How do I go about diagnosing a speedometer failure on an old Ford LN 8000. We have two of these trucks on the farm and neither one has a working speedometer. I didn't know they weren't working until yesterday when I had to use one of them. Then the regular driver is like "yeah, it hasn't worked for awhile, it doesn't work in the other truck either". Ok. I also didn't find out that governor had crapped the bed until I fired it up as well. Was never told about that issue either and it's apparently been a month or more since it started acting up. Got governor replaced easily enough, but I'm not sure where to start for sorting out the speedometer. Both trucks have the same drive train, 7.8L Ford engines, Allison MT653 transmissions. I'm not sure about rear end gears, but I can figure that out easy enough when I get back to farm this afternoon. Any insight would be appreciated as I'm not too sure where to start, what the common failure point to look for is (if any), etc.
My first thought would be that there is an inductive speed sensor at the rear of the transmission, The ones most of the truck manufacturers used were prone to failure.
How old is it? Does it have a mechanical drive. I know the old Louisvilles used to break cables. My father used to keep extras on hand.
I used to drive Louisville Fords and I'm pretty sure the speedos were cable driven. If I remember right, you remove two screws at the top of the speedo panel and the panel will then fold towards you, it's hinged at the bottom. You should be able to tell right away if it's cable
Rutt Roh. I had an 1986 f-150 and stopped in at the Ford dealer (they also did the big trucks) The gear at the back of the tranny shredded. The parts guy pulls out a thick book, looked like a NYC phone book, plops it down, and says "We won't get this right the first time". Looked up rear end ratio, serial numbers, etc. Ordered them in, and yup....not right, took maybe 3 trys.
If its not a simple fix (sender unit on rear of transmission) or a obvious cable disconnect- I would just put in a GPS aftermarket one. We have one in one of the cranes, and its accurate enough for no faster than we travel. Which is probably the same for you with a 7.8 and allison, you aren't touching a 65mph speed limit....
If you are wanting to check the cable drive (if its cable drive), I have built a temporary drive, to drive the transmission end off a drill, to see if the gauge, cable, or drive gear on the transmission was the problem. I was using it on a tach drive, but its the same deal, turn it on one end and see if it turns on the other.
That’s what we did, works quite well. Used it today in our F7000 (6.6l with Allison). I tried a used cable, lube etc but just couldn’t get the mechanical drive off back of tranny to work well. Put the gps sensor on the dash, near windshield…
Neither our 7.8 or 6.6 (current) are able to exceed 65mph…the gps speedometer actually works really well, there is a 30 second countdown while it locks onto satellites.
Michigan truck max speed limit is 65mph. I don't think either of these trucks can achieve that loaded. I used my phone with a GPS speedometer app the other day and it works quite well. It just eats my battery fast if I run for very long with it on. I mostly want it fixed as we pass through a lot of 25-35-45mph speed zones that are heavily patrolled when we are hauling. I don't need one of our part time drivers getting pulled over for something stupid like 5mph over in a reduced speed zone. I'll look into the GPS units this weekend.
I think ours was about $90 (Canadian). It does seem to read a bit high…my phone with (GPS tracks) says 97, speedometer will read around 100 km/h. Let’s just say I have no need to worry about a speeding ticket! We also have heavy speed enforcement & CVSE here so really nice to have a working speedometer. My ‘good’ used speedometer cable off my parts truck (obviously) not ‘good’ just resulted in a fluttering speedometer needle…
I just use the RPM's and the gears I'm in once I figure it out from looking at the phone of what correlates to what. Owned this dump truck two years, never worked since day one.
I do that with our Volvo semi. I know 5th gear is 25mph, 7th will run 53-55mph. Easy enough for a manual transmission. These automatics get driven by a lot of different part time and full time people for us and trying to train each one would be a bigger nightmare than fixing the speedometer. The one truck is getting serviced tomorrow morning so I'll have a chance to get an idea of what's going on.
Then a GPS unit from Autometer it is. I would just make "L" bracket and mount it on the dash like I did my Volvo.
The odometer stopped working in my truck, but the speedometer continued to work so I recently had JJT Speedometer repair the speedometer in my KW for 1/2 the price of a new one. Peterbilt Speedometer Repair -JJT Speedometer Specializing in semi truck speedometer and odometer repair. www.jjtspeedometer.com They were great to deal with: price, communication and turnaround time. So, if you find that it's the head unit, I would highly recommend them to fix it for you.