Need a switch to tell computer when in 4-low - any ideas?

Daveo91Burb · Apr 21, 2021

This has to do with my quest to upgrade from the first iteration of 4L80e PCMs (7060) to the next generation. (7427) Without going in to too much detail, the first gen used three speed sensors: trans input, trans output, and t-case. The PCM knew it was in 4-low if it saw a difference between trans output and t-case. The next gen in '94 eliminated the output trans sensor for the 4wd trucks and the input to the PCM for 4-low was handled by a simple switch at the T-case lever that would ground out when in 4-low. I need to replicate this input on my truck, but of course, I have the typical NP 208/241 lever that was used in squares, whereas the GMT400s had a different set up.

My plan is to go to the junkyard and hope to find a GMT400 with lever t-case and see if I can adapt the ball switch to my lever. Unless anyone has any better or simpler ideas?

Matt69olds · Apr 21, 2021

Is there a indicator light on the dash for 4WD or 4LO? If so, maybe you could use those indicator light to trigger a relay to ground for a input to the trans.

I’m not all that familiar with the different transfer cases or what transfer case came in each model. I just know I have seen some trucks and instrument clusters with a 4WD indicator.

yevgenievich · Apr 21, 2021

There should be a switch already given there is indicator already. I have a gmt400 manual shift lever that I can look at this evening as well

Bextreme04 · Apr 21, 2021

I'm doing something similar in my truck when I swap to the 4L80E. I was just planning on putting a simple lever switch on the shifter itself to determine 4lo, my NP205 has a switch already, but it engages for both 4lo and 4hi and you need it to differentiate for engine braking and shift point differences in the 4lo tables.

Daveo91Burb · Apr 21, 2021

Bextreme04 said:
I'm doing something similar in my truck when I swap to the 4L80E. I was just planning on putting a simple lever switch on the shifter itself to determine 4lo, my NP205 has a switch already, but it engages for both 4lo and 4hi and you need it to differentiate for engine braking and shift point differences in the 4lo tables.

Yep, that's it. I meant to put in my original post that yes, there is an indicator light for 4wd (works fine) but that it's the same switch (at least I think it is) for both 4wd modes, low and high. And 4-low is what the PCM needs to know since the gear ratio is different. If there were an additional light on the dash for 4-low, I'd have no problem.

Are you talking about a "user activated" switch on the shifter or a switch that the shift lever will engage automatically when in 4-low? I was considering going easy by just putting in a user-activated, but don't want it to be hoaky. Maybe I'm overthinking this......

Thanks for the responses

Bextreme04 · Apr 21, 2021

Daveo91Burb said:
Yep, that's it. I meant to put in my original post that yes, there is an indicator light for 4wd (works fine) but that it's the same switch (at least I think it is) for both 4wd modes, low and high. And 4-low is what the PCM needs to know since the gear ratio is different. If there were an additional light on the dash for 4-low, I'd have no problem.

Are you talking about a "user activated" switch on the shifter or a switch that the shift lever will engage automatically when in 4-low? I was considering going easy by just putting in a user-activated, but don't want it to be hoaky. Maybe I'm overthinking this......

Thanks for the responses

Just a switch that is activated when it is in the 4-lo position. Since mines a 205, my 4-lo position is all the way down so it makes it easy to have a switch that activates in only that position. I haven't really looked into it too much yet.

Camar068 · Apr 21, 2021

I'd look at mounting a switch that is activated only when in 4-lo. Have you taken the floor cover off the shifter off to see what mounting options you have? I've never had mine off so I can't tell you either way.

I need to fix this on mine as well. Currently I just don't put it in 4low because of this. I don't want a toggle. Depending on the setup, you'd think there'd be a way using a u-channel with a switch (pointing out the center of the U) that is depressed when it's shifted back. I'd want it pretty damn sturdy and consistent at switching.

Look forward to your results and don't care to help with ideas. I may pull the cover off mine later today and have a looksie.

yevgenievich · Apr 21, 2021

Switch similar to this one mounted on the bracket where shift lever would hit the arm in 4lo
https://www.newark.com/honeywell/v7-1x2ad8-048/microswitch-lever-spno-6a-250vac/dp/39B2877

Camar068 · Apr 21, 2021

or something like this:

https://marvac.com/products/spst-15a-12v-dc-off-on-automotive-marine-momentary-push-button-switch

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Waylon · Apr 21, 2021

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Maybe this?

Turbo4whl · Apr 21, 2021

Since the switch will most likely be installed under the floor on the shift linkage a weather resistant switch might be better.

Cole Hersee:

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The old Chevy brake light switch is a momentary off. When the brake pedal arm moves to rest (not applied) the switch is off. This Cole Hersee switch is available momentary on or off.

Daveo91Burb · Apr 22, 2021

Thanks, all, for the switch suggestions. I took the boot and indicator assembly off today to look at the gates for the shifter. The shifter is in 4-low in this position -- I'm thinking it should be pretty easy to mount a push button switch topside at the bottom of that gate that gets pushed when it's in 4-low. As long as A) it's mounted in exactly the right position so it gets closed and doesn't interfere with low engagement and B) the switch is low profile enough that the boot assembly goes back on.

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Matt69olds · Apr 22, 2021

https://www.digikey.com/en/products/detail/omron-electronics-inc-emc-div/SS-5GL2/137204

I would think this would work.

Camar068 · Apr 22, 2021

Daveo91Burb said:
Thanks, all, for the switch suggestions. I took the boot and indicator assembly off today to look at the gates for the shifter. The shifter is in 4-low in this position -- I'm thinking it should be pretty easy to mount a push button switch topside at the bottom of that gate that gets pushed when it's in 4-low. As long as A) it's mounted in exactly the right position so it gets closed and doesn't interfere with low engagement and B) the switch is low profile enough that the boot assembly goes back on.

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yeh you could either drill into the metal there, or fabricate a bracket and use the rear bolt to mount it. I'd make sure there was no flex, I don't know what would happen if the switch disengaged while driving.

Camar068 · Apr 22, 2021

Thanks for the pics btw.

Need a switch to tell computer when in 4-low - any ideas?

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