I'm working on a 91 w250 with an automatic trans. My cruise control will not function and my speedometer shows about 10 slower than actual speed. Also, 70 is about top speed I can achieve. My trans downshifts from OD at the slightest touch of the throttle pedal and then feels as if it is governed until i slightly let off and let OD kick in again. I need more speed... Can some of you knowledgeable guys share some knowledge with me on where to start to remedy these problems? Any help is greatly appreciated.

 

Try adjusting the TV cable for the trans .it control's the shift's and should help some .as for speedo maybe it has a different size tire ?

 

Usually if the speedo is off that much it is mis matched gears.
The VSS gear in the NP-205 is a different gearing than what is in the axles.
Sometime I will do this on purpose to deal with massive differences in tire size.

I used a 3.54 geared VSS in my truck on 37 inch tires running 4.10 gears in the axles.
It made my speedo almost perfect with the huge tires.

 

I'm no good at auto stuff, but the speedometer stuff I can help...

Is the speedometer off 10mph across all speeds or is it say 5 off at 30mph and 10 off at 60? If it is a variable amount, swapping the speedometer driven gear assembly or the ratio box can do that. Right, wrong or indifferent, the way I found to get the closest to actual speed to read on the speedometer was to use a GPS speedometer app on my phone, put the VSS directly on the speedometer driven gear assembly, and drive it. I would then take a drive where I could cruise at a certain speed, say 60mph (GPS) and compare that to what the speedometer was reading. That will give you an amount to work with. Say the GPS was 60, and the speedometer was 45, you'd need to increase the speedometer reading 1.333, which by adding the correct ratio adapter would do that. Over the years I have collected several speedometer driven gear/ratio box/VSS assemblies and I was able to get close to what I need. There is a company around that makes new ratio boxes for about $65, you can even special order the exact ratio you need.

Hope this helps you out.

 

TV cable has nothing to do with the OD shift point. It governs hydraulic pressure, and therefore firmness of shifts so you don't wreck your neck at low throttle, or burn clutches when you get on the hammer.

The OD symptoms you describe fit the description of a mis-adjusted TPS. It could also be related to the VSS gear ratio problem that Ike and Ollie describe. Both inputs are used by the PCM to operated the OD solenoid in the transmission.

To adjust the TPS, get a voltmeter on the center lead. Key on, engine stopped, adjust for 1.0V at closed throttle. Operate the throttle and observe smooth transition to about 3.5V at full throttle. It's adjusted by twisting on the plastic coupler under the sensor with a 10mm end wrench.

 

I think (know) you meant to say ratio adapter, not VSS. the vss is universal, however, the ratio adapter is not. Ikes is spot on with his example of 60/45= 1.33 which would require a 33% increase ratio adapter. the company is PATC. I purchased one of their units to correct the speedo on my truck.

just don't want the OP going to the parts store asking "do you have a vss that fits a truck with 3.54 gears?"

 

Search on YouTube cummins cruise control servo fix. It's been a couple years now, but my cruise went out and that dude on that video shows step by step how to disassemble the cruise control servo, clean the innards, and re lube the innards. Worked perfectly. Don't understand why the shop manual states the servo is a non serviceable item and must be replaced

 

Before diving into potential mechanical solutions for the cruise control and speedometer, I'd work with John Martin on possible electrical problems (see Post #5 above).

I recently had intermittent non-charging issues (as shown by the in-dash gauge) during which time the air conditioning and overdrive would both cease to operate -- while everything else worked fine. John helped me chase it down and it turned out to be an intermittent malfunction in my crankshaft position sensor ("CPS"). Go figure.

These trucks seem to have lots of electrical gremlins that can manifest themselves in very peculiar ways...

 

Thanks everyone for the responses. I will attempt to use what y'all have suggested to remedy my situation. Again, thanks...

 

Thank you for the info.

 

So do I understand that theoretically if I get the TPS adjusted correctly I might gain some throttle and thus more speed? As well as remedying the shift pattern? I apologize if I sound uneducated in this arena, I am just now getting into this Cummins thing. I have 3 sons that all have 2nd or 3rd gen Cummins' and of course I think I need to keep up so to speak. I don't wanna race but it is nice to mash the pedal and be able to get out of my own tracks.

 

The OD shift is done by the PCM. It need to know:
1. What is the throttle position?
2. How fast am I going?
3. Is the engine running? (CPS signal)
4. Is the engine temp above 65°? (sensor under intake manifold)
5. Is my internal OD enable/disable toggle in the right position? (OD switch and light, light must be off.)

Apparently your issue is with the first two items.

 

Thank you j martin, I appreciate the info. I'll let y'all know if I am successful...