Optional 4-Hi TOD conversion using a hazard (4-way flashers) switch

Do you wish your TOD equipped Trooper had a full-time 4-HI option?  TOD not working due to a bad transfer case speed sensor?  Here's a fix.

One of my transfer case speed sensors failed and shorted, thus preventing the TOD system from sensing speed and activating power to the front wheels.  Everything engages fine, but the "CHECK" light flashes at all times and no power goes to the front wheels.  This modification allows me to use 4HI and ignore the TOD functionality.  There are other causes for the TOD "CHECK" failure that this modification will not work around.  I would prefer to also fix my TOD system, but the speed sensors and installation costs are prohibitively expensive.

This modification is based off of the Planetisuzoo.com article entitled "4-HI conversion for TOD equipped Isuzus".  The link takes you to an archive.org copy of the article, as it is currently inaccessible on the web site, or has been removed.  Please read and study that article carefully, then consider this as a replacement for the relay and switch in that article.  This does not incorporate any of the "switch sequence change" section at the bottom of that article - either be very careful to engage TOD before engaging this switch, or use the part of the article above to control the 12v + line coming into the switch via LT GRN/BLK per the diagram below.

As with anything like this, continue at your own risk.  If you harm up your Trooper in any way, shape, or form...don't blame me!

Per the article above, you will need a fuse holder and 10 amp fuse, along with two 10 ohm 10 watt resistors.  Instead of the project box, relay, and 4HI Switch, you will need a hazard switch from a donor vehicle - it will take the place of the relay.  Ebay or a junkyard is probably your best bet, but the switch you get MUST have the wiring and plug included, otherwise, you will have no way to connect everything to the switch, much less know how to connect the pins.  It appears that the same switch was used from 95 or 96 through 2002, but you will need to make sure of that, though it should be easy to tell as I recall the 92-94 hazard switch is a totally different round button.  The switch will install into the open spot in the dash to the right of the existing hazard switch and 4WD AUTO button.  I have not yet figured out how best to do it, but it would be advisable to label the new 4-HI "hazard" switch so you can tell it apart from the real hazard/4-way flasher switch.  You will need to find a way to feed the completed assembly up into the dash, or perhaps easier, just remove the dash trim to install it.  That will require removing the center forward console trim (see notes in the Brake Controller Installation page for some notes on removing it), then the trim around the radio, and up into the rest of the dash.  It's a bit of a pain, but not terrible, then you can easily route the wires under the center console to under the passenger seat, as well as tap the cigarette lighter power outlet or other 12v + and - source.  Our Trooper is wired for a trailer brake control, so I tapped off of those lines.

This is a scanned image of a section of the service manual that shows the wiring diagram of the hazard switch.  I tested the switch and it appears to be a "break before make" type switch, so there should be no shorting as it turns on and off.  The text in RED is my notes.  If you follow it through logically, it should make sense, but I'll try to explain.




Under the passenger seat, as noted in the article referenced above, is a wire that goes from the TOD ECU to the clutch solenoid in the transfer case.  Two wires will need to be run from there to this new dash switch.  The wire coming from the TOD ECU connects to the BLK/RED wire and the wire going to the clutch solenoid connects to both PNK/BLK and RED/BLK.  I chose to cut the wire under the seat and attach a male spade lug to one side, and a female to the other (be sure to get the type that has a plastic/rubber surround to prevent shorting).  This would allow the modification to be removed if desired, and the wires simply connected back together.

In the off position, nothing has changed from the original factory wiring.  The signal from the TOD ECU comes in via BLK/RED and goes out to the clutch solenoid via RED/BLK (which is tied to PNK/BLK and as you can see, does nothing in the off position).

In the on position, everything changes:

--The connection to the TOD ECU switches to connect it to GRN/ORN which routes through the resisor pack and then to ground, which fools the TOD ECU into thinking everything is just fine, so no errors will appear because of the modification.  Please note that the two resistors are wired in series - GRN/ORN switch lead to the first resistor, through it to the second resistor, through it and then to ground.

--The 12v + coming in via LT GRN/BLK is now connected to RED/BLK and PNK/BLK, which sends 12v to to the clutch solenoid and locks the transfer case into a 50/50 power split and a full-time 4-HI configuration.

--Lastly, 12v + coming in via LT GRN/BLK also routes to GRN/BLK which connects back to the switch to RED/GRN, which illuminates the light inside the switch.  This is of course optional, and largely useless during daytime.  At night, you will see the switch illuminated when it is turned on.  I would suggest taking the switch apart to access the bulb, then pull off the red rubber covering over the bulb.  This will make it glow white, even though the switch marking is red (yes, I know, you would think it would shine red anyway, but it comes through white), thus making it easier to differentiate from the "real" hazard switch in the dark.

A side discussion and thoughts:  The switch I acquired had a bad bulb in it, and thus would not illuminate.  Being a very odd bulb, I was unable to find a replacement in short order, though I did later find suitable replacements available online.  If you search for "3mm twist bulb", that should put you on the right track.  That may not be the right size, but you can start there if have one in your hand (I currently do not)...if you find the right thing, let me know and I'll update this page.  With a little work and a resistor, I was able to replace the bulb with a small white LED out of an odd set of LED christmas lights.  If you want to try this route, I can get you the info (or I may update this page to include it), and may even be able to help you out with an LED, if I can find the extras I have.  It wouldn't be so easy to just purchase a plain LED as you would need plain flexible wires to connect through the bulb base, which this light set already had soldered to the LED leads.  What I wrote there may not make any sense until you take one apart and see how the bulb plugs in via an odd twist-lock type base.  Another option would be to tap the 12v + illumination line from the other adjacent buttons (I think that would be GRN/RED per the wiring diagrams, but you would need to confirm) such that it illuminates whenver the headlights are on, but then you have no indication that the 4HI mod is engaged.  To match the positive and negative on the illumination on the switch to be the same polarity as others dash lights, you may need to reverse RED/GRN and GRN/RED from how I have laid it out in the diagram.  With an incandescent light, it doesn't really matter which connection is positive or negative, you can switch RED/GRN and GRN/RED in the above diagram as either will work.  LED's are sensitive to polarity and must be connected the right direction...if you go that route, hook it up and test before putting the dash back together in case you wired it up backwards!

Here's an image of the installed switch.  The hazard switch to the right of the 4WD AUTO is the "4-HI" switch that I added.  On mine, there is also a blank to the right of the clock that could be used instead, if so desired.



Here is the wiring under the passenger seat.  You can see the TOD ECU on the right, and the spade connectors I added in on the left.  The orange and blue wires are the ones I added to go to the switch.  Flaps of carpet normally cover this up, so it should be sufficiently protected:



That's pretty much it.  I hooked the wires all up, then used zip ties and electrical tape to hold them together with the resistor packs all in a bundle behind the dash and switch.  Sorry, I did not take photos when I put it together several years ago.  You will have four wires coming out of it, one to positive 12v, one to ground (negative), one to the ECU, and one to the clutch solenoid.  I would like to find an easier way to do the "switch sequence change" as noted in the referenced article to prevent incorrect activation of the 4-HI modification, but I've not yet taken the time to figure it out or implement it.  Long ago, I read something referencing another connection to do the job coming from a positive voltage source, I think having to do with a 4-Low indicator, but I don't recall the details and can't seem to find it again.  If you have any suggestions, I would love to hear them.

Any questions/suggestions/additions/corrections/etc, please contact me at webmaster@megley.com.

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