Coming back from a camping trip today, I made note of all the gauge readings and wondered how they compared to others. The outside temperature was 95 , I was on I-20 terrain very modest rolling hills. MY observations:

• GPS speed 60 MPH Speedometer speed 65 MPH
• Tachometer- 2275 RPM
• Oil Pressure-70 PSI
• Water Temperature- 180 degrees
• Transmission Oil Temperature- 190 degrees
• Voltage- between 13.5 and 14.0
• Turbo Boost- Down incline 0-1, level 3-5, up incline 7-10.

Wondered how these reading compared to others. The temperatures seem a little low to me.
Nick

Everything looks normal to me except the voltage seems a little low.

Since the transmission is partially cooled by the radiater, the temp of the engine and trans will be about the same under normal cruise.

Nick: If you are really at 65 mph and have 2275 rpm you are not in 4th gear! In my "sister" coach I can baby the accelerator and run at 1790 rpm and 62 mph and get 10.4 mpg. I suspect you are getting about 8 mpg which is what I experienced before I began driving by the tach and not the speedometer. Other readings are about in line with mine

I forgot one more item. The Cat engine book and the local Cat Heavy Dealer Service Manager tell me the turbo should read about 13-15 inches under a heavy load - that is any hill that causes a downshift or under acceleration onto the freeway. I just replaced the exhaust "elbows" that connect the exhaust manifolds to the inlet of the turbo and that fixed a low boost caused by cracking of the bellows in the exhaust risers.

Tom, I am fairly certain that the transmission is shifting into 4th but I have never been comfortable with the RPM range that I run. The engine will turn up to about 2400 to 2500 rpm in 3rd gear and shift into 4th at about 45 mph. If running at a cruise speed the transmission will stay in 4th until you slow down to about 45 and it will down shift into 3rd. The engine-transmission combination seems to want to stay in that 2200 to 2500 rpm range. That just seems too high to me. I don't think I have ever seen 15" of boost. I will monitor it close next trip to see. And yeap, your about right on the mileage. I'd love to see 10+.
Thanks for your input.
Nick

Hi Guys, I have to chime in on this one. We are 2200 miles into a 6000. mi trip. I have lots of time to watch the gauges. GPS 60 MPH Speedo says 65MPH, Tac is 1950, oil pressure 70psi, water temp says 200, trans temp cooler on flat land runs about the same in the mountains, voltage always just above 14 volt after a rebuild, turbo boost 10 at best, 0-2 on flat land. milage sucks but running 60-65 towing a jeep wrangler. BUT, having a ball and sniffing a lot of roses. will be in yellowstone tomorrow then start heading south. See Ya Tom

I have no clue, but the low boost and high cruise RPM, having had those symptoms on my AMG 6.5L/GM 4L80E tranny were caused by:

1. Low boost: Dirty air filter, which caused more throttle to hold speed.

2. The throttle position sensor wouldn't let the torque converter lock up, higher RPM.

I have no idea how the Cat/Allison combo works, so this is a WAG...

quote:

GPS 60 MPH Speedo says 65MPH,

Tom, what size tires are you running and what size came on the coach. I currently have exactly the same error ratio in the speedometer. Data tag says the coach came with 255x80x22.5 but the coach currently has 255x70x22.5 tires on it. The specs I find say this tire is about 36.75 inches in diameter and turns about 560 revs per mile. I'm going to go to about a 40" tire that will turn about 515 revs per mile. That should correct the speedometer and hopefully a slight improvement in mileage.

Rusty, did a new or clean air filter correct the converter lock up?

Nick

I don't think there is a lock up torque converter on the 4 speed Allison.

You could tell if you have a lock up torque converter if you lift and don't see an immediate RPM drop just a reduction of RPM that directly follows speed.

Thanks for the info Ed, I'll add that to the things to watch. I'm not positive but I think I have noticed that the RPM does not drop when you take your foot off the accelerator. I'll watch next time .
Nick

I know very litle about diesels. ( I had a Jetta diesel once!) when I hear about differences between RPMs and MPHs I think gear ratio. Is I possible that there could be options on differential ratios?

quote:

Originally posted by Nick Cagle:
[Rusty, did a new or clean air filter correct the converter lock up?

Nick

Changing the air filter corrected the boost (and a related EGT) issue, and partially the torque converter lockup. The final solution of TC lockup was made by adjusting the TSP (throttle position sensor).

These showed up after installation of the new engine, which, due to the new turbo, is a heavier breather than the old. I now change air filters (NAPA Gold - made by WIX) every 6K miles instead of every 12K. I'd like to install a Donaldson, but room is tight on that side of the engine.

Nick, I have 275/80 R22.5 That I put on about 6 Months ago. Tom

Sorry Nick, My coach came with 10R 22.5X-LRG Tom

Tire size, rear end ratio, and transmission all contribute to the different RPM v MPH that one sees on their coach. In the one coach is a GPS that properly shows the true MPH.

If your speedometer is off, this could explain some of it.

These coaches could have been spec'ed or have been retrofitted with different gears. Unless one goes out and jacks up the coach and spins the tires you really don't have anyway of guessing what the correct ratio is.

If you are so inclined, you can read about the procedure here.

And, as Tom and Nick have shown, the 22.5 came in different sidewall sizes.