DME Relay question

[ianc]

Hi folks,

I had a DME relay failure last Wed night. Luckily I had the spare in the glove compartment, but nonetheless it considerably unnerved my (heavily pregnant!) gf at the time since it was raining cats and dogs out. After an easy replacement, it has run without issue since, but this, of course, prompted the failure analysis.

I cannot find anything seriously amiss with it after examining the wiring diagram and checking continuity, but I do have a question. Below is the relevant section of the diagram from the DME Test Plan:



You will notice that 86 is the control input for the main relay (on the right), which grounds through 85. Pictured after 86 is what looks like (to my limited circuit knowledge) a diode. I understand the function of a diode, and I'm supposing this prevents current backflow from 85 -> 86 in the event that the voltage at 85 happens to be higher than that at 86 (why this would occur I don't know).

Now looking a bit more closely at the relay itself, I actually see two discrete components there; the black cylindrical object labeled '0336' (1 -> 2 in dia), and another tan component to the left of this with stripes on it (stripes and '4' not visible in pic; 3 -> 4):



looking at the traces on the bottom of the PCB and comparing with pic #2:



We see 86 pictured at upper left. It leads straight into the black cylindrical component, which connects across 1 & 2 (labeled 0306 in 2nd pic). The tan, striped cylindrical component connects from 3 to 4, which appears to directly short connections 5 & 6, which are the connections across the main relay coil. I measured a resistance of 71 Ohms across this tan component, which is exactly the same as that measured across the coil (5 & 6).

Even from this not too detailed picture, it is evident that the soldering on the bottom of the circuit board has fatigued and cracked (mfg date Nov 87), so I am planning on reflowing the connections and adding more solder to it, then keeping it as the spare.

My question is, can anyone explain the purpose of the two components which I've mentioned and what function they perform in the circuit? If my original assumption is correct about the black component being a diode and preventing current backflow, then what is the role of the tan one?

Thanks alot for your thoughts,

ianc

[slodave]

The "tan one" is a resistor and you are correct on the "black one". Re-solder all joints and keep as spare.

[ianc]

The "tan one" is a resistor and you are correct on the "black one".

Thanks Dave.

What is the purpose of the resistor across the main coil, and under what conditions might the voltage at 85 be higher than 86?

Thanks for the help,

ianc

[slodave]

I'm going to guess, that the resistor value is 68 Ohms. Most resistors have an error factor of either 5% or 10% +-. A total guess - it's dropping the voltage on each relay. Looks like there is another resistor under the number "1" in your picture. I don't have one lying around to really look though.

Dave

[911quest]

Originally Posted by ianc

Thanks Dave.

What is the purpose of the resistor across the main coil, and under what conditions might the voltage at 85 be higher than 86?

Thanks for the help,

ianc

85 is the ground 86 is the + to the relay coil which draws down the contact the diode is used to keep for back feeding into whateversuspect the

are you sure that those are resistors they look like Zener Diodes

[ianc]

I'm going to guess, that the resistor value is 68 Ohms.

Measured at 71 as mentioned in first post.

A total guess - it's dropping the voltage on each relay.

Don't think so. It's connected in parallel with the control current for the main relay coil only, not the fuel pump relay.

Looks like there is another resistor under the number "1" in your picture

As mentioned, '1' is the input side to the diode from 86.

Anyone else?

ianc

[slodave]

It can measure 71, but I don't see a 71 Ohm resistor on my chart.

[ianc]

After further thinking, my thought is this:

Under normal circumstances, current will flow equally through the resistor (3 -> -4) and the main relay coil (5 -> 6), since they are connected in parallel and both have the same resistance. Hopefully this gives the coil enough current to activate.

If the main relay coil fails, you'd still get current (2X) through the resistor. This would send current to ground, but not activate the main coil, so no current from 30 would be passed to the fuel pump relay, injectors, and ICV.

What is the point of this?

ianc

[vintage guy]

The relay coil is in effect an inductor. When the current to an inductor is interrupted, a high voltage spike is generated across it. I believe the resistor and diode are there to prevent this transient voltage from damaging the electronic components driving the coil (in the DME?). The resistor limits the voltage spike to a safe level that the diode can block. I'm not sure why the other relay coil does not have the same protection. It could be that the driving circuit is just not as sensitive.

[dshepp806]

referred to as "relay chatter" prevention (the diode across the coil, that is) Quite typical in relay circuits...

[rusnak]

Hi Ian, your questions are way above my pay grade. I just wanted to post to say "congratulations" and that I sincerely hope everything goes well.

[ianc]

Well, I always get curious about this stuff. I hate filing things under 'don't understand it, but there it is'.

Vintage Guy's explanation seems to make the most sense to me, so I'll run with that one, unless anyone else wants to chime in. Thanks for the responses!

ianc