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I knew the 70 felt fast - now I know why

Started by Dan LeBlanc, November 13, 2016, 08:54:13 PM

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Dan LeBlanc

My speedometer is off by about 10mph at any speed. Seems like I need a speedo driven gear with 6 less teeth (60 reading is 70 or about 16% - each tooth less speeds the cable by about 2.5% in the THM400).  Are the speedo driven gears in these transmissions typical GM fare?
Dan LeBlanc
1977 Lincoln Continental Town Car

76eldo

I'll have to check mine with a phone Nav app that tells you your speed by GPS.  I know my 66 Toronado is also off like you described, indicated 70 is really closer to 60.

Maybe GM was playing games to make people think these cars were quicker than they are?

Brian

Brian Rachlin
Huntingdon Valley, Pa
I prefer email's not PM's rachlin@comcast.net

1960 62 Series Conv with Factory Tri Power
1970 DeVille Conv
1970 Eldo
1970 Caribu (?) "The Cadmino"
1973 Eldorado Conv Pace Car
1976 Eldorado Conv
1980 Eldorado H & E Conv
1993 Allante with Hardtop (X2)
2008 DTS
2012 CTS Coupe
2017 XT
1956 Thunderbird
1966 Olds Toronado

Dan LeBlanc

That's how I checked mine - GPS app. Tried it in the Deville and it was spot on.

I did pick up a cruise control unit on ebay. Put in the transducer and now I have working cruise again.

Tweaked the timing with a vacuum gage today also as I noticed a little pinging on hard acceleration. Idle is even better and, well, the car handily lays down some black marks now.
Dan LeBlanc
1977 Lincoln Continental Town Car

35-709

Surprised that yours would be off that much.  You may need both a different driven gear as well as a different drive gear to make up that much of an error.  Depends though on what you have in there now --- the gear color will tell you.

http://www.tciauto.com/tc/speedometer-gear-calculator/    Scroll down for TH 400 gears.

Sometimes the speedometer itself can be the problem, the speedo needle actually moves because of a magnetic field and sometimes the magnet can be too weak.

Here is another solution --- https://transmissioncenter.net/shop/ratio-adapter-other-ratios-available-call-jim-1-888-2012066/   Once at this site don't click on GM, just scroll down.

Good luck, Geoff
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible - Sold
1950 Packard 2dr. Club Sedan
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

The Tassie Devil(le)

Ha ha, you have found out why everyone in the old days always thought they could easily break the magic 100 mph mark.

It wasn't till modern cars got Computer controls that the speedos' became more correct, and even more-so these days with the Electronic Fuel Consumption figures and Cruise Controls.

The old cars always had a 10% over rating n the Speedos' and this was an agreement with the car makers, so that they never got it the wrong way, and therefore it was not the fault of the makers if someone got booked for speeding.

Tyres make a big difference, as in rolling diameter can increase, or decrease the speedo reading.

One way to really check your own speedo and its' gearing, is to drive the car over a measured distance, and see what the odometer is reading.   If it is close, then it is a simple matter of adjusting the hairspring in the speedo head, and or re-gearing the cable.

I adjust my own, using a battery drill, a corrected speedo, and the vehicle one, but it is a bench job.

Bruce. >:D

'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

76eldo

Could also have a rebuilt or exchanged transmission in it with the incorrect gear.
Brian Rachlin
Huntingdon Valley, Pa
I prefer email's not PM's rachlin@comcast.net

1960 62 Series Conv with Factory Tri Power
1970 DeVille Conv
1970 Eldo
1970 Caribu (?) "The Cadmino"
1973 Eldorado Conv Pace Car
1976 Eldorado Conv
1980 Eldorado H & E Conv
1993 Allante with Hardtop (X2)
2008 DTS
2012 CTS Coupe
2017 XT
1956 Thunderbird
1966 Olds Toronado

Marcel75

Mine (Eldo 70) indicates 2-3 km/h less than actual value. Calibrated with digital speed indicators you can see along european roads.

bcroe

#7
There are a lot of ways to check a speedometer/odometer.  Every "new" used car
here gets a precise check.  Then pull the trans gears, a little calculation, and new
cable drive gears put it right on.  Some try to just look up the gears their car should
have, but that method is quite error prone.  Bruce Roe

Scot Minesinger

Both my 1970 Cadillacs, one with rebuilt everything no cruise, one all original with cruise (replaced transducer) are right on the money accurate speedometers as checked by GPS and radar.

Yes the 1970 Cadillac will lay rubber, I try to never do it because it is not good for the car.  One time I was in a hurry and hit the gas just a little too hard, that is how I know. 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Dan LeBlanc

I'm siding with Brian. Transmission could have been replaced and the wrong gear installed. Gear could have failed and the wrong gear installed. Who knows? A lot can happen in 46 years.

So, it sounds like the gears are not Cadillac specific and are standard GM parts bin stuff.

And no, I'm not one for laying down black marks either. I just gave it a couple of punches yesterday just to check to see if the pinging was solved and I got a surprise.
Dan LeBlanc
1977 Lincoln Continental Town Car

bill06447

Quote from: Dan LeBlanc on November 13, 2016, 09:26:21 PM
That's how I checked mine - GPS app. Tried it in the Deville and it was spot on.

I did pick up a cruise control unit on ebay. Put in the transducer and now I have working cruise again.
Glad it worked out for you. Don't forget to leave positive feedback ;)


Bill

bcroe

Quote from: Dan LeBlanc

So, it sounds like the gears are not Cadillac specific and are standard GM parts bin stuff.

All TH400 speedo gears interchange.  TH425 interchange Toro/Eldo, not
many options there.  Bruce Roe

76eldo

When I am in the mood I smoke the tires on every car I own.
If something breaks it was bound to break soon anyway.
I'm 59 on the outside but still 18 on the inside.

Brian
Brian Rachlin
Huntingdon Valley, Pa
I prefer email's not PM's rachlin@comcast.net

1960 62 Series Conv with Factory Tri Power
1970 DeVille Conv
1970 Eldo
1970 Caribu (?) "The Cadmino"
1973 Eldorado Conv Pace Car
1976 Eldorado Conv
1980 Eldorado H & E Conv
1993 Allante with Hardtop (X2)
2008 DTS
2012 CTS Coupe
2017 XT
1956 Thunderbird
1966 Olds Toronado

Jeff Rosansky CLC #28373

Yep. Life'S too short to not have fun.
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

The Tassie Devil(le)

You should witness the smoke that comes of the front wheels of a '72 Eldorado.

The smoke slowly comes out from the side of the front fenders, and slowly curls up and over the top of the fenders, and as one is in a forward motion, it envelops the side of the windscreen, and up and over the roof.

The best sight is seeing the eyes of the person in the car next to you looking in disbelief on seeing the front wheels of this 40-plus year old car making smoke from the front wheels.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Dan LeBlanc

Quote from: bill06447 on November 14, 2016, 08:07:04 PM
Glad it worked out for you. Don't forget to leave positive feedback ;)


Bill

Of course I did ;)
Dan LeBlanc
1977 Lincoln Continental Town Car

Dan LeBlanc

Quote from: bcroe on November 14, 2016, 08:27:08 PM
All TH400 speedo gears interchange.  TH425 interchange Toro/Eldo, not
many options there.  Bruce Roe

Thanks Bruce.  That's what I needed to know.  Seems like an easy enough job
Dan LeBlanc
1977 Lincoln Continental Town Car

bcroe

The insert holding the driven gear, that the cable attaches to, comes in
2 versions.  One is for holding the smaller diameter driven gears closer
to the drive gear, the range of the number of teeth is cast into each.  I
didn't realize that when I put my 1st TH400 in my 68 Cutlass (after
blowing up the TH300), the gear was so far away it didn't touch.  So I
turned the insert around and it worked.  Now that I do my own TH400s,
all the gears plus some external gear boxes are on the shelf. 

Setting the speedo gears in daughters TH350, I couldn't get exactly
the ratio I wanted.  I set my speedometers a lot more accurately than
most people, engineers want super accuracy.  So I made up a gear
from a TH400 to go in the 350.  When I had to have a shop swap
the trans (I don't do TH350s), I informed them that they HAD TO
TRANSFER OVER my speedo gears.  They did.  Bruce

Dan LeBlanc

I'm starting to think the issue may be in the speedometer head itself.  It's not off by a percentage, it's off by a constant value.  If it reads 30, I'm doing 40.  If it reads 60, I'm doing 70.  I can lock in the cruise at 25 (should be 35).  So, I think the spinning of the cable is correct, the head is off.  I wish I had a known good one to swap out and compare.
Dan LeBlanc
1977 Lincoln Continental Town Car

bcroe

Quote from: Dan LeBlancI'm starting to think the issue may be in the speedometer head itself.  It's not off by a percentage, it's off by a constant value.  If it reads 30, I'm doing 40.  If it reads 60, I'm doing 70.  I can lock in the cruise at 25 (should be 35).  So, I think the spinning of the cable is correct, the head is off.  I wish I had a known good one to swap out and compare.   

The needle shaft hairspring has an anchor that can be adjusted, shifting all readings some
mph number.   Note that none of the hairspring turns should touch.  its best if the odometer
and speedometer are in agreement.  I have used a drill press to drive the cable, switching
belt positions to change speeds.  I had to reverse the motor, and the speedo head needs to
be properly positioned.  A variable speed drill isn't stable enough.  Use a stopwatch to check
how many seconds move the odometer a mile; divide that into 3600 to get cable MPH. 
Bruce Roe