Instrument Cluster Replacement

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mudshark

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Nov 15, 2014
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Hi All
I have been having trouble with the "service engine soon" light on the bus. I replaced both speed sensors on the transmission and it helped but it still comes on every two or three trips as we explore the country. The speedometer goes to 45 mph as soon as the engine starts so it is my belief that it is confusing the on board computer causing it to post the service light. My code reader finds the same error code each time, "PO720".

In Arizona RV Salvage I found an instrument cluster. My question is what am I to expect from swapping out the instrument clusters? What should I look for, be aware of? How much trouble will I have? Problems that can occur? I am a bit hesitant to begin. We will be hunkering down for the winter so the bus will be stopped for a couple of months. Any suggestions?
 
Why would you replace the entire instrument cluster to repair a "service engine light"?.. that's like replacing the rear differential because you have a flat tire.>>>Dan
 
Sort of a shotgun approach, I guess. Replace enough things and maybe one of them will fix the problem. I'm very skeptical, though. I don't see what on the dash cluster would cause (or fix) the P0720 code, but I'm not expert of that.
If the Powertrain Computer isn't getting the speed sensor signal, or the signals are out of range, that problem needs to be isolated and fixed.  I suppose it is conceivable the computer itself is bad, but you don't get a new computer module with the dash cluster.  Since new speed sensors didn't resolve the problem, I am inclined to think this is a problem with the wiring harness that feeds the signal to the computer.  Poor quality connections there are fairly common, especially on older vehicles.
 
Hi Guys
The clue I am following is the fact that the speedometer reads 45 mph while the bus is standing still. It reads 45 until the bus speed gets to 50 mph. From there, it matches the RV GPS. Since the error message always shows up when the bus is accelerating from a stand still, I suspect the speedometer is feeding bad info to the on board computer. I will take any info or advice before I start. Keep the comments coming...I accept any and all help!
 
You haven't replaced the VSS on the diff, right? I think that'd be cheaper and easier than the cluster, and it's a somewhat common problem.
 
I didn't see what chassis you have.If Ford,the speedo sender could be on the trans,in the tailhousing.On GM the speedo signal is processed in the speedo,and sent to the engine computer,sender also in tailhousing.
 
Talking about the speed sensor, is it possible some dirt has entered the sensor or connection, and it is now giving faulty readings? May be worth inspecting.


Bill
 
Our bus is on the Ford F53 chassis. I did replace both speed sensors on the transmission. There is one on the tail shaft and one up front, just behind the bell housing. It did improve things but did not solve the problem. Instead of the light coming on every time we moved now it may come on every two, three or four moves. So the problem exists. I will check to see if there is a sensor on the differential.

I was careful putting the new sensors in. There should not have been any crud pushed in. The front sensor I could get right on top of through the doghouse. No problem there. The wire connections were both clean and tight before and after replacements.

The diagnostic list for the  PO720 code ranges from simple to complex including replacing the computer module. I am narrowing down the simple stuff. I still think it is the speedometer. If it is sending the module a speed of 45 mph while the bus is just starting to roll it could be confusing it.
What happens at this point is the bus gets rough running and bogs down and is slow to roll, almost like the trans is in a higher gear. If I stop the bus, shut off the ignition and restart, things straighten out. Then the light stays on until I reset it with the code reader.
 
Most speed-o-meters get their signal from the power train computer, not the other way around.  I would inspect the wiring connections on the computer for moisture/corrosion before replacing parts.  I had a 1985 Chrysler that would go nuts during any rain event, even the dealer could not figure it out.  Turns out the moisture would collect in the cowl and when I made a right turn, if enough had collected it would run through a poorly sealed seam.  This water would drop down on the computer connection and all He!! would break loose.  The dash would go nuts and the engine would start missing, though the symptoms were never the same each time it happened.  Codes would be different every time.  Just shear dumb luck I found it.  The connection looked ok until I unplugged it and then I could see the moisture when I held the light just right.  Plenty of Dow Corning DC-4 cured that problem as I could never find the bad seam.  Kept the car for 17 years until it blew the third head gasket.
 
There are THREE speed sensors. Two on the trans and one on the diff (on a 2000 F53). The one on the diff has been an issue for many. Without a proper speed signal (like you are seeing on the dash) the trans can't shift correctly.

You could verify it's not the cluster by unplugging it. It will run without it.
 
Might try unplugging the speed sensor on the diff.  If the speedo drops to 0, it's not the speedo, it's the sensor.

I found some interesting reading over on the Ford Truck Enthusiasts forum.  I googled "p0720 on a Ford F53".
 
Also realize that the sensors changed around from 1999 to 2004. 1999 Had no TSS, and 2004 has no sensor on the diff. I think they have been the same since 2004 though. As you read other threads, keep this in mind...
 
And....

There is a TSB out there regarding grounding the PCM around that model year. Forgot about until I was googling a bit. Grounding TSB

There was also a guy that had a bad alternator inverter causing issues.

Here's a post of his. It was his inverter making his speedo read 40 MPH while stopped (sound familiar?).  He jumped in to the middle of another thread. A couple other folks with issues chimed in too...
 
Thanks Guys for the info!
Man, the links sent get deep into the electrical/computer side of things. Things that are way past my limited knowledge of such things. I would probably blow things up if I tried some of the things mentioned. I will try finding and disconnecting the sensor at the differential to see if that helps. I might just have to live with the problem. Thankfully I can return the cluster for a refund within 30 days.
 
OK, I disconnected the sensor at the differential and turned on the ignition. No change, speedometer still reads 45 mpg without engine running and stationary. So that's not it.

If forum consensus is the speedometer is not the problem, maybe I will return it, get my money back and live with the problem.
 
Ha Ha Ha!!
Gary, I have been told that on several occasions...not bus related!!
 
The Speedo in my 2004 Chevy Silverado did that (stuck on 45) a few months ago.  I took it to my local independent garage. The owner said yep, happens a lot., it's the stepper motor in the Speedo. $70 and a couple hours later it was fixed.
 
mudshark said:
Hi All
I have been having trouble with the "service engine soon" light on the bus. I replaced both speed sensors on the transmission and it helped but it still comes on every two or three trips as we explore the country. The speedometer goes to 45 mph as soon as the engine starts so it is my belief that it is confusing the on board computer causing it to post the service light. My code reader finds the same error code each time, "PO720".

In Arizona RV Salvage I found an instrument cluster. My question is what am I to expect from swapping out the instrument clusters? What should I look for, be aware of? How much trouble will I have? Problems that can occur? I am a bit hesitant to begin. We will be hunkering down for the winter so the bus will be stopped for a couple of months. Any suggestions?
Click to expand...

A couple Items to check,
1) Biggest thing to check when electrical issues arise is the ground connections. Many different areas to clean, people have found a bad ground (harness to frame) causes the PCM to search somewhere to ground.
2) While cleaning grounds, check the wiring harness for pinch points or chaffing. Ford had problems with the harness early on.
 
Hi All
Looks like I will be returning the speedo next week. We found a place to winter here in Arizona so I will have 3 months to chase grounds while the bus is stationary. If I find it has to be the speedo, at least I will know where it is.
 
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