Battery switch on (heater fan, lights work) no gauges or engine crank?

[oliver28732]

Battery switch on no power- Hammer to PCB and 40 minutes later works!

Just like the title says. I flip the battery switch to the "on" position. Heater fan in LOW works along with vehicle lights but no gauges and engine does not try to crank. When I flip the battery switch off I hear a thud left front of steering wheel (PCB?). I am printing out the "All vehicle electrical systems inoperative" and "starter motor inoperative; no solenoid thump" from TM 9-2320-272-23-1 trying to figure it out. So I am "RTFM" but would appreciate any additional advice. Truck is a 1990 M925A2 and last 5 or so times starting it had the same issue but a few flips of the battery switch back and forth brought it to life. Thanks!

 

[Artisan]

I am no master mechanic but a few "suggestive" q's;
WHAT IS THE VOLTAGE OF ALL 4 INDIVIDUAL BATTERIES Check It.
ARE ALL 4 BATTERIES FULL OF WATER W/ PROPER ELECTROLYTE LEVELS? Check It.
WHEN WAS THE LAST TIME YOU CLEANED EACH AND EVERY POS AND NEG BATTERY POST AND CONNECTOR W/ A WIRE BRUSH? Check It.
MAKE SURE ALL BATTERY NUTS AND BOLTS ARE TIGHT Check It.
If you do the above thoroughly it will help others by eliminating this.

 

[oliver28732]

Ty for above suggestions. I checked out the PCB connection, performed the reset per TM and tapped the box several times with hammer. No joy. Went inside for about 40 minutes, came back out because of a SS member (ty Adam) for calling to help and truck started right up. So this probably means the PCB is going?

 

[Artisan]

I have an M925A1 so things are different but the heater, on LOW in my truck, is wired OE to only use 12V. On high it uses 24V. I suggest? it is the same for your truck, you should check, look at the electrical schematic in the #4 Maintenance TM on page PDF 820 I think for you, it is right after the A1 schematic. FWIW. I highly recommend doing the above and when you open the battery box make sure nothing is grounding out. Make sure all rubber isolators are in place per drawing. Another issue I had and so have others is the battery wires from the battery box to the starter rub on I think it was the left rear corner of the engine exposing bare wires. MINE was really bad, I only found it when I unbolted all wires from the starter to add a ground wire from starter to block AND CLEAN the cable ends. I did a quick fix by slipping a 14" piece of dishwasher rubber hose OVER the entire #6 or #7 (I am not sure today) wire covering the bad spot. It is almost impossible to see the battery wires from under the truck and or thru the tranny fluid check access hole in the cab but they are know problems. It is on my list to move my batteries so I will address this soon w/ new wires. There is probably 2 small wires that parallel the big battery wires that you need to make sure are sound as well. I think it was the 569 wire (to heater for 12V I think) and 60 wire that runs to the Amphenol Diagnostic connector under the passengers knees so when you plug in a diagnostic tool you get a voltage reading. I am kinda bad w/ electricity bad necessity is the mother of invention and I have been learning a thing or two pouring over my truck.

 

[greywulf4570]

I concur with Artisan...
I wouldnt say the PCB is going bad just yet. Electrical connections suffer the worst effects of corrosion literally everywhere in the vehicle despite the best efforts to keep moisture and crud from finding their way in. after reading the TM in the starting circuit, the main thing to check is whether the connections have corrosion issues. Use the multimeter tests to read the voltage drops like the book says. Go from point to point and rule them out one by one...the truck has several "safety interlocks" that must provide solid continuity to get the starter to spin.
On your starter solenoid, clean those connections and then spray them down with Bullfrog or WD40... corrosion related problems can be a bugger... kill the rust... clean nuts, washers and points of contact... it will make a huge difference. Cheers. One of the beauties of this vehicle is in its simplicity of circuitry and how easily (relatively) it can be repaired.

 

[Neophyte]

I have the exact same issue. I picked up a PCB and plan on putting it in this weekend (I'll let you know how it works). Does your ABS light come on also?

 

[Retiredwarhorses]

I had that issue with a 923, it was a PCB....I found one used for 50.00 and no more issues...make sure you get the right one...800 series look the same but won't work.

 

[Neophyte]

Put a new ECU in today....problems solved (intermittent starting, ABS light on). Does anyone repair these?

Thank you

 

[oliver28732]

Replaced PCB today and starts like a champ now. On to the ABS light staying on and certain lights not working! Ty to those who posted and IM'd me about this issue.