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#1 |
Senior Member
![]() Join Date: Aug 2008
Location: Cedar Park, Texas
Posts: 7,498
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Trouble with underhood harness with HEI conversion
A while back i bought a new underhood harness for my 72 as the old one was a rats nest. Since i was planning on going with HEI, i bought a harness that is supposed to be "plug and play" from Brothers. I started the truck maybe twide then tore it apart for paint, having never actually driven it. I am putting the teuck back together and reinstalled the harness. I started the teuck fine and drive it about 100 yards and it quit. I checked and have 12 volts at the pink power wire plugged in to rhe dist with key on not cranking. When cranking it drps to 8-9 volts. I had to unplug the pink wire at the dist and hook a hot wire from the battery to the dist to get it to run. Why am i losing voltage at a spot i figured they had researched as having adequate voltage at cranking?!?! On the underhood connector it comes from the far left side second from top.
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#2 |
Registered User
Join Date: Apr 2011
Location: Klein Texas
Posts: 3,852
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Re: Trouble with underhood harness with HEI conversion
What's the battery voltage while cranking? I wouldn't be surprised to see 10 volts.
That means you have a couple volts of drop in the harness. One tenth volt per connection is considered normal. I'm a little surprised that the HEI wouldn't fire with 9 volts. Have you checked the spark quality?
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My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe |
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#3 |
Registered User
Join Date: Sep 2012
Location: Columbia, SC
Posts: 549
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Re: Trouble with underhood harness with HEI conversion
Sounds like plug and play for a points distributor. The drop in voltage is the same thing the original resistor wire does. 12 v constant is what you want with HEI so you solved the problem just not from the harness point of view
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Life is too long to go slow... ![]() 71 GMC 1500 Super Custom LWB 350/350 AC(replaced w/vintage air)/PS/PB 2.5 CPP Spindle, 1" CPP Drop Spring, CPP Front and Rear Sway Bars/DJM Flip, new General Spring KC 5 spring pack, HEI /IntReg Alt 20x8.5 OldSkools 5" BS, 275/45 Falkens |
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#4 | |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
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#5 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
I dont know how much voltage the hei needs when cranking. Its a new skip white hei with 0 miles. Runs fine with hot wire from battery. Maybe its not the wire harness (?)
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#6 |
Registered User
Join Date: Sep 2012
Location: Columbia, SC
Posts: 549
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Re: Trouble with underhood harness with HEI conversion
My Skip White is wired straight to the ignition unfused on the fuse block. Maybe your harness connection plugs to something else on the fuse block side?
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Life is too long to go slow... ![]() 71 GMC 1500 Super Custom LWB 350/350 AC(replaced w/vintage air)/PS/PB 2.5 CPP Spindle, 1" CPP Drop Spring, CPP Front and Rear Sway Bars/DJM Flip, new General Spring KC 5 spring pack, HEI /IntReg Alt 20x8.5 OldSkools 5" BS, 275/45 Falkens |
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#7 | |
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
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#8 |
Cluster King
![]() Join Date: Nov 2012
Location: Junction City, OR
Posts: 5,263
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Re: Trouble with underhood harness with HEI conversion
Do you have a DVOM? (digital volt ohm meter) Or just a way to check true voltage drop? Try it directly on the battery while cranking and you will see 9/10 volts while cranking and yes that is enough to lite it up with an HEI. What is the path of the pink wire from the hot side of the distributor. It goes to the junction block and what happens inside? Like others have mentioned, this needs to be on ign unfused. It should have battery voltage when the key is in the on/run position.
Does your ignition switch on the dash have an extra lug for ign? (hot only when key is on) If yes, try and run your pink wire directly to the ignition switch and see if that fixes the problem. |
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#9 |
Registered User
Join Date: Sep 2012
Location: Columbia, SC
Posts: 549
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Re: Trouble with underhood harness with HEI conversion
Should be 12 v constant. Here's a thread, there are many good threads on HEI conversion with great information. http://67-72chevytrucks.com/vboard/s...d.php?t=536484
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Life is too long to go slow... ![]() 71 GMC 1500 Super Custom LWB 350/350 AC(replaced w/vintage air)/PS/PB 2.5 CPP Spindle, 1" CPP Drop Spring, CPP Front and Rear Sway Bars/DJM Flip, new General Spring KC 5 spring pack, HEI /IntReg Alt 20x8.5 OldSkools 5" BS, 275/45 Falkens |
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#10 |
Registered User
![]() Join Date: Dec 2012
Location: Chandler AZ
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Re: Trouble with underhood harness with HEI conversion
bad ignition switch?
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1970 Longhorn, Front Disc, 350/4 bolt, 882 heads, HEI, Edelbrock, 700R4, HO-52/4.11. 1996 Corvette, Collector Roadster, LT4, 396, 450RWHP, 6sp, 4.11/Dana44/posi 5 point roll bar And let us not grow weary of doing good, for in due season we will reap, if we do not give up. Gal 6:9 |
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#11 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
That was my first thought. I have a wiring diagram and did some checking. I found that on the Brothers harness with hei provision, the pink wire for the dist plugs in to 3A on the firewall power distribution block behind the fuse panel. On the inside, a pink 12 ga wire runs to the ignition switch pigtail to position 3B. The corresponding terminal that this plugs into on the ignition switch is labelled "batt". There is power at this terminal when cranking, verified with a test light. I have not checked how much voltage with my volt meter yet but will do so tomorrow. BTW, I did verify that there is voltage present at the "sol" and "ign" when cranking.
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#12 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
I am thinking about picking up some dielectric grease to give the connections a better, uh, connection. Everything is pretty dry right now.
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#13 |
Senior Member
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Re: Trouble with underhood harness with HEI conversion
I was thinking about adding a relay to get higher voltage straight from the battery and using the original pink wire oy to trigger the relay. I guess i am not the first one to think of this!:
http://www.hoigto.com/tech/powerHEI.htm Posted via Mobile Device |
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#14 |
Senior Member
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Re: Trouble with underhood harness with HEI conversion
Back in the 80's when i was a teenager i swapped an HEI into my 71 El Camino. I asked my older brother about using a relay as above and using the old plwer wire with resistor to trigger it and he said it wouldnt work! I proved hi wrong by doing it anyway and driving the car for 7 more yrs before selling it. I havent done an hei conversion since then as everything i have had since then already had one. I didnt think i wod have to bother with it by buying the brothers harness. I guess i was wrong. I still cant figure why it ran for a little while but now won't. Im going to do what everyone always says and "check the grounds!"
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#15 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
Thanks everyone for the input. I have a few things to try tomorrow. I will keep you updated.
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#16 | |
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Re: Trouble with underhood harness with HEI conversion
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Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" ![]() "Be American, Buy American" ![]() |
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#17 | |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
Quote:
I used the wrong term. I have some "no-lox" for electrical connections, but even it is really intended for dissimilar metals. I was pretty worn out yesterday! Someone on an RX7 forum summed it up pretty well: one thing people have tried is dielectric grease on various electrical component connections. this is NOT a good idea! dielectric grease or "tune up grease" as you may find in auto parts stores is ONLY meant for high voltage connections such as spark plug wires, that is about the only place it belongs on a car. using it on electrical connections, eventually you may as well pitch the harness into the garbage.. it's almost impossible to get the stuff out once it's in there, dielectric grease does not conduct electricity very well and will in fact cause resistance issues and eventually failed connections. the best thing to do for connections is to clean and dry them with cleaners like brake cleaner or carburetor cleaner and compressed air. a nice dry solid and sealed connection is about the best you can hope for. any other electrical connector grease compounds are not easy to locate so dry and clean is your best bet. don't let the name fool you(just put the first 3 letters together and think about it) or let the store clerks tell you otherwise, they're not the ones who have to deal with the aftermath of diagnosing intermittent issues or replacement parts aside from grabbing them off the shelf and ringing them up for you. in fact it's meant as a voltage inhibitor, to keep the spark from leaking out of the spark plug boots and to also keep them lubricated for easy removal. also seen people use it for the contacts on the notoriously bad window switches, it's at least easy to clean off in there but if you really like tearing the switches apart regularly then go for it. Posted via Mobile Device Posted via Mobile Device |
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#18 |
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Location: Falls City, Nebraska "100 Miles From Nowhere"
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Re: Trouble with underhood harness with HEI conversion
"The Good Stuff!"
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Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" ![]() "Be American, Buy American" ![]() |
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#19 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
While checking grounds I found that the body shop attached the negative cable to the frame so I moved it to the exhaust manifold. I also have two new ground straps from each valve cover to firewall. No paint under screws. Still same voltage and no start with harness plugged in. ??? Looks like time for a relay or pull the skip white dist and install an old oem and see what happens. I wonder if I have a defective module (?)
Posted via Mobile Device Last edited by WIDESIDE72; 08-17-2013 at 02:38 PM. |
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#20 |
Senior Member
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Location: Kennesaw, GA
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Re: Trouble with underhood harness with HEI conversion
Well, if you can get it to run with 12v directly to the HEI, you know three things..
1. The HEI is working just fine 2. The battery should be in good shape. Not too much voltage drop while cranking to affect the HEI module 3. The ground at the HEI is sufficient. So, that leaves us with the rest of the circuit that powers the HEI. So check the following... 1. The positive lug on the terminal next to the battery that feeds the harness. 2. The interconnects where the Alternator feeds into that power wire 3. The connectors on the harness plug that sends power into the cab, as well as the wire that comes back out to the HEI. 4. The connection to the ignition switch which is where the fuse box gets the 'ign on' feeds How do all of your other electrical devices work? Headlights, charging system, gauges and such? I've seen lots of issues with the alternator interconnect wire where it was badly oxidized and the system wouldn't charge, and the feed voltage was low going to the cab. The only fix for poor connections is to remake them and make sure the wire you're using isn't green or black (oxidized) You may find that you have to replace a good bit of wire before getting to a good clean stretch of copper. Hope this helps Shawn
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#21 |
Senior Member
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Location: Cedar Park, Texas
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Re: Trouble with underhood harness with HEI conversion
Can someone with an HEI tell me how many volts they have at the dist power connector while cranking? More than 10 volts? I would be surprised if fhere was more than 10 volts while cranking. A starter is going to pull a lot. I have 10 volts at the connecteo while cranking which i would think would be enough, but it wont start, leading me to believe its a dist/ module issue.
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#22 | |
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Re: Trouble with underhood harness with HEI conversion
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#23 |
Senior Member
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Re: Trouble with underhood harness with HEI conversion
I also bought an adapter to run an internal reg alternator so i am going to check all of that out as well.
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#24 | |
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Re: Trouble with underhood harness with HEI conversion
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#25 |
Registered User
Join Date: Apr 2011
Location: Klein Texas
Posts: 3,852
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Re: Trouble with underhood harness with HEI conversion
I assume you are checking the voltage at the HEI with it disconnected rather than back probing the connector.
Back probing while connected will give you the true available voltage and can tell you if their is voltage drop in the circuit. Checking the circuit in the same manner toward the battery will likely find the problem. If you have 10 volts while back probing, I'm with you, something in the distributor (coil) is not right.
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My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe |
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