Factory installed Diode backwards

[funkstuf]

I recently delved into my Delorean because it has never seemed to idle correctly when cold. I've had it looked at by various mechanics, and had checked out numerous other possibilities like micro-switch, idle motor, etc. etc. but nobody ever came up with the answer.
I pulled the diode up that leaves the idle conrol ECU, It sits just under the box in the wiring. Strangely enough, the Diode appeared to be pointing backwards.
I didn't have confidence in my diagnosis, but have several Deloreans so I pulled the Diode on one of the others.
Both Diodes had the same model and number on them and appeared the same but the male and female ends were on opposite sides.
This was a factory install. The connectors were the same on both cars with the wiring male connector on the side nearest ground, and the wiring female side on the wiring nearest the ECU. Even though you could see the diode picture that it was directed backwards, I checked continuity, and open circuit, and sure enough, it was backwards. Whoever installed them, placed them in the connectors the way they fit, but, since the diodes had male and female on opposite ends, one was backwards and not providing a ground. This is a consideration worth noting in cars with long term unresolvable cold idle problems.

[Jonathan]

Interesting discovery. Thanks for sharing.

Just for sake of clarity, are you talking about this diode (seen in the bottom portion of this photo)? (or top portion of the image depending on which way the computer feels like showing it...)

ECU thermistor jumper 2.jpg

[funkstuf]

Interesting discovery. Thanks for sharing.

Just for sake of clarity, are you talking about this diode (seen in the bottom portion of this photo)? (or top portion of the image depending on which way the computer feels like showing it...)

ECU thermistor jumper 2.jpg

That looks like the diode I'm talking about, but it was under the ECU housing in my vehicle and I had to pull it out. I'm not sure if there's another one in that vicinity. It was interesting. The VIN that had the incorrect diode was 4984. I checked it against 1176. I thought perhaps they got a batch of diodes with male/female end switched and just kept installing them the normal way as someone else probably installed the connectors. The connectors on both my vehicles are exactly like the ones shown in your picture.

[funkstuf]

Also, Based on the schematic, this diode appears to control the portion of the ECU controlled by the thermistor so that explains why it's generally only when it is cold that it's a problem, but idles perfectly once it warms up just a bit if I'm reading it right. (Idle ECU, not Lambda ECU).

[Bitsyncmaster]

That backwards diode would have to be the fault of the harness maker. He put the wrong ends on each wire.

The diode goes to the idle micro-switch. I don't know why that diode was used in that circuit. I'm guessing it was done for an electrical noise problem.

[mluder]

So were you able to reverse it and if so, did it solve your problem?

Cheers
Steven

[funkstuf]

That backwards diode would have to be the fault of the harness maker. He put the wrong ends on each wire.

The diode goes to the idle micro-switch. I don't know why that diode was used in that circuit. I'm guessing it was done for an electrical noise problem.

Here's the situation on the harness: They all appear to be made the same way. When I looked at my other vehicles, and at Jonathan's photo, the harness has male towards ground, and female towards the ECU, and the diode fit appropriately with male and female on the opposing ends. This particular diode had the male and female ends opposite of the other ones so unless they were customizing each harness with the diode in front of them, this diode would not work in the standard configuration. My guess is that they assumed that all diodes had the same male and female ends but some of them didn't. This diode does provide ground through microswitch and I believe the diode is to prevent feedback to the ECU from the vacuum solenoid noise as it's filtered by an RC filter when the microswitch is open, but a direct ground with the microswitch closed.

[funkstuf]

So were you able to reverse it and if so, did it solve your problem?

Cheers
Steven

I reversed it by concocting a connection with opposite male/female ends on the diode so I wouldn't have to change the standard harness. Now the circuits test correctly, but, I haven't had a chance to start the car cold yet. I'll update as I go.

[rdarlington]

I had a cold idle issue as well. Really weak. No power. Just angry. Once warmed up all was good. Turns out some joker had 2 fuel lines attached to the fuel distributor swapped when I bought it. One went to the WUR, the other to the cold start injector. "oops". Would've been a really fast an easy fix if I didn't drop my last copper gasket/washer.

-Bob

[funkstuf]

I had a cold idle issue as well. Really weak. No power. Just angry. Once warmed up all was good. Turns out some joker had 2 fuel lines attached to the fuel distributor swapped when I bought it. One went to the WUR, the other to the cold start injector. "oops". Would've been a really fast an easy fix if I didn't drop my last copper gasket/washer.

-Bob

Good thought, the cold start injector appears to be correct on mine. After fixing the diode, the cold idle is still a bit rough and I still have to feather the throttle until it switches circuits. then it's actually spot on at 975 rpm. I need to check for any vacuum leaks and possibly the WUR.
Thanks,