M880 Electrical Problem?

kg5252

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Clarksville Ohio
My M880 has been put back together and I have driven it multiple times since. The truck runs, starts and idles well. All of the gauges and all of my lighting works. I removed the entire blackout light system from the truck. The problem I am having is that in low rpms, everything on the electrical system of the truck is very dim, and when coming to a stop to make a turn, which would be in low rpms, my turn signals will not function. When the truck gets revved, the electrical system brightens up. Even when the truck is at idle, the alternator gauge is reading that it is charging and we have not had a problem with the battery being discharged.
When I bought the truck, the previous owner gave me a new electronic control unit and a resistor that is mounted next to it on the inside of the left fender. The ECU on the left fender and another (similar looking unit?) on the firewall have both appeared to have gotten hot and melted the substance that is inside of them. Which of these units controls the motor and which one controls the lighting. I am afraid to put the new pieces in because I do not want to melt them too.
When the vehicle is running, we tried beeping the horn and got 15 volts, but while testing the headlights, we only got 8 volts. I have read that some of these trucks have a low voltage coil that only put out around 8 volts. Is this my problem as to why I am having such dim headlights and turn signals?
 

NDT

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The firewall mounted electronic device is the voltage regulator for the alternator. The fender mounted device is the ignition module. I have seen many of these where the substance inside (potting) has run out sitting on a shelf, so that does not mean they have overheated, just gotten old. Check the connections on your ammeter, that is a common problem area that could cause your voltage droop. The entire system voltage should be 13.5 or so any time the engine is running, at any RPM.
 

Flyingvan911

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Kansas City, MO
My dad had a '74 Dodge pick up. Basicly the same truck. He always kept a spare ballast resistor and ignition module in the cab as both could fail without warning and leave you stuck. The distributor cap was also prone to cracking.
 
Last edited:

NDT

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The amp gauge in the instrument cluster. Known to overheat and cause high resistance to electricity flow.
 

m38inmaine

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Maine USA
The electrical design is weak and a poor design. It's best to bypass the ammeter and run the main power lead direct, as designed the full load is running through the meter, a cause for many M880 electrical fires. There is a website that gives a tutorial on how to do it, just give a search. The alternator on these acts more like a generator and dims at idle, unlike a true alternator.
 

M543A2

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Agree that the ammeter in the dash is a weak design. The connections depend on plastic mount ot keep them tight. The plastic will melt at the gauge terminals, causing a bad connection which then builds more heat causing more problem and possibly a fire. Pull the dash and check the amp gauge mounting for signs of melting. You may be able to repair it with washers and double nuts to be sure contacts stay tight. On one I did run a direct wire as suggested by another response. If you want an amp gauge you can mount one in the cab and run through it with the direct wire.
 

N1265

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Fremont, Ohio
Most of these old Dodges have that "headlight flair " at idle:

1) preform the mod as described in the link above from madelectrical, even if your unit is not as bad as the one in the pictures it is good insurance going forward.

2) double check all connections and make sure they are clean

3) double check the curb idle RPM and bump up if needed ( I run 900 )

4) Tuff - Stuff makes a direct replacement 100 AMP alternator to replace the stock 60 amp, it is a big improvement and built inside the stock case so it looks original, be sure to replace the Voltage Regulator on the firewall and make sure the case is grounded. ( stock VR is fine but try to get an original mopar part )

5) If you are going to do any plowing with this truck and have an electric plow pump, skip the 100 AMP alternator and go straight to a GM 140 amp 1 wire ALT conversion with a dual battery set up. ( a bit of fabrication needed, but you wont be sorry )
 

michigandon

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Wake Forest, NC
Our truck has had the OEM ammeter bypassed, and an aftermarket Stewart and Warner unit now resides where the 24V voltmeter used to be.

It was that way when we purchased it.
 

Snow Chief

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Location
milwaukee wisconsin
My M880 has been put back together and I have driven it multiple times since. The truck runs, starts and idles well. All of the gauges and all of my lighting works. I removed the entire blackout light system from the truck. The problem I am having is that in low rpms, everything on the electrical system of the truck is very dim, and when coming to a stop to make a turn, which would be in low rpms, my turn signals will not function. When the truck gets revved, the electrical system brightens up. Even when the truck is at idle, the alternator gauge is reading that it is charging and we have not had a problem with the battery being discharged.
When I bought the truck, the previous owner gave me a new electronic control unit and a resistor that is mounted next to it on the inside of the left fender. The ECU on the left fender and another (similar looking unit?) on the firewall have both appeared to have gotten hot and melted the substance that is inside of them. Which of these units controls the motor and which one controls the lighting. I am afraid to put the new pieces in because I do not want to melt them too.
When the vehicle is running, we tried beeping the horn and got 15 volts, but while testing the headlights, we only got 8 volts. I have read that some of these trucks have a low voltage coil that only put out around 8 volts. Is this my problem as to why I am having such dim headlights and turn signals?
Would like to remove the blackout wiring and put back to stock,is there any tricks or advice as it looks like a hot mess.Thanks in advance I know this is an old post but don't know where to start.
 

Sweet5ltr

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Location
Jacksonville, Florida
Also, check out LMC Truck for the headlight harness. Instead of pulling power directly through the bulkhead & headlight switch, it will pull headlight power directly from the battery. This is a direct 'plug-in', with no modifications required. Plug your old harness into the new one, that's it.
 

deathrowdave

Active member
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Location
falmouth, ky
The AMP meter is toast as mentioned , the firewall plug is more than likely crusty also they are not in the best location . Remove it clean the connector , look closely at the main power connections ( Amp Meter ) in and out , 90 percent change they are cooked also . Remove these terminal , drill through the connector, pull new wires and solder outside of the junction box , bypass the AMP meter you will see a great change in electrical system operations .
 
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