Hello RX7Club. I have been sitting on this document for some time now. I contemplated not releasing these details due to a forum member selling a similar product on eBay up until a year or two ago, and more recently due to the impact of COVID-19 and not knowing if I would need a way to generate some extra income. However, today I will provide the details of how to create professional quality PnP t-harnesses for adding 451M relays to your power window switches. I only ask that this information be used for DIY...not for a business to show up selling these kits a month later . If this is beyond your scope, I may consider building some harnesses for the community, but I don't think there is enough demand to make it worthwhile, which is why I am providing this information out in the open.

Why are you doing this? There are several threads and diagrams detailing how to perform the relay mod?
I'm tired of seeing cars getting hacked up by people who have very little electrical competence; there is no need to cut into the factory harness, period. Creation of a PnP harness has also never been spelled out in detail. Secondly, I wanted to give back to this community and help further simplify the solution to a common problem for FC owners. Some very old threads have good information buried dozens of pages in, and some even have incorrect/incomplete information. I will also note, I only endorse doing this the "correct" way, with relays in both driver and passenger door. I won't get into the details, but I wanted to make this so easy that everyone would perform this mod 100%, and not feel the need to take a shortcut by only isolating the driver's switch.

Evolution of this mod:
The concept of adding relays to relieve the heavy current load carried by the switches is not new. Originally this was accomplished with several discrete 5-pin relays; this was difficult for the novice to implement without error, as well as bulky/difficult to package cleanly. Next, the concept of using DEI 451M relay modules was introduced; these are fantastic and convenient dual relay packages used in the aftermarket remote start/alarm industry (I was a professional/Master Installer in my previous life before becoming an automotive EE). The use of the 451M allows for a cleaner and easier install, but the problem still remained that most people opted to hack up their stock wiring, which made a mess and was still error prone. Finally, a forum member/business owner was manufacturing and selling PnP harnesses on eBay using the 451M. The price he charged was very reasonable considering the cost of materials, proper crimp tool, and knowledge required to implement. I do think the execution could have been better, but regardless I'd like to give credit where credit is due.

Why do we need to add relays in the first place?
Early car electronic systems commonly used mechanical switches to physically and directly interface with high current loads; the FC RX7 is no exception, and is the reason why the window switches are prone to electrical contact failure over time. In a modern car, the switches merely carry low current signals to relays and modules which are much more robust and can handle the high current switching; if a relay were to fail, it is also cheaper and easier to service. As you can see from the FSM excerpt below, the mechanical switches of the FC directly carry the load current of the window motors.



How do relays actually fix the problem?
As explained above and depicted below, relays can isolate high current loads. They only require low current signals to energize their coils and electro-mechanically switch. A typical automotive relay only takes ~150mA to energize, vs. several amps it takes to run the window motor! The diagram below gives a high level view of how we will accomplish this isolation, and is color coded to the 451M's wiring.



I've seen similar versions of the above diagram, and I still don't understand how to implement the relay mod without destroying my car?
For those needing more details than the diagram above, see the following detailed diagram for the PnP harness. This diagram covers factory wiring colors/pinouts, images, 451M wiring colors, signal function, part numbers, it even visually represents which signals are high/low current so you can ensure you use the appropriate gauge wire without creating an unsafe or excessively conservative harness.



This is amazing. What parts do I need?
- 451M relay modules (one per door)
- TE 172777 (14-20awg male pin)
- TE 172779 (18-22awg male pin)
- TE 172504 (5 pin receptacle housing)
- TE 172505 (7 pin receptacle housing)
- TE 172773 (14-20awg female pin)
- TE 172494 (5 pin plug housing)
- TE 172495 (7 pin plug housing)
- Ring terminal (I use high quality solid brass terminals from EasternBeaver)

*Note, these are open barrel terminals. You will need an open barrel crimper to terminate correctly.

What should a professional quality t-harness and install look like?
There are no butt/bullet connectors, tape, solder, etc. All gangs are joined together in the crimp at the pins in the connector housings, there are no splices or extensions in the middle of a wire run. I personally finish mine off with some Tesa protective tape, which is not necessary, but adds a nice touch. The 451M is secured to the door via a machine screw into a captive nut (on the S4, one door has a pre-existing nut, the other door I installed a rivnut in the existing hole). The ground terminal in the passenger door is making good contact with bare metal via a star washer.






Good luck, and I hope this information will benefit FC owners, new and old alike.

 

Bravo, good sir! I did a 451m mod in my window switches and while I thought I got it right, must have messed something up, because the windows did not work after that. I'll use this as a double check to see where I may have gone wrong.

And, as an aside, I used a couple of junk switches and some spare door harnesses I had acquired to make the system plug-n-play.

 

I wanted to do this years ago but I could never find the right part for the housings. great job

 

This is an old thread but a great write up. Hoping a new post generates some views for a question I have:
My switches have been disassembled and cleaned and still barely work. I have tested the motors directly off the battery and they work great. What kind of voltage should I look for with multimeter to determine if the switch contacts are too far gone for this to still work?

 

You should measure the resistances of the switches themselves. Too high resistance reduces the voltage to the motor first. Resistance of the switches themselves should be near zero, 1 or 2 ohms would be about as high as you would want it to get. You should also not only measure the incoming voltages, which should range 12-14v depending on if the car is running or not, but also measure the resistance from the battery to the switch, switch to motor, and motor back to battery. If any of these are over an ohm or two, you likely need to replace the wire.

Remember, if the circuit is disconnected, even with high resistance in the wires, voltage will eventually get to equal that of the source, which would give you the false impression that the line is in good condition if you just went off voltage readings. Voltage drop only shows up within a closed circuit. An absence of voltage to a part of the circuit indicates a complete break in the circuit.