a large snap ring. It takes a large pair of snap ring pliers to ensure you don't damage or stretch the snap ring. I replaced my snap ring while I was there.  

sorry to jump in ... but if the speedo needle is bouncing...

it is usually the cable its self...     pull it out of the casing...    examine it from end to end...    flex it over your fingers... you are looking for a hard spot... where it layed over the exhaust or went around a tight corner and has lost its flexability usually in just one spot......   this causes the core to whip as it goes around the corner or bend....  

measure the cable core...  any speedo shop can make you one in minutes... or order one and have it mailed to you...      some napa stores have a speedo cable krimping machine... to make custom length cores...

universal cores can be had at any parts store...

it could also be too much grease... you might use some spray cleaner shot through the casing with the core out..

then a little bit of grease...     white lithium  or i use moly graphite assy lube.. on the side of the cable .. not on the end... it will get pushed into the hole and the cable cannot fit all the way in.....  

waynep712 : I have already replaced the cable with a brand new cable from the local auto parts store. There are no major kinks in the cable, and it is far from the exaust.  Also, when I initially put the cable in, I used only a little of the lube they sent with it. When it didnt get any better, I put the rest of the lube on it.  

You need to make sure that the original and replacement cable are of the same length. Not only the outside cable but also the flex cable. If it is just a shy too long then it will be crammed into the back of the speedo head and what that will do is allow the cable to whip like a jump rope, hitting the sides of the cable housing.

Also you stated that you have a new speedo head installed. Even though it is new lack of lubrication at the head gear can also cause an issue. If the speedo head is "dragging" the when teh cable is driven by the drive gear and the speedo head grese is "gummed" up the the flex cable will again whip round causing the jump. Most of the time the jumping decreases when the speeds increase.

Personally i still think it's your cable. You indicated that when you added more lube that it helped a little. The lithium lube is ok but still allows for friction due to that it is a light weight lube. They do sell cable lubes which are thicker, But in the past what I would use is High temp bearing grease. When you lube the cable remove it completely and as you push it back in pack the cup area with grease the whole time until all of the cable is reinstalled. This is not too much grease because the cable will "Grab"  what it can and carry it through the whole length.  If this helps more than you need to focus on how th cable is routed. The more curves you put in the cable then the more the cables is forced to flex adding to the problem. Also make sure you do not any cable clamps too tight as that will also crush the housing.  

I dont actually have the original cable, however this is the length that was on it before. I dont know how else that I could run it that it would be any longer or shorter. It seems to be the right length. The inside cable is the same as the outside. I am using the cable lube that came with the cable, so that should be ok. So you are suggesting that I should lubricate the speeodmeter head itself? If so, how. q-tip? Also, do you think that it could be the drive gear in the trans?  

FYI If you ever remove the gear on the output shaft, there is a large "BB" that is used as a keyway...be careful not to lose it. It is very possible that your problem is not in the trans, but if your gear is damaged, I would replace what you know is damaged. It may not repair the problem, but a worn drive gear cannot help your problem.  

One easy way to test if it is or is not the drive gear. Remove the cable from the tail shaft housing and remove the gear from the cable. take your electric drill and run up the chuck. Spin the drill the correct direction and check the needle. Do this with the cable through its normal routing. Its should at least produce enough RPM's to find your problem if it is the actual cable.


If you can spin it fast enough you will diagnose the cable, routing and speedo head all at once. his will narrow down the most likely problem. Also you can look into the hole where the cable goes and see the actual Drive gear. If it's worn severely you will see a large dip in the center of the drive gear.  

Remember, unless you are the original owner or you know for a fact the history of the car it's safe to assume that a lot of your mustang has changed. Someone could have changed any piece of that the speedo circuit and the result would be mismatched parts.

Attack the problem with test that include multiple pieces at once similar to the test I stated. That way you eliminate more pieces from the puzzle at one time.
Read this article some else posted. Scroll down to the speedo section.

http://www.mustangandfords.com/techarticles/75259_instrument_electricity/index.html  

Ok. I have isolated the problem. I realized I could stick my finger up the gear hole and feel if it was worn. It felt pretty worn, so I removed the tailshaft. The gear is worse that I thought. I will post a picture if I get a chance. Now my next problem is finding a drive gear for it. It is a BorgWarner T10. I checked with the Ford Parts counter, and they cant get one. I have tried quite a few Mustang stores, and I cant find any. Does anybody know where I might be able to find one? Thanks!

Brennan  

is it a 4 speed top loader...     there is a tag on the side... riveted on...  that may help.....


some c4 driven gears may fit....    


what color is the gear...

how many teeth... count on one side.. use a marker to keep track...

how many splines on the output shaft...     25, 28... more..???

diameter of the area on the output shaft where the gear fits...

this is also a good time to replace the output shaft bushing and seal...  full  gasket sets are cheep.   under 5 bucks at a a gears and shafts in los angeles..  

It is not a toploader, it is a BorgWarner T10

The gear is black

The inside of the gear isn't splined. I dont believe it is a Super T10

The tag says: HEK AD

The inside hole where the gear goes on the output shaft is about one inch and a quarter. Here are some pictures: http://www.brennanu.com/speedo_gears/

The pictures are really big. If I get a chance I will make them load faster.  

Believe it or not, I called Summit Racing and told them what I had and got one from them. I called their tech support line and they hooked me up.