hour meter questions

Ok, first off, I did a search and found nothing on this subject, so bear with me if this has been discussed before.

The outfit I work for has got an M813A1 (converted to W/W and running 6 14.00 x 20 Michelin X radials on HEMTT rims and unflipped rear hubs) that we use for off-road cargo hauling. Usually we use it for moving our own stuff around, but lately we've started doing more commercial work with it, and usually the places we go are places where only a 6x6 truck will get the job done. (More than once I've been thankful that the USMC put lockers in our rear axles!)

Anyway, when it comes time to charge the people who hire us, we've been puzzled by the hour meter--it never seems to read what we think it should. Sometimes it seems to read half as many hours as we think it should, other times it reads almost twice as many hours as we think it should. The tachometer always seem to be accurate, however.

So here are my questions about hour meters on M813A1's:

1. does the hour meter run the entire time that the engine is running, even if the truck is parked and in neutral? It seems like it should as it is in the same gauge as the tachometer.

2. does the hour meter measure standard 60 minute hours? (rather than the 100 segment hours that I have seen before on some time clocks)

2.5 does the hour meter measure time at a constant rate or does it measure time faster or slower under certain conditions? For example, is the hour meter supposed to move 1 hour in 1 hour's time regardless of whether the engine is turning 600 rpm or 2200 rpm?

3. I can't imagine why this would be, but another M 809 series truck operator here claims that putting the big tires on his truck threw the hour meter off. Naturally the odometer would be incorrect with the super singles, but surely not the hour meter?

4. is this one of those situations where "none of them are accurate and nobody cares so get used to it"? If so, has anybody come up with a good way to measure the time that their 809 series truck gets used? We charge by the hour for cargo hauling, so accurate time keeping is sort of important to us.

5. is the hour meter mechanically driven off of the tachometer (like an odometer on a speedometer) or is it driven some other way?

Thanks for your time!

I was told the hour meter is true at 1500 RPMs. I have not verified this. I am going to be going on a trip in a few days, so I will check it out.

If it is run like an speedo, it should be able to be true at all RPMs. The Tach is cable driven like a speedo.

I can't recall where I read this, but I seem to recall 1400 RPMs = 1 minute on the clock.

Regardless of what RPM the meter is calibrated at it is slaved to engine RPM, not clock time. It is not electric. It is intended as another way to track engine usage, other than mileage, particularly for off road use and jobs that require a lot of idle time such as wreckers.

Since the tachometer hour meter does not measure clock time you would be better off using a trip log book or sheet with the starting and ending time noted or add an electric time "totalizer" hour meter and track that.

Lance

I think Bjorn covered this in a previous post. I can't remember the topic, but you might try searching under CRANETRUCK for his previous posts...or PM him.

It is rpm based for tracking engine usage, not real time. Calibrated to a specific rpm, not sure which for these.)
A lot of OTR trucks show it as 'engine mileage' rather then hours.

My suggestion for you, get a REAL hour meter ran by the 24v when the ignition is on (or have a relay setup for when the motor is running.)

Okay, thanks everybody! I've got good info to work with now.

So now I'm looking for an electric hour meter to add to our truck. I'd really like to find one that has total hours as well as one that is resettable. Can anybody point me in the right direction for finding that kind of thing?

So I ended up going with an ENM T14BG gauge, mainly because I found out that we had a brand new one laying around that everybody had forgotten about, (except one guy, of course). I ran the positive side of the gauge to a wire spliced into the switched hot side of the OFF/RUN/START switch and ran the negative side to a body ground under the dash. Now the hour meter only run when the master switch is "ON" and the OFF/RUN/START switch is in the "RUN" position. All good!

I mounted the gauge slightly to the right of the center of the dash, just above the fording knob. The way we do it, it's really the co-driver's job to record the time when we go out on a job.