Testing relay boards...., Can you bench test a 914 relay board....??? Options

[Superhawk996]

(IMG:style_emoticons/default/av-943.gif) at Ian's perfect description of what happens when the random parts swapping occurs . Truer words never written (IMG:style_emoticons/default/type.gif)

I'm happy to support you in the endeavor to find root cause.

The fastest method of troubleshooting almost anything is the half split method. Know the system you're working with. Find a way to divide it in half, part A & part B Is the problem in Part A? No? Then it must be in part B.

Now divide part B in half. Let's call them parts C & D. Problem not in C? If so, it must be in D. You get the idea.

In the case of your "suspected" issue of not having fuel pressure, the relay board is a perfect place to half split the system.

Let's divide it as power into the board as part A, and power out of the board as part B.

In your case, the intermittent nature of the problem when it's hot makes things a bit trickier but at least we know that you can duplicate the problem when it's hot. That's a great start.

So let's start by verifying (with Schematic & DMM) what all power conditions are when its running cold. Both on Part A (power input) side of the board, and part B (power output)

Then warm it up until you can duplicate your no-run condition.

Then recheck Part A (power into the board) -- if good when hot -- your problem isn't on that side of the system. If not good, then we work to figure out why it changed and where root cause is in Part A.

If the power output in Part B is not good when hot, we'll work our way through the Part B side until you find it.

Bottom line, you can do this, and I'll help in anyway possible, but it needs to be done logically, and methodically. (IMG:style_emoticons/default/grouphug.gif)

[Superhawk996]

Let's go back one more step, to the basics.

Before we even start assuming that there are electrical problems, with the fuel pump (only) when it's hot and not running, and you can't hear the fuel pump, have you pulled a spark plug and verified that it is sparking when hot?

Engine needs three things to run
fuel, compression, spark.
(Let's assume for the moment that we can ignore hot compression)

If you just assume that it won't run when hot due to no fuel pump noise, and haven't verified that it will spark, you're potentially missing an important clue on how we might start troubleshooting this. That is to say, we need to know for sure, when it's hot and not running, can we ignore the ignition circuit and focus only on the fuel pump? If we can ignore ignition side, that makes the troubleshooting work even easier.

What's being done here is to half split the system to the fuel side (Part A), and the ignition side (Part B). If by chance both fuel pressure is missing (no pump noise - Part A is bad) and we have no spark (Part B is bad), then we start looking for what is the common denominator between BOTH A & B.

This is a very important starting point.

Hopefully, this is making sense. Logical, and methodical.

[dr914@autoatlanta.com]

Here is an old guy explaining the function

https://www.youtube.com/watch?v=YLgQxpNHzIU

[second wind]

Well guys...thank you again very much and I have a new clue to share. What an automotive hell hole I have had today! So my Chevy S 10 pick up with almost 400,000 miles acts like there is no battery in the car. It has done this once or twice before....I tried charging the battery but the charger fell off the truck and didn't work after that. I figured I could jump it with the 914 but when I started it (first thing in the morning ice cold....but LA ice cold) it only ran 20 seconds and then no fuel pump sound. Hmmm, well I could take the battery out of the 914 and hook the truck up too it and that should work....not even close....barely lit up the dash even though it worked on the 914. So I am standing in the street with jumper cables in my hand and in two minutes I am up and running because of my new best friends that stopped to help me. So the new clue is that the fuel pump doesn't run even when cold.....just sharing. I think I will get the multi-meter out and start following some direction and start testing as an intelligent person....I will report back and I really do appreciate all of the help. BTW truck starts all day long now and might for a few days....but then ?? I am going to disconnect the battery at night and work on that issue later...914 in first spot.
All the best,
gg

[second wind]

Here is an old guy explaining the function

https://www.youtube.com/watch?v=YLgQxpNHzIU

Thank you very much George...great video! At the end I thought you were going to show us how to test a relay....is there a procedure for this? I have some of the new 914 Rubber relays....they usually make quality parts....have you had any experience with those? Thank you again,
gg

[Superhawk996]

At the end I thought you were going to show us how to test a relay....is there a procedure for this?

Take a look at the SirAndy board graphic on page 1 of this thread.


The schematic for the relay is shown.

Pins 85 & 86 are the solenoid coil. I have a good one in my hands it reads 69.7 ohms and draws .15A when connected to my power supply @ 13.6v

Pin 30 is the uncontrolled side of the relay. It is where the circuit to be controlled (usually 12v power) is usually fed into the relay.

Pin 87a is the normally closed contact. It is connected to pin 30 when the solenoid is not powered.
(this should read 0 between pin 30 and pin 87a ohm when the solenoid is not powered

Pin 87 is the normally open contact
(this should read infinity between pin 30 and 87 when the solenoid is not powered)

When the solenoid is activated by applying 12v power across pins 85 & 86, the resistance between pin 30 and pin 87 should change from open (infinity) to zero ohms. it doesn't matter what polarity goes to which solenoid pins - a solenoid doesn't care, it's just there to act as an elecromagnet.

The pins are clearly marked on the old German relays.


The solenoid can be bench tested with a couple test leads and a 12v battery connected across pins 85 & 86 while looking for the resistance changes noted above.
@second wind

I just tested 3 that came off my old relay board. 2 not working. Moral of the story - bench test to know you're dealing with a good relay. And don't ASSUME that if it works cold that it will work when hot. There are electrical connections and moving parts in the relay that may change behavior or become intermittent with temperature. They shouldn't - but they can.

This sort of testing is why a DMM is CRITICAL in my opinion. You could rig up the same bench test to work with a simple lighted test probe, but it's more work. Yes, you could see if the relay is working, but you can't measure resistance with a simple test light.

[Superhawk996]

Well guys...thank you again very much and I have a new clue to share. What an automotive hell hole I have had today!

Hmmm, well I could take the battery out of the 914 and hook the truck up too it and that should work....not even close....barely lit up the dash even though it worked on the 914. So I am standing in the street with jumper cables in my hand and in two minutes I am up and running because of my new best friends that stopped to help me. So the new clue is that the fuel pump doesn't run even when cold.....just sharing.

Sorry to hear about the added drama of the S10.

Going back to not assuming anything - are you ABSOLUTELY sure your 914 battery is good? Based on description above I wouldn't assume that. Yeah, a 914 battery might not be up to the task of cranking a S10 but it needs to be verified. So much of the troubleshooting you're going up against is going to need a verified 12v power source.

It is very possible for battery to show 12v but then collapse when a significant load is placed on it.

Again . . . just sayin' A bad battery could be why you're now noticing that the fuel pump dropped out even when cold. I don't know this for sure, but assuming the battery is solid is not a good baseline to start from given what you've just laid out.

If there is any question about its status, have it load tested at FLAPS.

[Superhawk996]

OK -- so now about the S10.

Sounds an awful like you may have a pretty large short somewhere in the S10.

1) A short would draw down your battery to some level lower than 12v and of course will drain the battery over time if left connected.

2) A short in the S10 COULD have been what collapsed your 914 battery (assuming it was good as you've assumed) to the point that it could barely light the S10 dash lights but yet the S10 could be jump started with a larger current source (the other car's alternator).


By definition a complete zero ohm short circuit will measure 0 volts if you measure the voltage drop across the short circuit.

WARNING - DON'T TRY THIS AT HOME KIDS!

If we were to take a good heavy duty copper cable about 2AWG and directly short the 12v + terminal to the 12v negative terminal while measuring the voltage of the battery with our DMM, it would read 0 volts. There will be a (IMG:style_emoticons/default/stromberg.gif) ton of current flowing but the voltage measured across the + and - terminals will be 0v (assuming resistance of the cable is ZERO ohms). The cable won't be absolutely Zero ohms but it will be darn close.

Think about that for a moment and let it settle in.

A short like that - in its most extreme case can pull a 12v battery very close to 0 volts. Lesser shorts can pull it to 11v, 10v, 9v . . . . all depends on the particulars of the short.

I've had to demonstrate this to people (wearing protective gear) because folks refuse to believe it.