I installed a tach a few weeks ago and have been wondering about the rpms at a given speed under different loads.<br>For example; at 70 mph on the flat it reads about 1800 rpm, at 70 pulling a long grade full throttle it reads about 2300 rpm, and at 70 going downhill with no loud pedal it reads about 1400 rpm. I realize that an auto is not like a manual which provides a direct mechanical connection, unless the clutch is slipping, but something doesn't seem right! What do you guys think?<br>Thanks, Chris<br>

Sounds about right depending on the age/ condition of your trans. Before my rebuild 70 on the flats was 1900- 2000. Now it is 1700. Try 55 in 3rd, it should be about 2000.<br><br>Pulling a load will depend on you trans and fuel settings. <br><br>Jay

I call that 'flare'... I know it's probably not the right term but it's a good word picture for me...<br>You are seeing the inefficiency in the TC, and to some degree, tranny slippage as well. The all contribute to that excessive RPM flare, both on pulling hard and on deceleration.<br><br>My RPM's are darn near identical to jbolts. He and I are both running nearly dead on identical systems.<br>Now, my truck starts pulling forward noticably at 1000 RPM, by 1400 RPM it is definitely pulling harder and by 1600 RPM she's pretty much holding it all.<br>To confirm your observations though, I noticed the same 'flare' as you did with my stock tranny system. It only had 70K Km on it too. So it certainly wasn't due to wear and old age, just inefficiency in the TC, Tranny, VB.<br><br>I'd like to suggest you do one more thing though.... put in a tranny temp guage..... and be prepared to get a surprise...<br>You might be shocked at the temps you're generating.<br>I know I was.<br><br>bob.

Thanks for the replies Jbolt and Bushy. I've been suspecting some slippage. The previous owner towed a 5th wheel and the &quot;brick&quot; now has 98K miles on her. When I'm pulling those long uphill grades at 70 mph, unloaded, that's usually all she has! The egt gets kinda high too! I need to do a valve adj. and bump up the timing also.<br>I have the trans temp gauge, I just haven't finished installing it. I'm not to sure where to install the temp sensor, I know there's some debate on that. What is the best place for the sensor?<br>Thanks guys, Chris

Hey Chris,<br><br>Tranny sounds typical for the year/ age.<br><br>As for the temp sensor I put mine in the cross over line. My truck came with the super duty auxiliary cooler under the bed behind the fuel tank. The cross over line was split to add in this cooler. I used a brass tee with barbed hose fittings and inserted that into the rubber hose connected to the transmission outlet side going the cooler. <br><br>The temp sensor in the pan will tell you what the fluid temp is going into the trans after it has passed through all the coolers whereas in the outlet line it will tell you what is coming out of the hottest part of the transmission. That’s what I want to know. <br><br>Jay<br>

Jay,<br>Thanks for the reassurance that the trans is probably OK.<br><br>I'm not to sure what's a crossover line, but I have old Power Wagon and Scout manuals that will remind me which line is hot. Is that aux. cooler thermostatically controlled? I've been wondering if one would be beneficial for a truck used mostly for driving to work.<br><br>Chris<br>

Chris,<br><br>The aux cooler was part of a factory tow package. I was told these were added at the dealers. They do have fans. Mine rarely came on unllsess I was pushing the truck hard with a load or sometimes offroading. <br><br>Strangely enough as I started to increase the HP the transmission temps fell some. Never could figure this out. I will tell you the upgraded tranny in my truck is outa this world. It is amazing how much power is lost through the stock tranny.<br><br>Do yourself a favor and get a factory service manual. About $65 from www.techauthority.daimlerchrysler.com/ <br><br>Worth every penny and then some.<br><br>Jay

Jay,<br>Yep, I know I need to get the manual- thanks for the reminder! How about the Powertrain-5.9L Cummins Diagnostic Manual? Is it a must have?<br>Chris

<br> How about the Powertrain-5.9L Cummins Diagnostic Manual? Is it a must have?<br><br><br>This booklet is directed toward diagnosing fault codes using a DBII scan tool. It's not essential but it does have some value. <br><br>Jay