Speedometer Cable

[centurion 455 ragtop]

Question and advise here

Over the weekend I had my Centurion out for a three hour cruise upstate NY and notice the speedometer & odometer wasn't working. OK I figured. I gassed up and drove home. But I'm also noticed my gas gauge still reads way over the full mark...GREAT, I screwed now. Can't figure when to gas up at all.

So today I took apart the dash replaced the gas gauge with a working spare. It went right past the full mark again. I am pretty sure the tank is not full, maybe 3/4's. Checked behind the dash and everything looks fine. Fuses look fine, wiring under the dash look fine. It was working the week before and I never had any problems before with it.

Onward to the speedometer. Removed the face plate and speedo. Looked at the cable and with needle nose pliers gently (very gently) pulled on it. Wow is easily started to come out. So I kept pulling, and pulling and pulling:confused4: until the broken end came out along with some 2 feet of cable. Bottom line the cable is broke.

So first up is I need a replacement cable. Haven't checked but are these like impossible to get? If not where can I get one? Next is how difficult are they to install?

My gas gauge? I figure its either a short or the float is stuck. I'll just run it down some more and see if it then works properly before I go nuts looking for a short. Gotta fix that speedo first.

Thanks

[Smartin]

Just get a cable from a parts car - I'm sure there are folks on here or on v8 that have access to those cables. Unscrew it from the trans to get the old piece out...put it back together. Slide the new cable through the hole in the dash.

BOTH of my gas gauges are doing the same thing you describe. I have heard many people say it is a grounding problem at the tank.

[centurion 455 ragtop]

Thanks Adam,

Also spoke with Duane he mentioned the same. To remove the cable at the trans side. He did say to hold onto the cable for a match and event try to locate a locate speedometer business or Parts store. Thats my project for this weekend, to get that repaired. The gas guage will have to wait. I do know I can tow for about 200 miles before I need a fill up. Not towing I can do much better.

Also the same with the gas gauge. But he thinks it's the sending unit on the tank that is where the short usually is. Unfortunately that huge 3 series hitch has to be removed before I can drop the tank. Fun fun fun:Dou:

Love these cars...:tonge2: Both break at the same time while sitting not being driven....:Dou: Gremblins, that must be it...

Late Night Update:
Did some digging found the diagram in the Buick book for 73's It states it's either the black ground from the sending unit to the body or the gray wire from the sending unit which .....:shocked: I just figured what it may be... That gray wire goes from the sending unit to the rear trunk body panel which is right under that rubber dust flap above the rear bumper on the left side. It plugs into a wiring harness outlet. As I just remembered the day before I noticed the tank being pegged beyond the full mark, I cleaned the car and I replaced a left rear bulb. I slide my hand behind the rubber flap to reach the socket. So I may have pulled the wire plug out of the socket by mistake....I hope:confused2: I'll have to check that in the morning.

[gr_speedy]

Rick,
speedometer cable is available at Autozone. For GM 1951-1986. 3 lengths available. When the rear connection under the rubber is disconnected or loose it will show FULL. If it is a loose or bad ground it will show EMPTY. If all the connections are good and it doesn't work you need a sending unit.

[Robroy]

I’ve seen NOS speedo cables for our cars on ebay not too long ago

[centurion 455 ragtop]

Thanks guys,

7 am and I was in the garage checking that connection and wiring to it. Unfortunately it was still connected. I unplugged it checked the contacts, everything looked fine. Re-plugged it in and the tank still shows full. Its raining today I will wait till Saturday to get it jacked up to inspect the wiring or as much as I can get to under the car with the tank and hitch still in place. (not enough room to work on in the car tent and its too clean to move out in the rain today;)). Starting to think it may be the sending unit.

Well if its raining I can still hit a local Autozone for that speedo cable.

[MichelHaak]

Hi Rick, can you tell me how to get the speedometer cable out? The cable in my car make a rattling noises so I guess it needs grease. Is it a hard job?

[centurion 455 ragtop]

Michel I think my cable is broke or rounded out at the trans as I will explain on the next post. To get at it I took the top two screws out from the instrument panel cover. Then moved the shifter to low 1 and the instrument face plate comes right out. Then I removed the lower plastic dash piece that the small ac vent attaches to under the steering column. I think there are 6 screws. Be carefull not to pry it otherwise it will crack. Then with the selector in low one on the right side under the dash you can see a wire attachment which goes from the dash indicator to the steering column selector. Just gently unhook it. Once that is done unscrew the three small instrument face plate bolts. The face plate can be gently removed. Now would be a good time to replace those instrument panel lights. Now you can see the speedo cable sticking through the back of the dash. With mine it came right out with needle nose pliers, BUT MINE I THINK IS BROKE. PLEASE DO NOT pull on yours I am not sure if they all come out this way or not.

To eliminate the speedo cable noise use some white lithium grease. DO NOT use vasoline. During cold weather the vasoline will bind up the cable, to grease the cable you would have to pull out the cable. The cable may not be able to be pulled out as there may be a plastic end piece on the transmission side. If that is the case maybe try to spray some WD-40 using the tiny straw that comes with the can, spray it down inside the cable case that houses the cable.

Maybe someone can add to this?

[centurion 455 ragtop]

Getting back to my bag of bolts. Need some help here.

The speedo cable has a white plastic attachment on the end that goes into the speedometer. The other end just has the cable, but it looks uneven as if it broke on this end.

I took the dash apart....:rooleyes2:again. I slid the cable through the cable case and when it was about two inches from the end I fitting it into the speedometer and gently mounted the speedometer and faceplate back into postion and bolted it in.

Went under the car and I saw the speedo cable sticking out of the speedo case on the trans side by as much as 1 1/2" . So I gently fitted it back into the trans and screwed down the cap that hold it it.

Road test: The speedometer works BUT only goes as high to 35 mph regardless if you are going faster. The odometer started to work for only 2/10 of a mile and then stopped.

Conclusion: Is it possible that the end going into the trans is either rounded not grabbing correctly or that a small bit of the cable did break off inside the trans. I am pretty sure that the cable has to be replaced, but would like to be certain.

NEXT PROBLEM SOLVED: The gas gauge. Got under the car in the rear unplugged the gauge wire from the harness which is attached over & behind the bumper above the license plate area. Placed a test light on it. No juice on the plug. Hmmm gotta be a short or broken connection, it's not the sending unit. I followed the wire harness inside the trunk and on the drivers side behind the trunk lid hinge is a wiring harness connector, a rather large one. Checked and discovered there is a single plug along with multi wire plugs. Well the single plug was disconnected. BINGO! That plug is for the sending unit. It must of worked it way loose. Plugged it it and my gauge now works.
OK one problem down, one to go.

[Robroy]

Conclusion: Is it possible that the end going into the trans is either rounded not grabbing correctly or that a small bit of the cable did break off inside the trans. I am pretty sure that the cable has to be replaced, but would like to be certain.
Rick, to me it sounds like a new speedo cable.
Feels more likely that the cable fitting would be the problem than a half-broken cable, but that’s just my gut feeling since I have no experience of such problems more than the famous rattling sound that is.

Glad to hear on your breakthrough on the gas gauge problem! :xyxthumbs:

[centurion 455 ragtop]

I did some checking. My speedo problem may be inside the trans, being the cable does not appear to be broken.

I was told to remove the speedometer gear housing on the trans and inspect the gear assembly. It is a simple proceedure. It may be that the gear which is plastic may be worn or part of the speedo cable may have rounded out the input on the gear. I checked the "good book" and the trans code BC (standard TH400 for the big cars) has the same trans speedo gear as the code BT trans (Riv GS and hi-performance big car trans). I do have a good running used spare TH 400 long tail, code BT trans, so this afternoon I'll remove the gear assembly from that and get a better idea on what to look for and I can use it as a spare.

[centurion 455 ragtop]

Update.

I removed the speedo gear in the trans. It looked perfect, I also inspected the cable end and inside the gear and it seems to grab, I took a look inside the trans and the gear inside also looked good. Being I had an extra gear I swapped gears anyway. Installed it all and took her out for a spin. Same problem, odometer does not work and the speedometer goes up to 35 mph and will not increase as the cars speed increases past that speed.

I will go back into the dash. I am starting to think its the speedometer. I do have an extra speedometer (it doesn't have the trip odometer option) I know it works, so I'll swap it and see.

[centurion 455 ragtop]

OK I am completely confused:Dou:

1. Swapped speedo gears in speedo cable housing that mounts into the trans. didn't help
2. Swapped speedometers with one I know works with old gear in speedo housing and with new gear in the speedo housing. Didn't work
3. Bench tested both speedometers using my speedo cable run on a drill. Both speedometers cranked up past 90 mph and odometers worked
4. Re-installed my old speedometer installed my speedo cable. Hooked a drill to the end of the speedo cable that goes into the trans to a drill. Speedo worked cranking past 90 mph and odometer worked.

5. Hooked the cable back into the trans. (the cable fits in snug) Took the car out road test. Didn't work
6. Swapped gears in trans again. Road test. still didn't work.
7. Had a 3 inch piece of an old speedo cable and inserted it into the trans and it would not spin by hand which means the speedo housing gear inside is grabbing the other internal trans gear. If is spun the gear then the problem would be one of the gears. Or if the cable didn't grab it could be the cable or gear.
8. When I was swapping gears in the trans looking inside the both gears looked good.

SO.....one last test (had to stop today, it started to rain) I will get the car on jack stands. Disconnect the speedo cable from the trans. Insert the spare 3" cable into the trans. Start the car and place it in drive and see if the spare cable spins. If it spins....:tonge2:I'll just shoot myself!!! Because I would not even know what it could be.:Blasting:

Two last thoughts....Inside the trans are 2 gears, one comes out with the speedo cable housing, the other is inside the trans case and this gear turns the speedo cable housing gear which turns the cable. Maybe for some reason that gear is not turning. Or the cable, the only thing I can guess is the cable is not long enough to go inside the trans. Who know...:confused4:

[dewbers]

Great thread Rick..... luckily I haven't had this problem, but if I do, I'll know where to go for the fix. Thanks for the updates... they are great!

[Robroy]

What’s the latest Rick?
Any breakthrough in the trouble shooting?

[centurion 455 ragtop]

Found the problem but repairing it will have to wait. The problem is the speedo gear that is attached to the output shaft inside the transmission. This gear slides over the output shaft inside the tail housing of the trans. Its not a difficult repair, just time consuming. There is a small metal clip that secures it to the shaft so that it spins at the same rate the shaft does. In my trans apparently that clip broke. So the gear will spin with the shaft until a greater load is applied then the shaft spins faster and the gear just slips on the shaft not spinning. This is why the speedo goes up to 35 mph. The gear is tight enough to grab the shaft up to that speed. Anything higher it slips.

So with the Norwalk event coming up this weekend this will have to wait. I was way too busy getting the race engine on the dyno and into the GS in time for leaving this Thursday. Oh and by the way the race engine is making 780 hp at 6900 rpm and 679.9 torque at 5300 rpms

[The German]

Again your story will be of help for all of us in case of a similar problem, Rick. Thanks for sharing !!! :xyxthumbs:
Sorry that we couldn´t help you, but glad that you found the problem to be not a big one !

Good luck with your race car !