Very Strange Behavior From Trailer Brake Controller

Hello to all on Tundras.com,

We are an aluminum truck bed manufacturer and recently picked up a low mileage 2011 CMSB Grade 5.7 4x4 for research and development (read: to play with). Long story short, the previous owner/their mechanic did some less than stellar repairs. After fixing their uh-oh's, it was time for the fun stuff i.e. Optitron gauges, drive monitor switch, ultimately leading to a front end conversion. Upon installing the drive monitor switch, it was discovered that 2 wires from JL1 to J28 were needed and subsequently installed using the correct pins for the harness connector. Finished the job, go to start truck and.............nothing. Not even a click. Literally followed every single starting procedure/wire checking continuity and signal all the way from the ignition to the ECU, all good. Starting relay, good. Battery, new and good. Assuming that I crossed or damaged something while pinning the harness for the drive monitor switch, I located a Tundra at a junkyard and went and picked the entire J28 down to the JL1 with the drive monitor switch wires already present! Also pulled the starting relay as a test/backup. Spliced the entire new J28 and JL1 connectors into our truck, plug everything back in and.............nothing. No start, same condition. Now to the fun part, while installing the 2 wires the first time, we un-installed an aftermarket trailer brake controller installed by the previous owner. Figuring we had nothing else to lose or try, we plugged the trailer brake harness back into the truck (not the controller itself, just the harness!) and BAM, fires right up. Unplug it, same no start condition. Called the manufacturer of the brake controller and talked to three separate tech support persons and none had any idea why this would be the case!

Therefore, here we are, any Tundra experts out there know why the truck would not start without an after market trailer brake controller harness plugged into the factory connector?

Thank you in advance for your best guesses!

AlumBody Team

 

Does the brake controller harness have an additional ground wire to a body ground spliced in parallel to the one in the connector?

Like this one:
https://www.tekonsha.com/product/3031_trailer-brake-control-harness

 

This sounds weird. Wish I could help. I'm about to install one on my 08....

 

Hi Landphil, thank you for the response.

Our harness does not have an additional ground wire coming out of the controller side pigtail as shown on the "3031" in the link you provided. It appears we have the "3040" harness with only 4 wires coming out of both the truck side connector and the controller side connector. However, whoever installed this must have been familiar with or thought they needed an additional body ground because they spliced a ground into the white wire and fastened it to the dash subframe. Here's the kicker, after plugging this harness back into the truck without the controller plugged into the other side, we held the spliced ground back to the dash subframe: truck fires up. Turned the truck off, pulled this ground away from the sub frame, turned the key and BAM, fires right up. this additional ground did nothing!

On Tekonsha's website page for the "3040" the view inside the truck side connector shows 5 males even though there are 4 wires pinned in the back (not that I trust file photos). Our 3040 most definitely has only 4 males and 4 wires pinned.

Thank you again for the response!

AlumBody Team

 

I may have one available real cheap shortly

I wouldn't let our trouble condition keep you from installing one. It appears from the installation videos we saw it really is plug ang play. Our issue is that we wanted it out! If you want it in you should be good to go!

AlumBody Team

 

The only thing that I can think from what you are describing is somehow there is something loose on the truck side of the harness and the pins from the trailer brake harness are bridging something. That would be my last ditch attempt, try to fire it up while jiggling the truck side wiring. This does sound like a super one-off issue hopefully you can get it figured out.

 

Thanks for the input APalmTree. Since the truck wouldn't start after removing the controller it was our first thought. Embarrassed to say we never plugged it back in to try it because after watching videos of a TBC install our thought was there is no way this has anything to do with it. Ran it by a two supercar master technicians in our network and their thought was the same, that it wouldn't really have anything to do with the "starting protocol" of the truck. Now after a month of troubleshooting to no avail and plugging the dumb thing back in as a last ditch effort, it sure is acting like some sort of "circuit interrupt" when taken out.

If nothing else, we will have one slick anti-theft device! No one will suspect you have to plug one end of a harness in to start it.

AlumBody Team

 

Well then... I'm no wiring guru, but, assuming the wires are not shorted in the harness (You all seem pretty thorough so I'm sure that was something you have already checked too, and it should have nothing to do with starting anyways as you said) I can not think of anything that 4 wire extensions to nowhere could possible do to the truck. Definitely one hell of an anti-theft device to be sure. If you don't know why it working that way, there is no way a thief could figure it out.

 

Hi APalmTree. That was our first thought exactly, is there a "jumper" somewhere in the harness pins? Going to bench test continuity from end to end and every combination on the TBC harness to test this. Also going to leave the harness out and cycle the ignition while metering the trucks TBC plug and see which pins do what.

We would (and may still have to) just clip the TBC harness to about a 1" pigtail and leave it plugged in down below. Our concern is the plan for this truck is make one sweet overland/offroad vehicle to test/showcase some new products we're developing and having an unexplainable starting issue doesn't bode well for being alone out in the wilderness!

Thanks again for your $0.02.

AlumBody Team

 

To those that may be interested or struggling with a similar issue we have solved our starting problem! Albeit, with a less than ideal answer, our opinion is as follows:

It appears that our ECU was looking for this TBC upon startup and when nothing was plugged into the truck's connector it would not start. Plug in the harness even without the TBC plugged into the other side and the truck fires right up. Assuming then there must be a "jumper" inside the TBC harness we checked continuity through it from both ends in every combination at least 2-3 times. All checks out, every combination only sounded off when in fact we were touching the other side of that specific wire, no jumper in the harness. Next was to start the truck with the TBC harness connected and then unplug it once running and see if it continues to run. Yep, truck continues to run once started with the plug in and then removed. Definitely only a starting protocol issue. Our only thought at this point is the computer is looking for resistance in those lines and having the harness plugged in even without the TBC connected provided that resistance for the ECU??? So how do we change resistance? Change the length of the wires. We cut the harness off about 1" from the connector removing about 2 feet of wire from the system. Plugged it in, truck fires right up. Next was to test the pins in the trucks connector and see what they did and if any of them changed during "start". One has constant power (B+) and the rest get nothing. None of the pins changed behavior when cycling the ignition. Tested continuity through the trucks connector and got some weird combinations and only in one direction. Then realized it is probably not strange to get continuity through them and it really wasn't telling us anything. Why we got continuity through one way and not the other we surmise is due to our meter sending test signal out one probe and there must be a diode(s) in that circuit somewhere. Following the 'Trailer Towing' circuitry diagram we were able to eliminate all but two of the five wires on the truck's plug. Figuring that the B+ wire is what powers this particular circuit and according to the diagram it runs straight from the fuse box to the truck's connector unlike the other wires which run through relays or other modules, we pulled the 30A fuse for "TOW BRK". Turn the key and BAM! Truck fires right up without the TBC harness plugged in. Ok, maybe this fuse is not present without a factory TBC and when an aftermarket one is installed a fuse must be put in this terminal to activate the circuit. Consult the TBC installation instructions and no mention of a fuse anywhere nor did tech support offer up this explanation. Sanity check, put the 30A fuse back in, TBC harness still unplugged, turn the key and BAM! Truck fires right up. Control test, let it sit over night, turn the key the next morning and BAM! Truck fires right up.

Although not the concrete solution we'd hope to find (like an unplugged wire or open fuse) we assume that by taking the fuse out of that circuit and starting the truck we may have "reset" the memory on having a TBC. When reading the installation instructions for the TBC vehicle specific harness, it primarily states: "Plug brake control adapter into mating receptacle on backside of brake control BEFORE attaching to the vehicle connector". Our interpretation of that statement leads us to believe that upon initial installation and subsequent first start, the ECU will "write" that it has the TBC and from then on be looking for it. Even though we had the battery disconnected many, many times during this whole process, it wasn't until we pulled the fuse from the circuit and cycled the ignition did it work correctly.

Much luck to those who may be dealing with similar issues! Thank you to those that gave their input and suggestions.

AlumBody Team