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98.5 24v engine swap back to 12V

2K views 5 replies 4 participants last post by  turbostang7 
#1 ·
I have a 98.5 dually 24v with @ 325,000 miles on it, changed the oil in it the other day, pulled a trailer with it the next day and all of the sudden it started blowing oil out the breather, within a 100 miles it barely showed on the dipstick. It's had some blowby for a while but never did this before. After refilling it a couple of times in a couple of days I decided I couldn't live with it anymore. Something had to be done. I use the truck to haul a small Cat dozer and need to get it going pretty soon. I have a 95 12v in a 3/4 ton with about 170,000 on it I decided to swap into the 98.5, engine sounds good, no blowby at all. Got both engines out now, I'll probably p pump the 24v after I rebuild it and put it in another 1 ton I have. What I need to know is what to do about wiring the 12v, could I remove the 24v wiring harness and put the 95 wiring harness in it's place or are the compatible? I see thee is a computer on the left firewall, could I use it or have to use the one from the 95? I beleive the 12v has 215 hp, both are 5 speeds. So I'm going to loose a little hp. Was running a programmer on the 24v, was putting out about 285 hp. Would like to get the 12v up to about 350-400 hp. All I want to do is pull my load with ease. Not worried about racing or sled pulling, just a good daily driver with this one. Can anyone help me?
 
#2 ·
95 5spd had a 175 pump. I'd switch the whole harness.
 
#3 ·
The wiring is not compatible. A 98 12v harness is a rare item. many have just swapped all the older style dash/wiring over when doing such a swap. The simplicity of a 12v does give the option of splicing the few wires you do need into the cab harness but nothing will be "plug and play" (short of locating 98 12v wiring). Them being 5 spd's makes it a lot simpler. the computer on the firewall will only be running cab related items and have no PCM duties.
Free 12v pump mods could get you to an equal HP range of your 24v.
 
#4 ·
About all you really "need" is the crank sensor signal. If you get creative you could take a tone wheel from a crank and your balancer to a machine shop and have them center it on it, then take the crank sensor and build a bracket to read it. That way your tach/charging system will work. Other than that the other sensors is oil pressure coolant etc, should just swap straight over and plug in.
 
#5 ·
Well been overseas for awhile, got back and completed my 24v to 12v swap in my 98.5, runs good but have no gauges only thing that works is my speedometer. I did notice when I took the 24v out there is a wiring connector between the starter and engine block, assuming this a crank angle sensor? Could I use these wires to run up to the crank sensor at the front of the 12v engine to get my tach to work? Haven't recharged my ac yet but I assume it won't work either unless I does something different also. Could anyone help me with this?
 
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