Nobody locally seems to know what the problem is, it comes and goes as far as working, sometimes when you drive it works fine, then the next time its not accurate, it seems to hover at the 25 mile per hour mark and stay there even when driving 55 or faster. This afternoon I drove it three hours, about a half hour it maintained the correct speed of 55, all the rest of the time it hovered near 25-35 and would constantly bounce around in that speed up and down the bulk of the three hours of driving. It never quits, and then all of the sudden to goes back to reading accurately for a few minutes and then drop back to that 25-35 zone again. We've adjusted the gap in the sender and that does nothing to change it. I think the truck is a 1998 year, mechanical engine. Its done it for years, and the only change is, now its not accurate more than it is, when this whole thing started, it would drop in speed maybe 10 percent of the time and was accurate 90 percent of the time, now its accurate 10 percent and inaccurate 90 percent of the time. Any idea's as to what's wrong and how to fix it?
Any other gauges acting up? Our dump truck does that and if you thump the instrument cluster it'll all start working which makes me suspect a ground or something along those lines. Bad wire? Bad plug at sensor? Sensor itself about to shoot craps? Maybe somebody can chime in on how to test one with a meter? Junkyard
Check the charging system just to make sure you don't have an over charging issue. Over charging can mess up the sine wave produced by the magnetic pickup. Truck Shop
Yep, saw that on an FL80 with a bad alternator ground. Both tach and speedo were acting up and would do it just revving the engine. Added a ground wire from the alternator to the block and cured it. Still not sure why the mount wouldn't ground it. But I had an International with a 466 do the same thing.
No other gauges are or have acted up, everything else works fine including the tachometer. I believe we have changed the sensor for the speedometer at least once, maybe twice and it didn't change a thing. The Alternator and starter have both been off since this whole thing started and both were rebuilt, same exact thing after we put them back on. As for a bad ground, the thinking we've had is, for it to be a bad ground, far more things than just the speedometer would have to act up, but maybe we're wrong, we have cleaned up some of the grounds in the past, or at least that's what I was told a couple of the local shops did to solve this problem and whatever they did, never changed a thing. I haven't had time the last couple of years to hash it over in my shop, been too busy with other more urgent repairs, but now we're getting down to the less urgent and more annoying things to fix, this is on the list. If your really in need of a head scratcher, I also have a 359 pete 1987 model year, about the last year they made them I believe, that speedometer is never accurate, it reads far more speed than your driving and the longer you stay at a constant speed, the closer to accurate it gets, but never gets to accurate ever, it always reads about 10-40 mph faster than your driving, no shop locally can figure that one out either and I've never had the time to look at it as of yet, we just use a hand held gps to get the speed off of that and jot down the mph at a certain rpm in each gear and have that taped to the dash for reference, how's that for a redneck cure. I've been told by every shop around what its doing is impossible to have happen in that model of truck, till they drive it and see for themselves.
It won't effect other gauges when it's an over charging issue, at least from what I have experienced. Only the speedometer. PacCar trucks seem to be the worst ones for this problem. But I suppose it could effect the tack also. Truck Shop
How does the speedometer work on that model Freightliner? Do the mag pulses from the pickup go all the way to the back of the speedometer itself or do they get computerized at some point? Something tells me they are computerized because I remember the gauges sweep all the way right then left when you turn the key on. On the 1987 Pete, is it a cable drive or electronic speedometer there?
I'll have to do some checking on this, I honestly don't know off the top of my head. On the pete, also I'll have to double check, but thinking its cable drive.
Drove the freightliner today and started it several times, none of the gauges sweep when you turn the key on, and chatted with the last mechanic that worked on it, he didn't think there was any computer on the truck.
There may have been older versions out there with different gauges than I am used to. Do you have a schematic?
I did have one at one time, but we can't seem to find it. I'll have to ask if the local freightliner dealer can print me one off.