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Help me spend $$ on my RD690


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I have had this truck for 8 years and have put very little into it. Next summer I may be working it daily and need to make it more reliable.

'91 RD690S

Maxidyne EM7 300 4 VH

12 speed TRTXL 1070B

Mack diffs w/ 4.42 gears (CRDPC92, CRD93)

Camel Back

Double frame

The syncro in the trans is GONE. Going up or down - everytime I move the button on the stick it grinds. It is particularly bad when I move the button to go down and the stick to go up a gear. My options are to replace it with a Mack 18 spd, replace it with an Eaton 18 spd (and have a new drive shaft made) or rebuild this trans. I like the Eaton idea as it will get me better fuel mileage, but the Mack 18 spd is built a bit stronger. Your thoughts??

I need more HP. I got spoiled with a 60 series 500 HP and now feel the Mack is under powered. I will be adding a Pittsburg Power muffler and maybe a turbo this winter. Any other hod rod ideas?? I want to pull a pup with this and will be grossing 92,000 lb. While a 300 may get me by in Ohio, it will be a real struggle at 5000' in the "hills" of WY and MT.

My jake brake is weak. The truck didn't come w/ this, I added it shortly after I bought the truck. I got the parts from a salvage yard in IL. I was told that there is a kit to increadse the oil pressure and get a stronger jake brake. I have no other info. If anybody knows how to get more braking power out of my jake, I am very interested!!

I also need several small parts. I recently blew a steer tire and it wiped out my passenger side headlight assy. The hood is mildly cracked but I can work with it. Other than the dealer is there a place that has Mack parts like this?? Aftermarket is OK as long as they are not chineese parts.

Sorry for the lengthy post, but I always had the $$ to pay a mechanic when I was running over the road. Now that I am semi-local I want to do more myself as well as I don't have the money to pay somebody else.

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First there is no synchro in the 12 speeds, check the air regulator to make sure you have the correct working pressure. Try to change the air filter on it also. Other then that it could be to much wear in the air piston that shift it.Putting an 18 mack in it is a major waste of money in my opinion. Get an hi flow oil pump, that should solve your jake problem and for more power, pitsburg muffler is another waste of $$. Call Harold at north west truck center for more power 800-665-9248

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Id keep the 12 and check what full fuel suggested at the most you would have to replace the air cylinder. The muffler is going to be a waste of money just take a slate bar and knock the baffles out and youll get the same result. And agreed the 300 is going to be slower than the 500hp trucks but then again they are useing more fuel to make to hp than you are. Plus when they pass you and try to show off doing so, you can do the same thing when their at the fuel island lol. I would turn the 300 up a little and if your going to be pulling them kinda hills, if your truck doesnt have one, GET A PYRO!!! Your pistons and wallet will thank you

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First there is no synchro in the 12 speeds, check the air regulator to make sure you have the correct working pressure. Try to change the air filter on it also. Other then that it could be to much wear in the air piston that shift it.Putting an 18 mack in it is a major waste of money in my opinion. Get an hi flow oil pump, that should solve your jake problem and for more power, pitsburg muffler is another waste of $$. Call Harold at north west truck center for more power 800-665-9248

Air Regulator? Where is it - what does it look like?

Air Filter - didn't know it had one. This could be the problem!!

Thank you for being gentle with my ignorance about Mack's.

Just got off the phone w/ NW Truck Center (Harold was out for the day). They recomended an injector upgrade and said the turbo should be OK.

Both Pyro and Boost gauge are on the short list. I already put holes in the dash for them.

Edited by Brandt
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Ha, yeah keep the 12spd its 10 times better than an 18. Again no Pittsburgh power muffler its a waste. Call about a set of injectors and set the pump up 10% over stock and you can swap out to a bigger turbo. I think the specs on your truck are just right all you need to do is add some HP with a little more fuel and air.

"Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin

"If your gonna be STUPID, you gotta be TOUGH"

"You cant always get what you want, but if you try sometimes you get what you need"

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What about the gears?? Am I OK there?? I was thinking about something in the 3.90 range. I don't know if changing the gears is cheaper or getting a set of cutoff's from a camel back.

If I can fix the 12 speed, I will keep it. I hope it is just the air filter or regulator. - I like easy!!

Edited by Brandt
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