Hopefully someone can explain how this relay works?

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inkjunkie

inkjunkie

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Not an A-body, it is my 02 Silverado. Truck cranks but will not start. Truck has a 4 pin relay that powers up several fuses which in turn run to the PCM and FCIM. Relay has me baffled. One pin is being fed from the ign c fuse, which is constant hot. Another pin runs to several fuses which in turn run to the PCM etc. This is the load side. One pin runs to an engine ground, which is obviously the ground. Which leaves one pin as the trigger, correct? Not according to the schematics I have as the fourth pin is also going to a constant hot. This relay has 2 constant hots, a ground and the load. Perhaps I am overlooking the obvious? Hopefully the attached images are readable. The relay is on 03

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You will need to crop the screen shots so only the wiring diagram is visable.

No detail as is now.
 
2 pins connect to magnetize a set of contacts to open or close. The other two are the contacts.
Typically this allows low amounts of energy to control a larger load
 
Dana67Dart said:
You will need to crop the screen shots so only the wiring diagram is visable.

No detail as is now.
Click to expand...
Fixed...at least they are cropped I should say

slickchick said:
fuel ?? spark ??
Click to expand...
It is a Duramax. But there is no power to the fuel injection control module.

Syleng1 said:
2 pins connect to magnetize a set of contacts to open or close. The other two are the contacts.
Typically this allows low amounts of energy to control a larger load
Click to expand...
This particular relay both of the + wires are constant hot...I have never seen a relay that the trigger wire was not switched.
 
I can't read those at all. Is it possible the DIAGRAMS are wrong? Have you checked with a meter to see what is actually hot?

What is the configuration of the relay?
 
67Dart273 said:
I can't read those at all. Is it possible the DIAGRAMS are wrong? Have you checked with a meter to see what is actually hot?

What is the configuration of the relay?
Click to expand...
Anything is possible. I got schematics from 2 different sources. They match. More pressing matter right now...will try and figure out how to post the diagrams when I get a chance.
 
67Dart273 said:
I can't read those at all. Is it possible the DIAGRAMS are wrong? Have you checked with a meter to see what is actually hot?

What is the configuration of the relay?
Click to expand...
Created the attached files in PS, saved as JPG...hopefully they are viewable. If they are viewable will someone let me know please?
 
1972GY8SE said:
Do you have a scan tool? If you do, are there any DTCs?
Will the engine light off an aux fuel source?
Please don't spray anything into the air filter.
Just a little wd directly into the throttle bore.
A little!
Click to expand...

Have scan tool, obd2 port is dead. Has power and ground. From what I have been reading if the CAN has no signal the scanner will not work.
harrisonm said:
Sorry, but how can I tell anything from those pictures.
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They show up on my phone just fine. After dinner I will print them and take a picture of them with my phone and post them.
 
If the relay in this image is the one you are referring to...

Screenshot_20220908-193839.png



The relay supplies power to ING 1 (red arrow )

The control of the relay comes from the "FICM relay control" providing a ground to energize the relay. (Green path to ground)
 
if the signal side of the relay is getting power, AND simultaneously it's ground side is grounded, it will read near battery voltage on both sides. You have to break the circuit to read it right.
 
AJ/FormS said:
if the signal side of the relay is getting power, AND simultaneously it's ground side is grounded, it will read near battery voltage on both sides. You have to break the circuit to read it right.
Click to expand...
???????????? Uh no.
 
Dana is on the right track. 79 C1 at the PCM is controlling the ground side of the relay which is a common method of switching. When the PCM pin "goes low" the relay triggers.

The relay top right terminal is fed from some main buss at it's 20A and branches off somewhere else. That is power to the relay coil

Power for the switched load is coming in at top left, the switched loads are supplied out the bottom left
 
They are simply switching the ground side of the relay instead of the hot side. Common when using an ecu to control a relay. So you’ll have two constant hots, a trigger side and a load side. Then a load, and a switched ground.
 
Well.....this is kinda baffling. Have checked this particular fuse at least a dozen times. Always has continuity. Checked the relay it is supposed to feed...nada. Probed the back of the fuse...one side is hot the other is not. So how does it pass multiple continuity tests but not pass current?

Still has codes. Guessing it is a p0875 (that is what started all this). From what I have been reading the internal harness on the Alison is frequently the cause, when the harness gets old the wick up fluid. I have a harness...
 
Odd thing is that code won’t cause a no start condition.
 
Dana67Dart said:
If the relay in this image is the one you are referring to...

View attachment 1715982741


The relay supplies power to ING 1 (red arrow )

The control of the relay comes from the "FICM relay control" providing a ground to energize the relay. (Green path to ground)
Click to expand...
That is indeed the relay.

TT5.9mag said:
The p0875 is pretty common in Allison’s. A faulty reverse pressure switch, faulty or dirty NSBU switch, or the internal harness are all common reasons for setting that code.
https://www.ecuforcetruck.com/produ...-reverse-pressure-switch-circuit-malfunction/
Click to expand...
Have replaced the pressure switch, the NSBU switch and the internal harness. The pressure switch also monitors the fluid temp...which is what started all of this. Trans temp, according to the TCM was 230*. Hit the trans sump with an infrared thermometer and it was 160*. Replaced the NSBU just because. Also replaced the internal harness. When I unhooked the external harness from the case it had a good amount of atf in it, so it needed to be replaced anyway.

TT5.9mag said:
Odd thing is that code won’t cause a no start condition.
Click to expand...
The no start.....this was a bit weird. Put an ohm meter across....need to go look to verify this...the ecmb fuse and it showed continuity. When I probed the rear of the fuse one side had 12 volts, the other did not. Replaced the fuse and the truck started. However, it still throws the generic code of p0700 along with p0875. And on a regular basis the shift indicator light will flash. Truck goes from slamming into gear to no engagement at all. I have read that the ignition switch can often be the culprit. But I am going to go probe around the NSBU wiring today, just want to make sure that when I was snipping the zip ties off of it I did not accidentally nip anything. When I cut the zip ties I was doing so somewhat blind...was not wearing my glasses.
 
If I do end up having to replace the ignition switch I am going to see if I can figure out what the toggle switch does that the previous owner installed does. I have had the truck since 07 or so. Seen the switch a few weeks after buying the truck. Left it in the position it is in out of fear of what will not work if I move it.
 
TT5.9mag said:
Sounds like a common NSBU switch fault to me.
Click to expand...
Just replaced the switch. The weather seal on the 4 pin connector is kinda a wreck. If I insert the connector with the seal on it the connector will not latch on. I left the seal out, reset the CEL. Did not matter. My old NSBU switch is under the truck still, I may try putting it back on, just to see.
 
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