M1088a1 disconnect switch relay problem

[abfisher1798]

The original relays finally quit so I replaced with the newer model, single box relax setup. We turn battery and master switch to on and dash powers up as usual. Push the start button and one of two things happen:
1. Everything goes dark, like a blown fuse. Cycle the battery disconnect switch and after a few minutes power is restored, but issue remains.
2. Relays at battery disconnect start cycling on and off at a regular pace; click-power on, click-power off, and so on.
I’ll post some pictures. I’m sure someone has ran into this before.

 

[NDT]

Sounds like intermittent connections over on the LBCD under the spare tire. 12 and 24 volt battery cables from the disconnect solenoids lead to this box and can be corroded at the ring terminals.

 

[Ronmar]

Yep, i suspect NDT is correct. a bad connection between that disconnect relay and the power panel in the dash. When you pull current thru a bad connection, like a kink in a garden hose, the resistance causes a voltage drop. This only happens under load, so voltage isn’t considered good until tested under load.

The added power drawn during the attempted start causes the voltage to drop below that which will keep the relays powered. The relays open, this causes less current to be drawn, the voltage increases, which pulls the relays back in and restarts the pulsing cycle.

Voltage drop under full load should be less than 3% from source to destination plus the 1/2-3/4v that the polarity diodes in the polarity box/LBCD knocks off.

 

[Ronmar]

Just got a chance to look at your pics. Thats a weird way to measure voltages. The black lead should be connected to chassis ground. Then one of those pathways should read 12v and the other 24v. The voltage should disappear beyond any open disconnect sw or relay. Measuring at the various connections(batteries, disconnect box, LBCD and power panel batt and ignition bolts) both loaded and unloaded you should be able to see where the issue lies When loaded.

You can lower the spare tire to access the LBCD terminals. There is a label on the side that indicates which terminal is which. pay close attention and insure the 12 and 24 batt terminals have 12 and 24v respectively measured to ground

 

[abfisher1798]

I checked that LBCD box (I assume that is also referred to as a polarity box). Both 24v and 12v read fine. Still I removed each cable and cleaned them off and retightened and the result was the same. I also checked the battery and ignition bolts in the fuse panel and they came back fine.
while the relay is cycling the on shows normal voltage and the off drops to 2-3.
I started pulling relays to isolate the problem and no change.
There is evidence of a short at the TCM fuse. Photo is attached. It was not there when I received the truck. The bottom of the circuit board is fine and other than that black spot, I don’t see any other questionable areas.
I don’t know if that’s even relevant or not.

 

[coachgeo]

.....
There is evidence of a short at the TCM fuse. Photo is attached. It was not there when I received the truck. The bottom of the circuit board is fine and other than that black spot, I don’t see any other questionable areas.
I don’t know if that’s even relevant or not.

hmmm.. that dont look good..... hope somewhere in there nothing got friend.

LBCD= Load Balance Compensating Device [something like that]

-eventually on A1 trucks the LBCD replaced the Rev. Polarity Box of the A0's. It has a few more tricks up it's sleeve compared to the RPBox. Think some of those involve partnering with the shut off solenoids on A1's

 

[abfisher1798]

During one of the “dead” moments when all switched were powered on and nothing was happening, I jumped the 24v relay underneath the control board and found that would “awaken” the truck, but only to continue its cycling on and off. I jumped both the 12 and 24 relay and no change.

 

[Ronmar]

So this starts its cycling when you push the start button?

 

[Ronmar]

What exactly is clicking? Which relays? Did the power at the 12 and 24 batt bolts in the power panel drop during the clicking?

i am trying to get an idea where to tell you to bypass, so you can stop the clicking and do a proper load test to find where your voltage is dropping off Under load.

 

[abfisher1798]

So this starts its cycling when you push the start button?

No. I apologize for being unclear.
Underneath the power distribution panel are the two large relays, a 12 and 24v. Each have a set of thick gauge cables that (I think) link the battery and ignition together. They also have a set of two small wires that connect to the disconnect relays.
When I jump that 12v relay, it wakes up the truck which then begins the initial cycling on and off of the disconnect relay.
If that’s still not clear, I’ll take photos in the morning and attach.

 

[Ronmar]

No. I apologize for being unclear.
Underneath the power distribution panel are the two large relays, a 12 and 24v. Each have a set of thick gauge cables that (I think) link the battery and ignition together. They also have a set of two small wires that connect to the disconnect relays.
When I jump that 12v relay, it wakes up the truck which then begins the initial cycling on and off of the disconnect relay.
If that’s still not clear, I’ll take photos in the morning and attach.

Those are the ignition relays, K1 and K2. They connect 12v batt to 12v ign, and 24v batt to 24v ign when you turn on the ignition switch. Those relays shouldn’t have any connection to the battery disconnect relays.

battery 24 goes thru CB45 thru half of the battery disconnect sw under the steering wheel to the ignition switch. When you turn on the ign switch it powers the ignition relays to energize the 12 and 24 ign circuits.

so are K1 and K2 ignition relays clicking? Is that new battery disconnect relay clicking?

 

[abfisher1798]

Those are the ignition relays, K1 and K2. They connect 12v batt to 12v ign, and 24v batt to 24v ign when you turn on the ignition switch. Those relays shouldn’t have any connection to the battery disconnect relays.

battery 24 goes thru CB45 thru half of the battery disconnect sw under the steering wheel to the ignition switch. When you turn on the ign switch it powers the ignition relays to energize the 12 and 24 ign circuits.

so are K1 and K2 ignition relays clicking? Is that new battery disconnect relay clicking?

The new battery disconnect is clicking. Either one or both of the ignition relays are also clicking. I can hear it clear as day. It also feels like they’re both clicking, but it also might be the vibration from one clicking that makes the other feel the same.
The ignition relays in my truck have wires coming from them that are hand labeled as “from batt disconnect” written on duct tape. That’s why I assumed they went.

I bypassed the battery disconnect using jumper cables to go from the batteries to the inboard side of the relay and the click on, click off continues.

 

[Ronmar]

Ok was going to say bypass the disconnect relay if it was clicking, but since you have done it already and the ignition relays are still clicking, then the issue must be the supply power feeding the ignition relays. Ig you jumper from the BATT to the IGN terminals in the power panel, that will keep the ignition power on. The relays however may still click. You need to measure the power reaching their coils. It could also be that one has a bad coil…

 

[abfisher1798]

Here are some photos of the ignition relays under the PDP. You’ll notice a set of two wires coming from both with the handwriting label that says BAT RELAY.

 

[abfisher1798]

Jumped the ignition to battery bolts. No change, still cycles on and off. I’m pretty confident that both the 12 and 24 volt ignition relays are clicking.
Everything powers up, dahs lights and low air alarm for 3 seconds. Them the 12v clicks followed immediately by the 24v relay, then all goes dark.

 

[Ronmar]

With the batt disconnect relay and the 12 and 24 batt to ign points jumpered up in the power panel, what voltage are you measuring for 12 and 24 at the power panel?

Regardless of whats clicking, With those 3 relays bypassed with jumpers, you should have steady power between batteries and the power panel. If you do not see within a volt of what you have at the batts, you have a bad connection between batteries and power panel that is breaking down/dropping the voltage when current is pulled by the load applied.

Those 2 small wires on the ign relays are how they are powered. Dont care what some misguided soul wrote on some duct tape(although it looks like they may have been trying to keep track of which ign relay is which). what do the metal band labels and wire numbers printed on the wires read?

 

[Ronmar]

The bands on the two small wires on the 24v relay(K1) should be labeled TL 505 & 506. The bands on the small 12V ign relay(K2) wires ahould be TL 168 and maybe TL 169(not labeled on the schematic I have).

TL 505 and 168 should be 24V from CB45 via the battery sw and ign switch. TL,506 and 169 should connect to TB2/ground…

 

[Ronmar]

Any luck? What I think you are experiencing is the same thing that hapoens when you kink a garden hose. You will have full pressure(voltage) all the way to the nozzle at the end of the hose, untill you open the nozzle. Then the pressure at the nozzle drops off drastically as you try and flow water past the kink.

I finally found some documentation on how that battery disconnect relay module is supposed to work. It clicking is an interesting clue which might indicate that your power issue is between batteries and that disconnect relay...

 

[abfisher1798]

Any luck? What I think you are experiencing is the same thing that hapoens when you kink a garden hose. You will have full pressure(voltage) all the way to the nozzle at the end of the hose, untill you open the nozzle. Then the pressure at the nozzle drops off drastically as you try and flow water past the kink.

I finally found some documentation on how that battery disconnect relay module is supposed to work. It clicking is an interesting clue which might indicate that your power issue is between batteries and that disconnect relay...

It has to something like that. I’ve ruled out practically everything else to no effect:
-bypassed battery disconnect relay
-swapped TCM with a known good unit
-recharged the batteries individually
-bypassed the LBCD
-added extra ground
My next idea is to cannibalize the battery disconnect and relay from a working FMTV and installing that in the clicker.
I’m really at a dead end. I’ve passed everything I can think of. Nothing I’ve done so far makes any difference. I could buy new batteries as well, but I don’t think that would be the sole problem. They all tested as good.

 

[NDT]

Looking at your pictures, I think you have the trouble-prone second generation PDP with the no-lead solder. Suprman knows a lot about these, I don’t. These boards have been known to have bizarre intermittents like you describe.