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About BlueDogDozing

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    Engine Started

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    Knoxville, TN
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    Owned and operated heavy equipment for awhile now, got a Mack to help me move it around! Got tired of working for the machine, so I bought a couple machines and work for myself.

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  1. Sorry it took so long to get back to you. My axle ratio is a 4.17. And I don’t see a rear support.
  2. That is true! I was hoping to find the correct part off the bat so I wouldn't have to fabricate anything as far as mounting brackets go, but it won't be too hard to do if it comes to that. However for now I just set it to draw air from outside only so it's cooler than from under the hood. I won't be using the plow anytime in the near future so I won't need to switch to inside air. And must be nice to have your trucks in a heated garage! Every time I work on it in the freezing cold I will think about your warm garage...
  3. That makes sense, there are a few switches without labels in the truck. I finally found a replacement pneumatic cylinder, for $435. Needless to say I won’t be replacing it. Good to know it’s not necessary so it doesn’t matter if I disable it It seems like an odd option. What is the point of choosing outside vs inside air? Besides air being cooler outside I don’t really see a difference between the two. This was formerly a DOT plow truck so I assume it has something to do with that.
  4. I have a question about a Donaldson air filter housing on a 96 Mack RD688S with an E7300. The air filter housing has two different air inlets, one on top under the cap (smaller of the two), and one that draws air from under the hood. Both of these air inlets have large butterfly valves and they're setup so that when one is open, the other is closed. The valves are actuated by a pneumatic cylinder that should be bolted onto the Donaldson housing...but my mount broke off...and the pneumatic cylinder doesn't move anyway so I was going to replace it. I've called three different service/parts departments at Mack dealerships and have yet to get an answer about what it is, what it does, etc. and when I tried to get a replacement part for it, they told me they had the part number, and that it was made by Bendix (part number 283023N), but apparently they didn't have a price, nor do they show any parts in stock. Since the little cylinder/mount is broken, and the larger of the two butterfly valves was shut, I temporarily set it up to where the larger valve stays open...seeing as how they likely put this weird assembly in for a reason (I assume it has something to do with operation under different throttle/idle conditions, I have an air throttle so this might work like the air modulator for the auto transmission) I'd like to figure out what it does, and restore the function to factory operation. Or just remove both butterfly valves so I have more air incoming all the time 😎 I've attached pics of the housing on the truck, the inside of it with the butterfly valves and linkage, and the pneumatic cylinder. Ignore the red paint on some of the parts that shouldn't have paint on them. Apparently someone resprayed the cab and for some reason they removed the hose from the air filter housing and sprayed paint into it instead of taping it off. I guess they figured the filter is there for a reason 😳. Any help or explanation would be appreciated!
  5. Sorry for the slow reply, been busy! I went to check the axle ratio and the sticker on the cab is not readable. Do you know where else the ratio might be located? I'll have a look for you. I know sometimes they're stamped on the differential somewhere...but if you can give me a hint that would save me a lot of time...plenty places that number could be!
  6. I cannot help you with part numbers and such, but I do have a 1996 Mack RD688S with an Allison HT 750 DRD in it from the factory. I can take some pictures of it if you need them. It is mated up to a E7 300. I would assume that the frames on our trucks are very similar. I am currently having some work done on the transmission, there is a company called Clarke Power that has facilities all around the United States. They specialize in Allison transmissions. You might give them a call, they’re doing my work and seem pretty knowledgeable. My local Mack has them do Allison trans work for them.
  7. After searching extensively I finally tracked down an old manual for the transmission and found out what the "problem" is! I'm mainly just replying here so that if anyone searches this they will find my post and perhaps it will help them out. The "issue" I'm having isn't an issue at all. It turns out that the Allison HT750DRD (and a few other Allison automatics) have lockup clutches where the transmission runs like a direct drive instead of through the torque convertor. When the transmission locks up the clutches, it feels like a gear shift...a temporary interruption in power. I thought it was a problem as I've never driven a heavy duty Allison transmission, I usually only deal with manuals. The way the transmission works is this: 1st gear: (only used for heavy loads) runs through torque convertor (TC) 2nd gear: runs through TC, then locks (L) later in the rev range 3rd, 4th, and 5th gear: fully L I hope this helps someone who thinks something is wrong like I did! It feels like a gear shift but only happens in 2nd gear (in my trans), but it turns out that all is well. Link to the manual that talks about this: https://www.wanderlodgeownersgroup.com/downloads/Allison Transmission Operators Manual.pdf
  8. Hello there! Long time lurker and avid search function user, but I cannot for the life of me track down a similar issue in the forums as to what I'm having, so I am hoping someone here may have some pointers. I have a 96 Mack RD688S dump truck with the E7 300 Mack engine and an Allison HT750DRD paired up to it. Truck has 130k miles/8600 hours. There is an air throttle on the truck, air modulator for the trans, and fully mechanical operation with both. The issue is this. When driving in 2nd gear (doesn't happen in other gears) under any acceleration/throttle input, the truck will surge for the lack of a better term. It almost feels as though the transmission changes gears in the middle of 2nd gear. Around 900-1000 rpm every time this happens. With more throttle input it happens a bit later, less throttle earlier. The truck will accelerate smoothly, acceleration and RPM's drop slightly for about 1 second, and then power will resume unimpeded until the shift into third/fourth/and fifth. Again, it feels as though it is shifting gears in the middle of 2nd gear, and after the "surge" happens the truck resumes at roughly the same RPM that the "surge" happened at. Motor function is spot on best I can tell, and again, doesn't happen in the other four gears. I recently purchased the truck and haven't put a load on it yet when driving as I'm currently going through and fixing all the little problems before putting it in service full time. It is possible that the RPM's where this happen are a little higher than I listed as the tach doesn't appear to be spot on, but the indicated RPM is where this happens, and it is very consistent. For experimentation I made sure the truck stayed in gear and I allowed the RPM to drop to below where the "surge" happens, and I'd accelerate back up all the way to the shift point and it won't happen again if it's already occurred...unless I let the torque convertor kick in and the RPM drops to idle speeds while I'm still rolling. At that point once I apply throttle and the torque convertor kicks back in in 2nd gear where it left off, if I accelerate past 900-1000 RPM I will get the "surge" again. I have a fair bit of experience with motor/transmission work but never with a fully mechanical automatic. I prefer manuals (for reasons like this) but this truck was a good deal and in excellent shape otherwise. I've had to pull valve assemblies and replace solenoids and such before in electronic transmissions, but never had a problem like this and don't know where to start considering the mechanical nature of the transmission. It doesn't help that it's extremely hard to find service manuals for this transmission. I have replaced the transmission modulator (air throttle) and that helped tremendously with shift patterns on the truck (it was the original modulator on it still) but this problem has remained the same. All other functions of transmission/truck are great. I considered a torque convertor issue but this only occurs in one gear so that would rule that out in my experience. Any ideas/suggestions? (Pic of the new to me Mack just b/c I'm proud of her)
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