Speedometer Swap

RalphG

RalphG

Well-known member
Well its done. I successfully transferred the speedometer from the 53 parts car into the 52 Merc. And it works! It is a little bouncy at lower speeds and the accuracy is a bit out even at high speeds. But maybe that is normal. I carried a Garmin hand held GPS for reference and it was only showing 50 mph when the car was showing closer to 60. I recorded some of the details and road test for future reference.
https://youtu.be/vtZJCltERDc
 

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Ralph, You violated the first rule of Parts Car Cannibalization: The parts you bought the car for are probably as bad as the parts you need to replace.
10 miles per hour is not "a bit out." You've got a speedo disaster in the making. Take that thing out again, clean & lubricate it properly, and check your speedo cable while you're at it.
 
As Alan mentioned the speedo is more effected by the cable then anything else... most likely. The wrong gears effect the accuracy. A cable gummed makes it bounce, more times then not. Easy to see, pull the cable to housing. run some gas or something through the housing then put he cable back in with lithium grease or graphite compound. Worth a try and that's ruled out;


I think you should put a motor in the 2dr 53 merc and run it :).
 
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Tinker said:
As Alan mentioned the speedo is more effected by the cable then anything else... most likely. The wrong gears effect the accuracy. A cable gummed makes it bounce, more times then not. Easy to see, pull the cable to housing. run some gas or something through the housing then put he cable back in with lithium grease or graphite compound. Worth a try and that's ruled out;


I think you should put a motor in the 2dr 53 merc and run it :).
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The 53 is a four door.
I had the cable out and well oiled already so no problems there I would think. The 53 had a standard trans while the 52 is automatic but I don't think there would be any difference in the speedometers.
 
flatheadmurre said:
If it´s bouncy the cable is the issue...a to tight bend on the cable can have the same effect.
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I'll check the routing on the cable. I had to pull it out of a retaining clip that held it to the lower steering column so I could get enough slack to thread the nut onto the speedometer. It was well oiled and runs quiet so I'm wondering if its something in the speedometer head itself that causes the bouncing. I've got an 81 GM pickup that has had a bouncing speedometer needle at low speeds for about 30 years now. I have never opened it up and it works fine at higher speeds.
 
Too late now, but some speedo's had a little oil hole on the top somewhere and a little light oil (3 ind 1) might help with the bounce.


As far as getting it to show the right speed, change the gear at the transmission end of the cable If I remember correctly, more teeth means a slower speedo, less teeth the opposite.
 
paul2748 said:
Too late now, but some speedo's had a little oil hole on the top somewhere and a little light oil (3 ind 1) might help with the bounce.


As far as getting it to show the right speed, change the gear at the transmission end of the cable If I remember correctly, more teeth means a slower speedo, less teeth the opposite.
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Yes, I noticed the hole with felt in it and added oil so that should be ok. As far as changing gears, I wouldn't even know where to start there. I'll drive it some more and see if it improves with exercise. That speedometer has likely not turned since the early 1970s. .
 
There should be gear tables around that give you the best speedometer gear based on rear axle gear ratio and tire size.
 
Check the odometer accuracy against your GPS. My experience with older speedometers is that the bushings wear and let the magnetic drum that drives the needle get out of sorts. Your speedometer gear is probably the original and correct for your car.
 
the standard will have a different tooth count on the gear than auto due to different rear gear.look up rear gearing standard vs auto
 
If you had to pull it out of a retaining clip to get enough slack to get I to go on to the speedometer then you probably have the cable in a bind. Sounds like the cable is too short and that may be the problem as it has to make a sharp bend.
 
54vicky said:
the standard will have a different tooth count on the gear than auto due to different rear gear.look up rear gearing standard vs auto
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Yes, rear axle ratio on my 52 Mercomatic car is a 3.31. The 53 with standard trans could have a 3.90 or 4.10. I have not actually crawled under the 53 and cleaned off the axle to see what it has. So that could explain the higher reading in the 52.
 
if you changed only the speedometer and cable the rear gear ratio should not matter unless you changed speedometer gear. or am i missing something?
 
marko39 said:
if you changed only the speedometer and cable the rear gear ratio should not matter unless you changed speedometer gear. or am i missing something?
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I think you are right on that as I have never heard of changing the gears in the speedometer head. Just the transmission end. And I did not change anything that far down. Just the speedometer. Kept the original 52 cable and it is not too short. Just the routing through that clip made it seem like I needed more slack to hook it up to the speedometer. Once everything was in place there is plenty of cable.
 
Cables need to be matched to the sheaths. Just a small fraction too long will jam at the speedo head, causing needle bounce. Also, speedo heads may differ, one may allow the cable sheath to screw on further than the other, resulting in the same problem of cable jamming at the speedo. This can be confirmed by slowly unscrewing the sheath head while driving on a clear road, or with wheels off the deck.
 
ford38v8 said:
Cables need to be matched to the sheaths. Just a small fraction too long will jam at the speedo head, causing needle bounce. Also, speedo heads may differ, one may allow the cable sheath to screw on further than the other, resulting in the same problem of cable jamming at the speedo. This can be confirmed by slowly unscrewing the sheath head while driving on a clear road, or with wheels off the deck.
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This cable has been in the 52 since day one as far as I know so it should be ok. I drove a few more miles this evening. The needle smooths out once it gets over 20 mph. Seems to measure the miles pretty close to accurate.
 

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Working on uploading a video of the speedometer as I drive the Merc but its taking forever. Another consideration, Speed Age magazine did a road test evaluation of the 52 Mercury and showed that the speedometer read a little high over the full range of speeds. Indicated 50 mph was actually 47 and a half mph.
 
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