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Old 04-08-2021, 08:35 PM   #1
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Limit Switch Advice

I'm trying to duplicate a limit switch device used to send a 12v signal from my accelerator to my transmissions electric modulator.

The original device is appears to be a wobble style limit switch. The relay the OEM signal goes through a 15a flat fuse. Since it's a simple on/off situation, I'm looking for the following:

SPST (single pole single throw)
12v DC
15a
wobble style

I searched using these parameters and all I got back was SPDT.

What I'd appreciate is verification that these are the correct parameters, and why am I not getting SPST in my search results??

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Old 04-08-2021, 09:23 PM   #2
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Why don’t you just use a Hobbs pressure switch on the intake manifold and set it to a manifold pressure that corresponds to 50-60% throttle. I used them on propane injection systems many years ago. They were very reliable. Then all the wiring is in the rear of the bus.
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Old 04-08-2021, 09:41 PM   #3
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Originally Posted by s2mikon View Post
Why donít you just use a Hobbs pressure switch on the intake manifold and set it to a manifold pressure that corresponds to 50-60% throttle. I used them on propane injection systems many years ago. They were very reliable. Then all the wiring is in the rear of the bus.
I don't know about pressure switches, but now that I do, I'm not sure why I would or wouldn't.

I get your point about not having to run wiring from the rear to the front of the bus, and it seems like it would be easier to adjust and more accurate than figuring out the proper placement of a limit switch to the accelerator.

How would I determine what intake manifold pressure equals percentage of engine load?
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Old 04-08-2021, 09:48 PM   #4
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Do you have a boost pressure gauge? Do you have a way to read the calculated load while driving?
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Old 04-08-2021, 09:50 PM   #5
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I don’t think that it has to be exactly 50% or 57% or whatever. Just in the ball park. I would error on the low side. If that makes sense.
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Old 04-08-2021, 10:18 PM   #6
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Here is one that is adjustable 1 psi to 20 psi. I think your max is 18??? My cousin had a 7.3 f250 with a pillar gauges and I drove it a few times and never saw 20 psi driving it BTTW. Close but no. I don’t know what Navistar did on the truck side. https://www.ebay.com/itm/76575-4-Adj...oAAOSwMH9dVLd5
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Old 04-08-2021, 11:06 PM   #7
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I've been pondering a Bluefire or Datascan to provide parameter readout, so yes, if I got one of those I'd be able to read boost.

I get boost is related to engine load. I did a quick search for how to determine how much boost equals how much engine load, but no luck.

Per the manuals, the modulator kicks on at ~80% engine load and off at ~60%.

How does boost pressure relate to the psi of the pressure switch?

How do I determine how much boost equals 80% engine load to activate the modulator?

How do I set the pressure switch to activate at ~80% and deactivate at ~60%?

How does boost psi get affected when on a flat road and wanting it to downshift and pick up speed, verses climbing a hill and wanting it to downshift for pulling power?
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Old 04-08-2021, 11:22 PM   #8
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I have a boost pressure gauge on mine and if I am running on flat road about 45 mph it will be about 10-11 psi and jump to 17-18 if I floor it. It won’t down shift to 2nd because it is going to fast. By 55 mph I’m at about 24 psi and then it shifts to 4th and drops to about 21 to 22 psi. On mine I would start at 15-16 psi. Some switches have both normally open and normally closed terminals on the same switch. I don’t think the 20% load differential between on and off will make any difference. That was done in the Navistar controller. The Williams switch was on or off. Same with this. Error on the side of more aggressive actuation ei lower load kick-down. With the adjustable switch it is easy. Just tee into the map sensor hose and go.
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Old 04-08-2021, 11:33 PM   #9
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These hobbs / honeywell switches are how they establish engine loading for some of the water / meth injection kits. You will be part way there if you want to hack water / meth injection to your ride. Add more gauges. I’m adding 8 to mine now. PITA on a rear engine bus.
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Old 04-09-2021, 01:57 PM   #10
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Quote:
Originally Posted by s2mikon View Post
I have a boost pressure gauge on mine and if I am running on flat road about 45 mph it will be about 10-11 psi and jump to 17-18 if I floor it. It wonít down shift to 2nd because it is going to fast. By 55 mph Iím at about 24 psi and then it shifts to 4th and drops to about 21 to 22 psi. On mine I would start at 15-16 psi. Some switches have both normally open and normally closed terminals on the same switch. I donít think the 20% load differential between on and off will make any difference. That was done in the Navistar controller. The Williams switch was on or off. Same with this. Error on the side of more aggressive actuation ei lower load kick-down. With the adjustable switch it is easy. Just tee into the map sensor hose and go.
Okay, that all makes sense. I'm going to throw that idea out to the drivetrain forum and ask for any pros and cons to that idea. But, barring any thing really negative it does sound like a much simpler and cleaner and probably more functional solution. Thanks!
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Old 04-09-2021, 03:46 PM   #11
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I haven't suggested this yet, but since you're still noodling on the solution, have you checked to see that the limit switches are not stuck/broken/on the edge?

These are the four switches (in my bus, yours may have more/fewer) that stop the travel of the door.
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Old 04-09-2021, 04:23 PM   #12
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Quote:
Originally Posted by Rucker View Post
I haven't suggested this yet, but since you're still noodling on the solution, have you checked to see that the limit switches are not stuck/broken/on the edge?

These are the four switches (in my bus, yours may have more/fewer) that stop the travel of the door.
They're currently is no limiting switch. The limiting switch is a mechanical workaround for the known problem with the OEM ECM/relay setup.

The pressure switch idea was suggested and it seems pretty reasonable boring no big negative feedback.
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