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bad connection or relay


that-son

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DSCN0015_zpsdaeaa0a8.jpg

 

shouldn't there be a relay where that jumper cable is?  is that the headlamp relay next to it?

 

last night i turned on my headlights and i heard a buzz then all the lights went out.  i gave the dash a whack and the lights came back on but the headlights were still out.  checked the fuses and they were good.  pulled out that one relay. put it back in and everything works again.  coincidence or is that really the headlamp relay?  my maual is underneath a whole lotta $h!t right now and don't have time to pull it out and look it up.  do these relays go bad or is there a known connection problem in the 720's?

 

thanks!

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My 1981/2 diesel wiring harness is the same when it comes to the headlight circuit, and my headlights don't come on for a long time when it is cold, sometimes it takes more than a minute, I found that I could help it along by tapping on the relays on the fuse block, but it doesn't always work.

Now some will say that neither of these relays are for the headlights, so why then will the headlights not work when the relays are removed I ask, and why does this wiring diagram say there is a headlight relay?

DSCN3403.JPG

Your relays should both be there, I cannot recall which is the heater relay and which is the headlight relay anymore.

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The headlight relay only selects high or low beams so one or the other will always work. It's the black one on the far right. The missing relay is for the heater and does not affect the lighting.

 

Pull the three fuses on the far right and either replace them or plug them in several times to clean the contact surfaces. Do the same with the black relay... this is about all you can do right now.

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help!!!

 

i found a relay.  part number ar210.  it fits, however the truck will not start with the relay plugged in.  it (the relay) just clicks , the relay above the passenger side kick panel clicks but the starter and the motor does not turn.  take the relay off and plug the jumper wires and it will start again but... no heater and no ac.  so... wrong relay?  faulty relay or faulty wiring?  i cleaned all the contacts where the relay plugs into btw.

 

also, where is my fuel pump relay?

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DSCN0015_zpsdaeaa0a8.jpg

 

This is a hokey set up and I have no idea what was done to it. I can only say what was installed in the 720 from the factory.

 

The ignition gets it's power directly from the battery and sends the start signal directly to the starter. There should be nothing whatever in the fuse box to interfere with this set up. It may not start, but it will always crank over with the key.

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Looks like someone rigged it in then. I would trace those wires and rewire your new relay to that blue connectors.

Theres not much to the starting system on these trucks.

 

relay is just a switch wire the ignition to one side of the relay and wire b+ and stater solenoid to the other. I bet someone was broke and couldnt

get a relay so the bypassed the thing directly to your ignition.

 

I'm looking through alldata right now and I dont seem to have a diagram for you. But I do have it on all the other 720's

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Follow those two wires and try to figure out where they go. The truck will run fine without the heater or heater relay in place. someone is using it for a voltage supply for something else.  If disconnecting those wires prevents the starter from working then maybe the fusible link that supplies the ignition is blown.

 

The fusible links are connected to the battery positive terminal. Your 720 should have three.... a Black and a Green somewhat paired together and a separate Green one with it's own connector. Inspect the Green one that's paired with the black one. If burned out replace it. This is the power supply to the ignition switch.

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Granted, mine is an "80, but the fuse block looks the same…or I imagine should.  I just snapped some shots of mine, which is very factory looking. My truck starts (when it is assembled and not motor frozen, but that is a different issue).  My pic is also with the cover off, as I was fiddling with them recently and the cover is still on the passenger seat. IMG_0342.jpg

 

IMG_0343.jpg

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Sorry :blush:  can't stop sometimes.

 

The '80 was the end of an era and the last of the L series L20Bs. It was under powered and not that great for emissions. The alternator was I believe still 35 amps output.

 

The '81 alternator puts out 50/60 amps and the fuse box had two fuses for the headlights so if one blew you still had the other lamp on. There were two coils and one was powered from it's own fuse. Rear window defogger had it's own fuse. Wiper (15 amp) and heater (20 amp) were now on their own fuses instead of on a shared 15 amp fuse. The Z series was a new pollution control revolution requiring more circuits and relays and more things in the future. California was even worse. .

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wow!  thanks for all the help!  i could not get to the relay today.  in fact i did not get much done today...

 

1. drain gas tank - gas was clear and no debris.  will install new fuel filter tomorrow

2. change oil/filter - could not get dip stick out.  aftermarket dipshit i mean dipstick broke off and now the rest of it is in the tube.

3. compression test -

4. check a/b valve

5. adjust spark plugs.  plugs look great but should have checked gap before installing them 3 months ago.

 

tomorrow...

 

1. repair egr leak

2. repair shifter boot

3. and adjust idle speed and air/fuel mix again

 

tuesday is the big day.  let's hope i pass smog.

 

thanks again.  i will get to the bottom of this relay soon...real soon. 

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I have a 1980 4X4 wiring harness I used in my 520, it is not like the one in the photo above(post 11), I use all the circuits except the front circuit/fuse, and I only have 2 relays.

DSCN3423.JPG

 

Damn wayno I know how much work this represents. Put a later 720 harness in my 620.

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i am reallay bummed.  could not even drive to the smog station today.  last night everything was perfect.  timing, idle speed, air/fuel mix... i even ended up replacing two fuel lines connected to the fuel filter.  my dad drove it around at least an hour, check everything before parking it in the garage and this morning...  timing changed again to 10 deg btdc.  prior to that it was 15 btdc.  what could cause that?  i tried turning the distributor by hand but it wouldn't budge cuase i made sure it was tight.  anyway, i set it back to 5 btdc, and adjusted idle speed and air/fuel again.   everything's perfect again until i drove off..  there was no power, backfiring wouldn't hold idle speed.  brought it back to the house, let it warm up even more, checked everything timing, etc... everything checked.  drove off again but still the same issues.  finally parked it for 3 hours, checked timing and wouldn't you know it... 20deg btdc.  wtf!?  turned distributor by hand and sure enough it's still tight and secure.  what do i do now?

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are you adjusting your idle after you set the timing? The timing curve changes depending on rpm. Also if you have vacuumm advance you should unplug it and cap it on the carb side. When plugged in make sure you dont have any vacuum leaks or problems. If you also have a mechanical advance check that as well.

When finished Mark a line on the distributor to make sure it's where you left it.

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doh!!!  i read it many times on the sticker under the hood and of course in the manual!  thanks for the help!

 

i must say there is a marked improvement from when i thought it was running great to tonight!!!  i took it on the freeway and there's this stretch going up the mountain... i usually pop it in third so i could keep up with the big rigs.  tonight i put it in 4th and i was actually overtaking the big rigs!  tried it in 5th and it held it's speed and i didn't have to put the pedal to the metal.  lol.  i hope it passes smog!

 

thanks again!

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  • 9 years later...

Day zen mike you are a god read everything 720 you are beyond knowledgeable. Recently I had a problem occur with my 84 z24 4wd fuse panel I have a dead spot in the middle where I have had a burnt up spot where blower motor fuse is now I have now power in the middle of my fuse box. This started by running my blower motor with a bad resistor on full blast just to get by it kept on blowing the 20s and all of a sudden I have no power in the middle of my fuse block is this a bad blower motor relay or if I replace my entire block will I have my power again 

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