Full digital gaugecluster idea.... need input

[kastein]

OK, I got the hardware needed to put a second 4.0 HO temp sensor into one of the tranny lines today. Got a couple questions for 4.0L/engine gurus:
* what's the temp at which the engine computer goes to normal mode instead of lean mode?
* what type of sealant should I use on brass fittings exposed to tranny fluid? I'm assuming standard plumber's pipe dope would be a bad idea, how about teflon tape? Anything else I can use? These are NPT thread and are definitely going to require some sort of sealant.

My intent is to install a pushbutton inside the gauge cluster next to the temp display that will toggle back and forth between tranny temp and coolant temp by switching the temp sensors in the thermostat housing and the tranny line using a relay. This should be compatible with any year except the dual temp sensor years (which are even easier to do this with, no worrying about lean/rich mode) as long as you only switch to the tranny sensor after the transmission fluid has gone above the lean/rich switchover temp - unless anyone sees a problem with this, in which case tell me!

For those interested, the current BOM is:
* 2x Watts A160 3/8 flare nuts
* 2x Watts A183 3/8 flare to 3/8 NPT fittings
* 1x Watts A758 3/8 NPT tee
* 1x coolant temp sensor from junkyard (clean coolant residue off)
* 1x relay + assorted electronics to switch it however you want (I'm going with a 7805 5 volt regulator, some capacitors, a 74LS76 flipflop with JK pulled high, a driver MOSFET, and two LEDs + resistors)

 

[5-90]

Finding an SPDT or DPDT press-button may be difficult (not impossible - just difficult,) you may have an easier time using a small toggle switch instead.

The Open Loop/Closed Loop thing, I am given to understand, depends more upon a valid signal from the HEGO than from the ECT. This is why EGO sensors are now HEGO sensors - heating elements were added in short order to improve the warm-up time on the sensors and to get the feedback signal that much more quickly (one or two minutes vice ten to get up to 700*C for valid readings.) I'm not inclined to believe that it has anything to do with the ECT - except for the fact that the HEGO signal may be "more valid" for certain ranges from the ECT (meaning that Closed Loop trim gets a bit tighter at operating temperature than on cold startup.)

At least, that's my thinking. I really need to get my grubbly little paws on spec docs for OBD-I and OBD-II...

 

[kastein]

Ahhh, interesting. Also, if you get your hands on OBD II docs (more interesting to me than OBD I) I definitely would like to peruse them also!

The circuit I'm thinking of isn't really that tough - I'm thinking a 74LS76 dual JK flipflop wired as a toggle flipflop (i.e. each clock results in the Q and Q' outputs switching states) with a debounced standard SPST pushbutton driving it. One output of the flipflop drives the MOSFET which drives the relay, both outputs also drive the two LEDs in the gauge cluster that tell you which temp you're looking at (or two halves of a bicolor LED if you want it to be fancy.) So I guess add some more resistors/capacitors, a 555 timer (for the button debounce circuit), and a resistor, capacitor, and diode or a dallas semiconductor DS1233 to reset the flip-flop to a default state on power-up.

EDIT: any idea on the transmission fluid fitting sealant? You seem like the sort of person who would know the answer to that...

 

[5-90]

Ah, I see what you're doing. A bit overcomplex, but workable.

As far as sealing against ATF, you can use a standard PTFE-based pipe dope (PTFE is proof against most petroleum products, save gasoline and probably Diesel) or RTV. I've used PTFE pipe dope before without any ill effects - use it on pretty much all of my drain/fill plugs, and most fittings. As long as you're not dealing with a corrosive fraction or something typically used as an organic solvent (gasoline, Diesel, in some cases kerosene,) you'll be fine.

I'm sure I'll announce it if I get my hands on OBD docs (preferably without having to pay for them. I wouldn't mind so much if I did, but that would make me a bit less inclined to "share freely" than to want a buck or two for copies. Gotta offset having had to pay for it, y'know...)

 

[kastein]

If it's under a hundred bucks, I'll split doc costs with you if I get a copy too I've been looking for that info for literally years now.

Good to know about the PTFE, I was considering PTFE tape as I use it for everything else. As for complexity... the chip I'm working on at work has over two billion transistors, so it doesn't seem all that complex to me really :eyes: I've seen SPDT pushbuttons, but they're expensive and require a significant bit of pressure to trigger. I figure I'll tear apart an old 97+ gauge cluster and take out the odometer reset button + actuator bar and retrofit that in for the "factory feel".

 

[5-90]

If it's under a hundred bucks, I'll split doc costs with you if I get a copy too I've been looking for that info for literally years now.

Good to know about the PTFE, I was considering PTFE tape as I use it for everything else. As for complexity... the chip I'm working on at work has over two billion transistors, so it doesn't seem all that complex to me really :eyes: I've seen SPDT pushbuttons, but they're expensive and require a significant bit of pressure to trigger. I figure I'll tear apart an old 97+ gauge cluster and take out the odometer reset button + actuator bar and retrofit that in for the "factory feel".

Meh, I'd just meant that you're overly complicating the task. What you're going to do with the 74LS76 and associated compoentry I'd do with a simple small toggle - done. A couple of small diodes to remove the possibility of feedback, if necessary. Seventy-five cents worth of components - done.

I take much the same approach to repairs. While I was doing the "industrial mechanic" thing, the boss bought a new work truck (PG&E surplus GMC Diesel) and we started having trouble with it a couple of months later.

While he's trying to figure out what's going wrong and how to work around it, I dug out the wiring diagrams and got to work. Twenty minutes later, I told him I'd be right back - I had to go pick up a diode. Five minutes, one diode, and a foot of 14AWG wire later, and all was good.

(His solution would have involved replacing a batch of stuff that didn't really need replacing, working around a bunch of others, and two days' work.)

Once I established that it worked, I noted the mod on the wiring diagrams in the manual and put the manual back on the shelf. Total cost of repair? Two dollars.

"Things should be made as simple as possible but no simplier." -A. Einstein