speedometer wierdness

[foresterdj]

Never noticed before, but this week speedometer has been acting wierd. I was up for an ice fishing trip to Tiber reservoir, weather not real cold, single digits in a.m. (As a side note, plug truck in for an hour before starting and it fires off like summer.) After warming up enought to let high idle kick off, I head down the highway toward the lake and tach plus gear (4th in the E4OD) would indicate ~ 65 mph, but speedometer reading almost 75mph. I check with GPS and indeed I am going 65. In the course of 10-15 miles the speedometer gradually slows to where it normally would be, or if close to that I would stop and start moving again and then the speedometer reads correctly. For my truck in 4th at ~ 1950 rpm speed is 65. Truck did this the last 4 days. On trips back to town, even after sitting all day at the lake, speedometer read correctly.

My guess, open to opinion of those on this site who know way more about these trucks than me, is that speed sensor in rear differential (I think that is what it is) is reading incorrectly due to cold rear end, which improves as rear differential and gear lube in it warm up? Your thoughts.

I have been meaning to change lube in rear differential anyway, and replace with fresh fully synthetic, so this may be the excuse to do so, just to have a look inside.

 

[79jasper]

Temps shouldn't make a difference. You can remove and clean the vss. Other than that, just clean the connections.

 

[LCAM-01XA]

Temps shouldn't make a difference. You can remove and clean the vss. Other than that, just clean the connections.

Except on a '91 the speedometer is cable driven, and cleaning the VSS on the axle will accomplish absolutely nothing.

Foresterdj, I've noticed a similar behavior from my '90 speedometer as well - it starts off reading a few mph higher than what GPS shows, then over the course of about two hours of driving it will slow down to a few mph below the GPS reading. I'd say it goes from +3mph to -3mph from the actual speed, for an overall swing of 6mph. There is no needle bouncing, readings are very stable (suggests cable spins nice and easy). I wouldn't blame it on any temperature differential, because my truck gets the cab real hot withing 10 minutes from starting the engine, and my transmission temperatures tend to be rock-solid at about 150F - meaning over the 2 hours or more it takes the speedometer to change its readings, both the speedometer and the cable that drives it see constant temperatures (albeit quite different from one another).

So, I'd like to hear some suggestions as well...

 

[79jasper]

Except on a '91 the speedometer is cable driven, and cleaning the VSS on the axle will accomplish absolutely nothing.

Even with the E40D????
Or is that starting in 92 only that use vss for speed?

For being cable driven, I would pull the cable from the casing and clean then relube everything.

 

[LCAM-01XA]

On a brick there are actually 3 vehicle speed readouts - one is the mechanical speedometer cable drive that lives on the trans or transfer case, coupled to it and is one VSS which is used for E4OD and cruise, and on the axle is a second VSS which is only needed for the RABS. The mechanical drive and the first VSS are one assembly and get replaced as a unit, but say a ZF5 truck w/o cruise will only use the cable drive portion of it. For '92 Ford eliminated the need for mechanical cable drive and ditched the VSS coupled to it, and retasked the VSS on the axle to feed signal to the RABS, the E4OD, the PSOM, and the cruise.

As for the cable - sticky cable will cause twitchy speedometer needle, and at least my truck does not show that symptom. A sticky cable wouldn't explain my gradual decrease in indicated speed - it needs good 2 hours of non-stop driving to cause the speedo to change its readings, I'd think a fast-turning cable would have warmed up and stabilized much sooner than that. My cable is also well lubricated and turns very smooth and easy...

 

[79jasper]

Yeah it's pretty weird. If you've taken a cluster apart you'll know what I'm talking about, inside the needle is actually on a wound spring, I could see high heat causing a difference there, But very unlikely. Or the gears up in there needing lubed, causing it to read under.

Now on another note that EVERYONE needs to know, GPS's are NOT always 100% accurate. Think of it like a cell phone signal.
If it is indeed reading off and not the gps, blame that circular thing with the lights hovering above you. :roflmao

How much of a difference in speed is there at a given rpm with the torque converter locked vs unlocked?

 

[Jake_IN]

My 94 exploder does the same thing. Pretty sure i just need to lube the cable because when its doing it i can hear the cable chattering. On my exploder it will be accurate and then all the sudden it jumps up 10-15 mph and starts chattering.

 

[79jasper]

Just thinking outside the box. As in if some torque converter slip might've been able to cause it.

It's hard to explain, but I can see how a slightly binding cable could cause it to read high.

Running two should give a pretty accurate reading. Lol But if something caused one to be off, what's the chances both would be off?
I've read that they're bad for being inaccurate while driving with quick changes in elevation.

 

[Wicked97]

The torque converter has nothing to do with speedometer operation.

Its more than likely a guage issue. My Taurus does this same thing.

If your speedometer is smooth then the cable is good. The vss on the other hand with go to **** without seeing the needle move around but that usually only drives you nuts when you use the cruise.

 

[79jasper]

For those of you that haven't seen inside the speedometer, here it is. Hopefully this will help everyone conjure some explanation on what causes it to be off.

This spring keeps the speedometer needle from bouncing and doing crazy stuff. Also returns the needle to zero.

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The circular thing in the middle has a magnet that goes across the diameter of it. The unit is attached to above said spring.

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This part sticking out is what's actually connected to the speedometer cable. There's one on each side. When it spins, it will "catch and release" on the magnets. In turn moving the speedometer.

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Maybe that will help, maybe it won't.
I would disassemble it further if more pics are needed/wanted.

 

[foresterdj]

O.K., all interesting and informative, so if I follow all these comments, on my truck, a 1991:
1. There is a mechanical cable from transfer case to the actual speedometer to drive it
2. There is a VSS from this same location providing speed info to the E4OD transmission, and
3. There is a VSS from the rear differential providing input to the RABS (I am not sure what that is, but gather it is not the speedometer needle in dash)

Thus, it would seem that if the speedometer in dash is the only component acting wierd (it is, the truck shifts fine, feels normal doing so, & the tach reads correctly for the gear in use at the speed measured by the GPS), then some part of the mechanical cable and the speedometer head themselves must be the source of the wierdness. I would also think it reasonable, since there is no chattering noises and the speedometer operation seems steady and smooth (though apparently the spring is there it moderate any minor jumpiness), that the cable is not to blame. I guess that would leave the speedometer head as the source? (Or, obviously in the transfer case end of the cable there is something driving the spin in the cable, could that play a roll?)

Anyway, it is wierd, never once noticed this before and I have paid particular attention to tach, speedometer, and pyro this last year after installing new baby moose and BB injectors. (As a side note, compairing the mixed driving for the 15 tanks of fuel prior to the new IP and injectors with the 17 tanks after, I show a 0.3 mpg increase on average.) The speedometer behavior is that I would hit the highway, accelerate normally, (except noticing that shifts came correctly by rpm, but seeming to be late by mph), then at what felt like my normal driving speed I look and think, "No way am I going 73 -74 mph." So I check with GPS and I am going 65 mph +/- 0.4 mph per variation from GPS. Then, while driving steady on cruise I see the speedometer slowly over 10-15 miles decrease until it is reading more-or-less correctly.

 

[Jake_IN]

In general if you hear the cable making funny noises, and the speedo needle start twitching, yes, chances are cable is the issue. However, your truck is a '94 - are you sire you even have a cable? The PSOM came about in '92 for the F-series, most passenger cars also switched to electronic speedometers at about the same time - I'd be quite surprised if the Explorers still retained the cable-drive in those model years....

Yup its got a cable, and it doesn't have a digital odometer either. I don't know about the automatics though cause my Exploder has the M5OD. Heres a pic of the back of the trans....it leaks a little
Clicking the link should open the picture in a new window.

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[ifrythings]

Speedometer Head: Customer Interest Speedometer - Indicates High Speed When Cold
Article No. 92-12-9
DATE: 06-03-92
SPEEDOMETER-MECHANICAL (ANALOG)-INDICATES HIGH SPEED AT LOW AMBIENT TEMPERATURES (ABOUT 20 DEGREES F OR -7 DEGREES C OR BELOW)- VEHICLES BUILT AFTER 4/1/90
FORD: 1990-92 ******, MUSTANG, TAURUS, TEMPO
LINCOLN-MERCURY: 1990-92 SABLE, TOPAZ, TRACER
LIGHT TRUCK: 1990-91 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES 1990-92 RANGER 1991-92 EXPLORER
ISSUE: Mechanical speedometers may indicate higher than actual vehicle speed at low ambient temperatures (about 20~ F or -7~ C or below). This occurs because of insufficient lubrication at lower temperatures. The speedometer will indicate speed correctly after the vehicle warms up.
ACTION: Replace the speedometer head with a service speedometer assembly. Speedometer part numbers are identified in the Parts Block/Application Chart. Refer to the appropriate Service Manual for removal and installation details.

I would try to relube it with some graphite lube and see if it helps.

 

[79jasper]

Wow, holy crap. That's great!!!!!!!

I have an explanation for "how" it happens with more pics, but I typed it all up an ****** some fairy dust made it disappear. Ticked me off. I'll retype it tomorrow when I do a different requested write up.

LCAM- we need someone with the newer cluster to pull it apart so we can look for specific differences.