Weird Voltage Fluctuation with Backfiring
#1
Nam Labrat
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Weird Voltage Fluctuation with Backfiring
I tried to work on the project after 3 months of being in a "PTSD SEMI-COMA".
Details:
Project-Car has been "de-bugged"/electrical upgrades made within the last year and ran perfectly for previous 6 months
Car sat in garage for 3 months untouched
1968 convertible
1970 350 engine with new aftermarket HEI Cable-Drive Distributor
New 130 amp alternator (14.9V max output)
New "heavy duty" charging wire @ alternator
Firewall wiring in good condition (maybe?)
New dash wiring harness
New fuse block (original fuse block was corroded internally)
New 800 amp battery
New Positive cable from battery to starter
New Negative cable from battery to frame
New starter
Edlebrock carb rebuilt/has worked perfectly/mechanical fuel pump @ 6 PSI
Engine idles perfectly at 700 RPM ...... Timing set at 12* TDC
________________________________________ _________________
Symptoms:
Engine backfires/loses power above 700RPM
________________________________________ ______________
Troubleshooting revealed this with digital multimeter:
Alternator works perfectly 13.0 to 14.9 V with NO fluctuation
13.0 to 14.9 volts at horn relay with NO fluctuation
13.0 t0 14.9 V at battery with NO fluctuation
HEI (BAT terminal) shows 0.2 to 13.0 V FLUCTUATION ABOVE 700 RPM.
ALL FUSES SHOW 0.0 TO 13.0 V FLUCTUATION WHEN ENGINE IS REVVED ABOVE 700 RPM
________________________________________ ______________
I will lift the car this week and trace/probe main wiring at the starter and at the firewall with the engine running to see if voltage fluctuation is happening BEFORE wiring enters the new fuse block.
Did I miss something.........Does anyone have an idea as to what might cause the fluctuation?
Details:
Project-Car has been "de-bugged"/electrical upgrades made within the last year and ran perfectly for previous 6 months
Car sat in garage for 3 months untouched
1968 convertible
1970 350 engine with new aftermarket HEI Cable-Drive Distributor
New 130 amp alternator (14.9V max output)
New "heavy duty" charging wire @ alternator
Firewall wiring in good condition (maybe?)
New dash wiring harness
New fuse block (original fuse block was corroded internally)
New 800 amp battery
New Positive cable from battery to starter
New Negative cable from battery to frame
New starter
Edlebrock carb rebuilt/has worked perfectly/mechanical fuel pump @ 6 PSI
Engine idles perfectly at 700 RPM ...... Timing set at 12* TDC
________________________________________ _________________
Symptoms:
Engine backfires/loses power above 700RPM
________________________________________ ______________
Troubleshooting revealed this with digital multimeter:
Alternator works perfectly 13.0 to 14.9 V with NO fluctuation
13.0 to 14.9 volts at horn relay with NO fluctuation
13.0 t0 14.9 V at battery with NO fluctuation
HEI (BAT terminal) shows 0.2 to 13.0 V FLUCTUATION ABOVE 700 RPM.
ALL FUSES SHOW 0.0 TO 13.0 V FLUCTUATION WHEN ENGINE IS REVVED ABOVE 700 RPM
________________________________________ ______________
I will lift the car this week and trace/probe main wiring at the starter and at the firewall with the engine running to see if voltage fluctuation is happening BEFORE wiring enters the new fuse block.
Did I miss something.........Does anyone have an idea as to what might cause the fluctuation?
Last edited by doorgunner; 07-11-2015 at 11:46 AM.
#2
Burning Brakes
with all of the wiring work and replacements, I would look at the dash harness and fuse block connections for problems first. you could also bypass the key switched power to the distributor and jump it straight from the battery and it should run just fine, pointing to the run circuit. of course, you'll have to yank the wire off by hand to kill the engine.
#3
Melting Slicks
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with all of the wiring work and replacements, I would look at the dash harness and fuse block connections for problems first. you could also bypass the key switched power to the distributor and jump it straight from the battery and it should run just fine, pointing to the run circuit. of course, you'll have to yank the wire off by hand to kill the engine.
#4
Nam Labrat
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Thanks Men.....The battery ground is new and shows NO fluctuation at the battery post. Is there another ground I should check?
I'll also check the ignition switch positive terminal for fluctuation before I take the new fuse block off to open it up and look for a loose connection that supplies voltage to it.
I remembered that for the last year there has been a 1 second voltage drop that would cause the engine to miss for a split second....
I guess the problem has worsened to the point that I must find the cause
I'll also check the ignition switch positive terminal for fluctuation before I take the new fuse block off to open it up and look for a loose connection that supplies voltage to it.
I remembered that for the last year there has been a 1 second voltage drop that would cause the engine to miss for a split second....
I guess the problem has worsened to the point that I must find the cause
#5
Le Mans Master
I dealt with a frayed ground wire at starter (on my '80, it attaches to one of the bellhousing bolts to engine block). It runs in the same harness to the starter area with the 2-3 wires thatconnect to starter. The wire feeding into the grounding ring was frayed and weak. I snipped it off and replaced with a new ring. Now, my C3 spins over hot or cold without issues. Also, it holds charge for months without noticeable loss.
NOTE: I have disconnected the OEM clock and I also went with a newer radio (with memory) that no longer appears to drain the (2-year old) car battery. New starter and battery installed together also (reman starter).
NOTE: I have disconnected the OEM clock and I also went with a newer radio (with memory) that no longer appears to drain the (2-year old) car battery. New starter and battery installed together also (reman starter).
#6
Melting Slicks
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Location: Downers Grove Illinois
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Thanks Men.....The battery ground is new and shows NO fluctuation at the battery post. Is there another ground I should check?
I'll also check the ignition switch positive terminal for fluctuation before I take the new fuse block off to open it up and look for a loose connection that supplies voltage to it.
I remembered that for the last year there has been a 1 second voltage drop that would cause the engine to miss for a split second....
I guess the problem has worsened to the point that I must find the cause
I'll also check the ignition switch positive terminal for fluctuation before I take the new fuse block off to open it up and look for a loose connection that supplies voltage to it.
I remembered that for the last year there has been a 1 second voltage drop that would cause the engine to miss for a split second....
I guess the problem has worsened to the point that I must find the cause
#7
Burning Brakes
I am wondering what is changing above 700rpm? Could this be when the engine gets enough momentum to shift towards the passenger side and cause a bad connection - probably ground?
I had a similar problem on a 67 Chevelle years ago, where I could turn a corner and it would die - electrical. Then it would crank right back up and be fine. Took me a while to find the + cable was missing some insulation and shorting just enough to shut down when I made a turn.
Good Luck
I had a similar problem on a 67 Chevelle years ago, where I could turn a corner and it would die - electrical. Then it would crank right back up and be fine. Took me a while to find the + cable was missing some insulation and shorting just enough to shut down when I made a turn.
Good Luck
#8
Nam Labrat
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Thanks Men....I'll check those areas by connecting the multimeter and positioning it so I can see it as I move each wire separately (I unwrapped the firewall harness to check for damaged/pinched wires...the purple wire had been repaired 3 times/it's getting replaced too!)
It seems to have gone from an intermittent slight miss and has gotten much worse when I replaced the starter yesterday....Hmmmmmmmm!
47 year old wiring...about time it failed...LOL
I did manage to put about 30 miles on those new tires before I limped home.
It seems to have gone from an intermittent slight miss and has gotten much worse when I replaced the starter yesterday....Hmmmmmmmm!
47 year old wiring...about time it failed...LOL
I did manage to put about 30 miles on those new tires before I limped home.
Last edited by doorgunner; 07-07-2015 at 10:57 PM.
#9
Nam Labrat
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After studying the wiring diagrams it seems the main supply wire to the fuse block must be damaged.
If I move the harness around with the engine running the voltage will stabilize at the distributor 12V connection...so the harness must have been pinched in the past during engine removal, which explains several repairs on smaller wires.
Rather than continuously dealing with failing harness wires/splices from previous owners.....I ordered a harness from Willcox to connect to the new dash harness (original was spliced/repaired in 5-6 places)
If I move the harness around with the engine running the voltage will stabilize at the distributor 12V connection...so the harness must have been pinched in the past during engine removal, which explains several repairs on smaller wires.
Rather than continuously dealing with failing harness wires/splices from previous owners.....I ordered a harness from Willcox to connect to the new dash harness (original was spliced/repaired in 5-6 places)
#10
Burning Brakes
Check the fusible links that feed the fuse box and the lights. They (2)connect at the starter terminal (stud) and run to the fuse box and the headlight switch.
#11
Nam Labrat
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Thanks.......That's what I thought when I read the wiring diagram last night. After unwrapping the complete engine harness, it's so "messed up" that I ordered a new harness (And I;m one who replaces original parts as a last resort)
#12
Nam Labrat
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I re-tested everything......same results.
I revved the engine a few times while testing the alternator to make sure it wasn't failing.....it's good.......no fluctuation....14.9V at start-up which levels off at 13.5V after a few minutes.......
The popping/backfiring is still consistent
The new harness should be here in a couple days.
The good news is.....I wasn't on the planned 350 mile road-trip when the backfiring began!
I revved the engine a few times while testing the alternator to make sure it wasn't failing.....it's good.......no fluctuation....14.9V at start-up which levels off at 13.5V after a few minutes.......
The popping/backfiring is still consistent
The new harness should be here in a couple days.
The good news is.....I wasn't on the planned 350 mile road-trip when the backfiring began!
#13
Burning Brakes
I re-tested everything......same results.
I revved the engine a few times while testing the alternator to make sure it wasn't failing.....it's good.......no fluctuation....14.9V at start-up which levels off at 13.5V after a few minutes.......
The popping/backfiring is still consistent
The new harness should be here in a couple days.
The good news is.....I wasn't on the planned 350 mile road-trip when the backfiring began!
I revved the engine a few times while testing the alternator to make sure it wasn't failing.....it's good.......no fluctuation....14.9V at start-up which levels off at 13.5V after a few minutes.......
The popping/backfiring is still consistent
The new harness should be here in a couple days.
The good news is.....I wasn't on the planned 350 mile road-trip when the backfiring began!
Sorry just read you have an HEI Dist, forget what I just said. Look into the HEI. Check ign. module, pick-up coil . I hope your not re-using the original resistance wire to feed the new Dist. The HEI needs a 10 Gauge wire feed from the fuse box . The old ignition wire must be removed completly back to the fuse box and a new 10 Ga. wire run to the dist. Thats all I have. Good Luck!
Last edited by 1Hotrodz; 07-12-2015 at 11:59 AM.
#14
Nam Labrat
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Look into the HEI. Check ign. module, pick-up coil . I hope your not re-using the original resistance wire to feed the new Dist. The HEI needs a 10 Gauge wire feed from the fuse box . The old ignition wire must be removed completly back to the fuse box and a new 10 Ga. wire run to the dist. Thats all I have. Good Luck!
I ran a new wire to the HEI, but I'll do the 10ga. (I think I used 14 ga.) All the HEI stuff tests good....it's just the whole fuse block/HEL feed wire have wild voltage fluctuations...Forum members are thinking the harness that branches off and feeds the fuse block is damaged/fried since the alternator/starter/horn relay have a steady 13.5 V going to them.
#15
Nam Labrat
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I'm still waiting on the new engine harness......
I checked voltage again at the fuse block.......
The fuse block IGN terminal now has NO voltage.....I disconnected the HEI lead from that IGN terminal and temporarily connected it to the Heater fuse
It seems like the members are correct----the harness connection at the firewall to the fuse block is failing for some reason (I cleaned it thoroughly when I installed the new fuse block-dash harness).
I checked voltage again at the fuse block.......
The fuse block IGN terminal now has NO voltage.....I disconnected the HEI lead from that IGN terminal and temporarily connected it to the Heater fuse
It seems like the members are correct----the harness connection at the firewall to the fuse block is failing for some reason (I cleaned it thoroughly when I installed the new fuse block-dash harness).
#16
Burning Brakes
I'm still waiting on the new engine harness......
I checked voltage again at the fuse block.......
The fuse block IGN terminal now has NO voltage.....I disconnected the HEI lead from that IGN terminal and temporarily connected it to the Heater fuse
It seems like the members are correct----the harness connection at the firewall to the fuse block is failing for some reason (I cleaned it thoroughly when I installed the new fuse block-dash harness).
I checked voltage again at the fuse block.......
The fuse block IGN terminal now has NO voltage.....I disconnected the HEI lead from that IGN terminal and temporarily connected it to the Heater fuse
It seems like the members are correct----the harness connection at the firewall to the fuse block is failing for some reason (I cleaned it thoroughly when I installed the new fuse block-dash harness).
#17
Nam Labrat
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I'm assuming you are removing the bulkhead connector at the firewall. It connects to the back of the fuse panel inside the cabin. (this bulkhead connector has a bolt in the center of the connector) Yes? This connector has to be separated from the back of the fuse panel to make the new 10 Ga. wire connection. Do not splice into the existing wire, you have to remove it completely. If I remember correctly its a blade terminal at the bulkhead connector.
#18
Nam Labrat
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I removed the original firewall harness. The waterproof "grease" that the factory coated the inside of the connector with had dried up and was allowing rain/moisture to soak the terminals between the two halves of the fuse block.
Apparently I forgot to clean/inspect the firewall connector when I replaced the dash harness/fuse block
Back to the garage...............................
Apparently I forgot to clean/inspect the firewall connector when I replaced the dash harness/fuse block
Back to the garage...............................
#19
Burning Brakes
I removed the original firewall harness. The waterproof "grease" that the factory coated the inside of the connector with had dried up and was allowing rain/moisture to soak the terminals between the two halves of the fuse block.
Apparently I forgot to clean/inspect the firewall connector when I replaced the dash harness/fuse block
Back to the garage...............................
Apparently I forgot to clean/inspect the firewall connector when I replaced the dash harness/fuse block
Back to the garage...............................
work on enough old cars and having those 3 around makes life a lot easier...I put the di-electric grease on every terminal I take off, headlights, a little on the alternator, battery, you name it...same with anti-seize, if it unscrews, it gets some...and brake grease is great on lots of rubber grommets and other stuff as well...
#20
Nam Labrat
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3 things to buy in big jugs/tubes at Napa: Anti-seize, Brake grease, and Die-electric grease!
Thanks for the info............
I've always been a fan of anti-seize and rubber conditioner.........
I just added dielectric grease to my list/connections yesterday......after being stuck on the side of the road 4 times this year and spending $200 on new wiring last week......I want everything to last!
Thanks for the info............
I've always been a fan of anti-seize and rubber conditioner.........
I just added dielectric grease to my list/connections yesterday......after being stuck on the side of the road 4 times this year and spending $200 on new wiring last week......I want everything to last!