Anybody have any ideas on replacing a broke speedometer in a 93 7.3l diesel? No cable to be found on the tranny though the shop manual says there should be !just a wiring harness to the actual guage , but i can't find a sensor on the transmission.

 

hankster,

I'm referencing a 1992 Ford manual on CD. It shows the anti-lock brake sensor located in the differential housing as what gives your speedometer it's signal. I quote, "The electric analog speedometer receives it's input from the anti-lock brake sensor."
You might want to pull the sensor and clean the magnetic end of it. I've heard it's not particularly expensive. Also might want to trace out the wiring all the way back to the speedometer to make sure it's in good condition.

These documents continually refer to a LCD (liquid crystal display) which displays vehicle speed and the odometer... I think.... I've not found direct reference to an "analog" type speedometer display but why would it say "electric analog" in the previous paragraph if that isn't what it meant? At any rate, I've got detailed instructions on how to test the speedometer if this is what you're looking for......R.A.

 

Dieselzen . Thank s for the imfo ! i live "out in the country" and have no close support or parts stores near by . I have removed that sensor and cleaned it but no improvement . My abs light is on also . When the speedometer went out , it acted like the old cable units use to ie...jumped back and forth for a month or two , then quit altogether just like a broken cable . I will replace the sensor and if that doesn't do it , it must be the actual gear head in the guage ?............hankster

 

hankster,
When you cleaned the end of the abs sensor, did you get any metal filings off it's magnetic end?
The reason I ask is not too long ago someone else posted about problems with the speedometer/abs light and metal filings in the differential gear lube. There is not only a magnet in the abs sensor but also magnets in an excitor ring which turns with the ring gear. The magnets on the excitor ring can get cluttered with filings also. You're best bet is to replace the diff. gear lube and clean the excitor ring magnets while you got the cover off (gotta make sure you use the limited slip additive if you have a LS diff. upon re-fill).

Also on testing whether the sensor and wiring between speedometer and sensor are good:

The connector which attaches to the speedometer should be a 12-pin connector but only 7 of the spaces are used. Looking at the instrument panel FROM THE BACK, Pin 1 is in the top left hand corner(that is, top, passenger side), Pin 6 is in the top right hand corner(that is, top driver's side)Pin 7 is in the bottom left hand corner( that is, bottom passenger side), and P 12 is in the bottom right hand corner(that is, bottom driver's side). Here is a list of the Pin numbers, (supposed) colors of the wires, and what they do:

Pin 1- Lt. Green w/yellow stripe; goes to the pos. of the battery.

Pin 2- Pink w/orange stripe; goes to ground.

Pin 3- White w/purple stripe; diagram says, "Run". I think it is hot when the ignition is on.

Pin 4- Red w/pink stripe; diagram says, "Speed In".

Pin 5- Lt. Green w/black stripe; diagram says, "Speed In Return".

Pin 6- not used.

Pin 7- Gray w/black stripe; diagram says "Speed Output".

Pin 8 (or 9, very crappy digital diagram)- Lt. Blue w/yellow stripe; this is either not used or is the "Dealer Mode Enable"

Pin 9- read pin 8 description.

Pin 10- not used.

Pin 11- not used.

Pin 12- not used.

To test ABS sensor and wiring, connect a DVOM to Pin 4 (speed in, pos.) and Pin 5 (speed in, neg.). Put DVOM in 20 volts AC range. Bring vehicle up to speed of 30 mph. You should see a steady increase in DVOM reading from 0 volts AC to 3.5 volts AC.

You should also check for:
A. 12 volts to Pin 1 as this is to the battery.
B. Less than 1 ohm resistance to the ground on Pin 2.
C. 12 volts to Pin 3 with ignition on.

Hope this stuff helps, R.A.

 

Dieselzen , No metal filings thank goodness . If there had been , it would indicate to me that there were other problems . I'll drive to town tomorrow when businesses are open again and purchase a new sensor . What do you think about the erratic needle before it broke ? Would an electronic part do that ? Makes me think the gear head in the guage is busted , but other people tell me that almost never happens . Thanks for the wiring schematic , i'll try that if the new sensor doesn't fix it . Just replaced the glow plugs today with delco . Found three different kinds from letting other people work on it in the past !....hankster

 

Dieselzen , don't know if you got my last message .Got a note to finish my message field whatever that is . No metal filings on sensor , thats the good news. Will buy a new sensor tomorrow and if that doesn't work , i'll try the using the wiring schematic you posted . Can't thank you enough for the help though ..................hankster

 

hankster,
Here are the causes of a jumpy analog needle (again, from the 1992 Ford manual on CD):

1. Damaged, missing, or bent teeth.
2. Metal chips on speed sensor (RABS sensor).
3. Open or shorted sensor.
4. Improper clearance between sensor and excitor ring.

Solutions:
1. I initially thought it was refering to something inside the speedometer itself but after reading some stuff, now think they're talking about the excitor ring as when you pull the sensor out of the diff., you're supposed to turn the axle while looking at the excitor ring through the hole and check for dented, damaged, or broken teeth.

2. Well, I guess you've eliminated that cause.

3. It may have taken awhile for the sensor (or wiring) to completely open the circuit. This would explain why the needle fluctuated.

4. The proper gap between the sensor and the excitor ring is .005" to .045" (.127mm to 1.14mm). To my way of thinking, metal filings on the excitor ring could cause the gap between sensor and ring to be off.

Note 1: If the excitor ring is removed it can not be re-used. A new one must be installed.

Note 2: Here is the procedure for installing the RABS sensor:

1. Clean mounting axle surface. Use care to prevent dirt from entering the axle housing.
2. Inspect and clean the magnetized center pole piece to ensure it is free from loose metal particles which could cause erratic system operation. Inspect the sensor O-ring for damage and replace if necessary.
3. Lightly lubricate sensor O-ring with motor oil, align the sensor bolt hole, and install. Do not apply force to the plastic sensor connector. The sensor flange should slide to the mounting surface. This will make sure the air gap setting is between .005" to .045" (.127mm to 1.14mm).
4. Install the hold down bolt and tighten to 25 to 30ftlbs (34 to 40N-m).
5. Inspect the blue sensor connector seal and replace if missing or damaged. Push the connector on the sensor.

Hope this helps, R.A.

 

Hi there,
I know this post is a little late but maybe it will help someone else it's a pretty common problem. We rebuild the psom (speedometer) for the F series trucks and thru the years of diagnosing the fluctuating needle problem what we have seen is this: If the speedometer ONLY bounces at low speeds (under 40) is is usually the PSOM module. If it bounces at high speeds (above 45mph) it is usually the exciter ring in the diff. I believe it is because the exciter ring becomes magnitized & at the higher speeds the vehicle speed (ABS) sensor cannot "see" the teeth in the exciter ring. Usually there is NO visible damage to the ring at all. If your ABS light is not on, don't bother replacing the speed (ABS) sensor. We have repaired many of these trucks and can usually pinpoint it just buy observing what the vehicle is doing. If anyone has any questions on diagnosing it give us a call or email us sales(at)dashusa.com 248 835 1332 www.dashusa.com
Hope that helps! Steve