Electrical Relays with diodes

I have a ron Francis harness with their tyco relays, and have installed them on another car with literally zero issue whatsoever.

Sean Canney said:

With the cobra 75 harness?

Click to expand...

They were pm-75 harnesses, built virtually identical to what you have.

I installed them how RF built them and never had any issues. That was 4 years ago

96pushrod said:

They were pm-75 harnesses, built virtually identical to what you have.

I installed them how RF built them and never had any issues. That was 4 years ago

Click to expand...

Maybe looks and how they meld all of the grounds but thats where they stop......The PM-75 harness is made for a PIMP Megasquirt ECU....

And even though PIMP Megasquirt sells the RF made harness doesnt make it hugh quality or even right...Ron Francis makes wiring harnesses for factory5 kit cars too and Im a member on that forum too and theres lots of complaints about the quality of the harnesses. and lack of correct functions.

Wheres this plug on your harness?..Thats right,,,you use a USB port now......



The EEC-IV system is way more susceptable to rf intereferences than the custommade Megasquirt systems that utilize the Ford EEC box and plug which is quite criminal if Ford didnt give their blessing..

Have a lil conversation with a real EEC expert ad they will tell you the EEC-IV system is very finicky and takes perfect grounding to work then tell them you have a harness with all of the grounds combined in one in a harness designed for an A9L ECU like the relay ground wire also the HEGO control ground wire and 02 sensor ground wires and the clean ECU ground wire on #40 and the clean ECU gauge wire coming off pin #60 are all hooked together and mounted to the cylinder head and tell them thats your professional opinion of how an EEC-IV system should be grounded and they will beg to differ and differ very highly to your statements ;-especially the guys over at HP academy.........LOL


Tell them about that PM-75 harness you have and send them the pdf....I bet they come back with corrections that need to be made or want to see a datalog and say you should of just modified the stock harness you had....

Put an A9L on that PM-75 harness and make a vid of your car running and doing a burnout......Ill bet it wont even run right with that RF harness especially of you have the wideband 02's hooked up...LOL...


Even Sean Canney will tell you the instructions are totally different so are the circuitry.



Heres the TDZ-75 fuse box and relay diagram...




This is the kind of quality you get from RF...Look at the crappy crimps also the lack of silicone gromets for weatherproofing.........



Heres what real crimps look like................



Heres what weatherproof relays look like too........

Sean Canney said:

The one I've got has been horrible. It's a weekend made harness I think.? I long for the salt n pepper shakers

Click to expand...

$200.00 on ebay gets them.....

Sean Canney said:

So I have the much disrespected cobra 75 wiring loom. I was told to put relays with diode protection built I. When I switch them out I now blow fuse 18 factory fuse. Why? I put the tycos back in and at least I can start the car. This fuse is got tons of stuff on it. Head lamp . Cigar lighter , speed sensor. And I can't do diagnostics without that circuit. What does the diode change? Mustang 5.0 from 1990

Click to expand...

Well atleast you got it to start up.....I think you said you never got it to run so your heading in the right direction....

Heres the direction I went after getting contacted by a professional via pm about the errors my harness still had and it really opened my eyes as to how ford got the cleanest grounds to the ECU and why pins #37 & #57 also pins #40 and #60 are important for the way the ECU gauges its power to ground resistances............


I undid the whole harness again and separated all of the systems and wrapped each in protective copper foil tape then wrapped over the copper foil tape with harness wrap...........Found out that I had to also separate pins #40 and 60 plus the EEC relay ground wires....Like this...





My harness now has 6 seperate grounding wires....one of the wires is the two 02 sensor wires melded into 1

Fuse #18?...............I dont have anywhere in my instructions that says to hook a wire to fuse #18 or any fuse in the fuse panel ............

My "KEY ON" wire I tapped direct off the ignition switch wires on the switchplug going to pins # I1 and I2 labeled Run Output for Engine.



The purple TFI start wire I tapped off the ignition switch too on wire on pin labeled ST for Start Output..........


The fuelpump wire should be hooked to the fuelpump relay trigger wire coming off the fuelpump relay plug under the passenger seat....You cut the wire off the relay and hook the pink wire to it...I do away with the inertia switch provision on the pink wire by cutting off the plug and hooking the pink and fuelpump output wires together because the stock inertia switch is wired into the FP relay youre wiring into..

Cant do diagnostics without the circuit?.. What circuit?... Are you referring to the MIL Lamp power wire being hooked to fuse #18?

FYI:..........The MIL power wire Does NOT Hook UP to ANY Outside 12v POWER source with a FUSE....The red Mil Power wire already has 12v power to give the 12v power to power the Check engine light bulb so in essence you're temporarily creating 24volts to that circuit with the diode with an immediate short to ground for the temp overvolt condition until the fuse blows...




Its just two wires that makes the check engine bulb illuminate and you have to make a bulb work and fit inside your cluster...The wires are red and tan...It doesnt matter which one is negative or positive as the T-10 style bulb doesnt care...

I took a regular bayonette style bulb holder from an old oil pressure gauge with bayonette bulb and modified it to replace the stock check engine bulb holder and mounted it in my cluster...you can simply use one of these too...

You get 2 for $10.00 on ebay...



The harness is a stand alone system and uses the diagnostic plug with seperate wire to make the check engine light blink.........

This is how you do diagnostics.............




View: https://vimeo.com/766330560

Oh and I also had to add a 22kohm 1/2 watt 1% tolerance non-polarized metal film resistor to the wire coming from pin #4 onthe ECU plug going to the IDM circuit and youre supposed to use a NON -CCD[computer controlled dwell] TFI module too....

I use ACCEL module #35369.

As you can see I got 100 resistors from amazon for $6.00 and 650pc shrink tubing kit for another $5......Ford wants $35.00 for their resistor and it has 5% tolerance...



As you can see from these examples you dont need a soldering iron.....alls you need to do is stretch the wire covering a tad by crunching it with needle nose and threading the needle like ends into the wire ends using a twisting motion then use heat shrink tubing to protect the bridge...



I made my resistor with female terminal ends so it can be easily replaced if needed and the connections with the male ends are protected by shrink tubing after being connected to protect the connection from moisture and oxidation...





The 22k resistor is very vital.....


Heres what the ones from Ford look like...