dshepp806

Loren was gracious enough to provide a circuit that would mount within the DME relay to help prevent catastrophic over voltage due to a runway alt/reg situation..............very similar to the defrost circuit but functionally opposite.

BIG THANKS LOREN!!!!!!

Doyle

dshepp806

This stemmed from my thread ""Simply,..a BIG Battery install,..1989"" for those that want to review that thread.

Thanks to Rusnak for the coaxing of a new thread........


Best to all!

Doyle

rusnak

I think I understand the concept of the design, but I'm no EE. I think it's a fine design though. Very clever!!!!

Lorenfb

"I think I understand the concept of the design"

The MPSA26 (basically a switch) provides the ground for the main relay which
gets it power from the ignition switch. Once the power-in (battery voltage)
reaches a voltage determined by the 24K & the 1K, the 2N2222A switches
'on' and turns 'off' the MPSA26 which de-energizes the main relay. Until
the battery falls below the set point (~18 volts), the DME ECM receives
no battery voltage.

Note: 86 - ignition sw., 30 - battery voltage, 85 - ground, 87 - pins 18/35 DME ECM,
85b - pin 20 ECM, 87b - fuel pump.

dshepp806

Ordered my parts today for this and will be looking into bench testing it once I decide how she goes together within one of my many (used spares) dme relays. I wondering if any visual indications could be placed within the circuit ( I would think so), driving an LED (or whatever) mounted atop the DME relay......without doing anything other than looking at this indicator (in a hard fail situation), one would know the status of the DME relay (be nice to have it latch the event,..but probably going overboard here) .

I'm always surprised at how many people don't have battery/charge indication additions to CONSTANTLY monitor system voltage while driving. I presently use my PassPort's voltage display functions to tell me what's going on, this, AT the cig lighter. Every so often, when servicing her, I'll doublecheck the battery terminal voltage then measure the cig lighter point to know if there exists any drops to account for (differential between these two points. The X50 is set to AUDIBLY alarm on the high voltage side at 16.5 VDC,..it will also alarm at 10.5 VDC on the low side. So far, I haven't had to make the decision to install a permanent meter (fine by me). I'm an analog sweep kinda guy but would appreciate digitals that can provide high/low historical data......

Loren: why would you select 18 volts as the trip point? Primarily, DME component(s) spec-related? What would be the weakest component link within the DME? (Knowing you've repaired many of these that died from voltage going too high on it's DC buss. Since I have a print, I think I may revisit that circuit just to sniff around.....have repaired many a circuit (non-DME) that failed due to overun of it's spec,...just curious of the circuit (DME) {{{{{{{{{{{{{{that I've repaired other's DME's only 3 times....twice with solder reflow,..the other was changing out injector driver related SS. No bench mockups like I'm sure you have, just basic testing of junction integrities on the SS devices ..easy to see when it's "shot"...but never have I fired up the function generator to establish drive signals then trace accordingly......}}}}}}}}}}}}}}}}}}

Anywaze,..just a few questions here. Thanks again for that circuit,..'tis SO simple, eh?

Best!

Doyle

whalebird

How very nice. I really like the LED trip signal idea Shepp, or maybe a gong/buzzer (audible). If I follow it correctly, the motor shutting off would certainly be a hint.

Also, Loren...posting the circuit nomenclature(30, 85b, etc.) is a very nice thing. I have this on a note pad stuck to my tool box. Word of advice all you DIYers, copy this down and refer to it often, you will be surprised how often you refer to it. Most cars (european for sure) use this same "code speak" for circuits. It makes schematics a lot clearer in diagnosis, and invaluable when jumping relays or installing new equipment like radios.
Nice work.

Lorenfb

"driving an LED (or whatever) mounted atop the DME relay."

That's a good idea. All you need to do is add it with a resistor to
the '87' output pin (maybe green).

"why would you select 18 volts as the trip point? Primarily, DME component(s) spec-related? What would be the weakest component link within the DME?"

It's the injector driver transistor which always fails when overvoltage
occurs, i.e. over 20 volts. It's only rated at 15 amps and it drives
6 low impedance (< 5 ohms each) injectors.

"No bench mockups"

Actually it's very simple to bench test a DME, contrary to what some have
posted and pictured over on the 'Dark Side'. Just a basic 12V power supply
and a A.C. signal generator with a basic o-scope (1MHZ like found on eBay
packaged in a iPhone case for $60 - with a probe). Amazing what the
Chinese now produce.

dshepp806

Thanks for the reply Loren.