24 Volts on Red GP Relay lead or 12?

[DLord]

TM-20 Manual says to dismantle and ohm check resistors if 10-15V is not present at red (top) lead to GP relay. Most threads here suggest that 24V is proper until load is applied. I have 25V at the top and 12.5 at the pink/blk (with key on "run"), and my relay clunks when the blue is applied- but I don't get anything at orange (bottom) therefore my GP's don't get voltage. I only measure 3.2 ohms across pink/blk and light blue, instead of the 6-10 that the TM-20 suggests, but I don't know if this is due to a bad module (card) or if another wiring devil is at work. I bought a replacement relay but still no voltage out of the orange leads. What am I missing?

 

[cucvrus]

Sounds like the relay on the firewall under the hood is bad. I have taken these apart after it was suspect to the problem ( it was the problem). Replaced it with a heavy duty solenoid and moved on. I hope that helps resolve the issue. I never refer to any TM's. I just solve the issue by a process of elimination. If that don't work I phone a friend. But all this point I resolve most things. Happy July 4th. Have a Great Day. I drilled the rivets out to look inside. It was the problem. See the arc on the one shoe. My Isuzu starters used to fail the same way. It was an easy fix.

 

[MarcusOReallyus]

Most threads here suggest that 24V is proper until load is applied.

That is correct.

I have 25V at the top and 12.5 at the pink/blk (with key on "run"),

That is correct.

my relay clunks when the blue is applied

What does "when blue is applied" mean? What are you doing to it?

but I don't get anything at orange (bottom) therefore my GP's don't get voltage. I only measure 3.2 ohms across pink/blk and light blue, instead of the 6-10 that the TM-20 suggests, but I don't know if this is due to a bad module (card) or if another wiring devil is at work.

Neither, likely. When you measured the resistance, were the wires connected? If so, your measurement is invalid. You need to disconnect the wires BEFORE you take a resistance measurement.

I bought a replacement relay but still no voltage out of the orange leads. What am I missing?[/QUOTE]


Is this a new problem, or did you just get the truck and it's always been this way? We need to know if there's a possibility that the wiring is incorrect. Are you sure you have the correct relay?

It's also possible that you have more than one problem.

 

[DLord]

Thanks for responses, and happy Independence Day, all.
So, truck is recently bought so i've not successfully started it yet. New batteries. New AC60's, Turns over solidly, good fuel supply through filter and at cylinder/injectors. Fuses are good.
Meant to say... "...when blue applied to ground (at module wire under dash) it clunks good". GP relay wiring is correct to be sure (working top to bottom: red; lt.blu; pnk/blk; oranges-bottom).
Thanks for reminding me to disconnect the wires to ohm check- I did NOT do that, so I'll recheck.
It seems like it has everything in order except voltage from bottom of at the GP's and perhaps a bad module card- which I'm in process of checking out via check list in TM-20 (which is where I got the BAD info stating there should not be 24V at top of GP relay). So, at the risk of making this response to responses even more tedious (sorry, guys!), Is it possible that the manual is CORRECT in stating that there should only be 12V at RED term. AND/OR, should i just go ahead and supply it with 12V from battery (as many have done here).

I'm thinking somebody's going to tell me to disconnect the resistors - but I'd really like to keep it 24V in the whole "spirit of keepin it real" kinda thing.

 

[MarcusOReallyus]

I have not read that part of the TM, so I'm not sure how it's worded, but here is the deal:

The resistor bank drops the voltage from 24v to 12v only when current is flowing to the GPs.

If there is nothing going to the bottom orange wires, there should be 24v at the top of the relay.

Let's try a test to clear things up:

1. Disconnect the blue wire. We'll ground that manually. This will leave the controller card out of the equation for now.
2. Make sure you have 24v at the top (from the resistor bank), 12v at the next one (key in run position). You should have nothing at the bottom two terminals.
3. If all of that is good, use a jumper to ground the terminal where the blue wire connects. Make sure it's a solid ground - we don't want any guesswork.

This should produce a solid clunk, 12v at the bottom terminal, and hot glow plugs.

1. If it does, your problem is likely with the controller card, or its ground.
   
2. If it does not, your problem is the relay itself or upstream of the relay.

Let us know.

 

[Merddin]

There is 24 v on the big red wire coming from glow plugs resistor until you engage the glow plugs and innitiate the wait light, then as the resistor comes under a load the voltage will drop to 12v. It is a two person job to check the resistor with a load test. You can check for resistance in the resistor with one person by disconnecting both ends and checking continuity.

So power comes In the big red wire. When your turn the ignition switch, power is sent into the electro magnet inside the relay which pulls the relay closed and current is transferred through and out of the big orange wires at bottom. The magnet works in opposition to a big spring g that is trying to keep the relay open. The small blue and pink wires are the circuit powering the relay. From factory blue wire is on top. But it really doesn't matter as long as they are both hooked up. When you engage the GPs there should be 12v on one of those wires. If there isn't the GP card could be bad.

Check for voltage there at the blue and pink wire. The relay should click when engaged. You should then have 12v at the bottom on the two big orange wires which go to the GPs. I would replace the relay, my original was bad and was sticking closed and sucking batteries dry. It was around 40-50$. Napa St-85 is a common replace ment.

 

[MarcusOReallyus]

The small blue and pink wires are the circuit powering the relay. From factory blue wire is on top. But it really doesn't matter as long as they are both hooked up. When you engage the GPs there should be 12v on one of those wires. If there isn't the GP card could be bad.

Not quite. The GP card provides the ground to the electromagnet. The 12v comes from the ignition key,so if there is no 12v, we need to look at that side of the circuit, not the GP card.

If we have 12v there and it still does not engage, then we should look at the GP card, the wiring to it, or its ground.

 

[Merddin]

Not quite. The GP card provides the ground to the electromagnet. The 12v comes from the ignition key,so if there is no 12v, we need to look at that side of the circuit, not the GP card.

If we have 12v there and it still does not engage, then we should look at the GP card, the wiring to it, or its ground.

Thank you for the correction, I couldn't find my glow plugs relay diagram and I did look. What I meant was power comes in the pink wire and is grounded through the blue wire via the card. I couldn't remember. My point was that the circuit must have power and a ground to energize the magnet and close the relay.

I like the test you suggested to bypass the GP card temporarily.

 

[MarcusOReallyus]

There ya go.

 

[DLord]

Thanks, gentlemen. Manually grounding the relay gave me the voltage at the bottom I was hoping for. I confirmed that the Lt Blu wire had continuity down to the module pin, so it apparently is the module. Now I have some run-away magic smoke from my resistor pack, so I guess I'll get those too- if not just direct-source the top terminal with 12V from the battery. Anyway, thanks again for walking me through this. Truck still doesn't start or even puff unspent fuel, but you've gotten me through this phase of it, so I "feel like" I'm getting closer.

 

[MarcusOReallyus]

Great news!

So, now you need a controller card. You came to the right place. One of our members has a company that makes replacement cards, with improvements. His website is here. (He needs to get a new hosting company - the site is down at this moment and it's not the first time I've seen it down. )

As for smoke from the resistor pack, don't sweat it. When the GPs are energized, those things get HOT. (I did the math once and IIRC it was about 1200 watts. Yeah, a room heater!) Bugs and dust will smoke. Nothing to worry about. Pull it out and hose it down, put it back.

Get your truck running, then decide if you want to bypass the resistors.