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> Grounding through throttle cable, Return spring burned the crap outta me! - SOLVED
clapeza
post Aug 8 2017, 07:14 PM
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Got the FI back in my car (1970 1.7L) and working recently. The PO had cut the throttle cable for the carb conversion, so I installed a new TerryCable.
I noticed that the throttle cable got hot. I figured that the ground strap was corroded or broken (I can see it, but can't get to it now.)

I installed a new ground from the rearmost engine case bolt to the battery ground stud on the passenger side of the engine compartment. I used a generic 4 ga battery ground cable from my FLAPS. One end is on top of the battery ground cable. The other is on the bolt that holds the oil bath air cleaner "shelf". This end of the cable is sandwiched between 2 washers on top of the shelf arm that the bolt goes through, with the nut holding it all down tight.

I cranked the car and it ran easily, and the throttle cable was no longer hot, or even warm. Success? Not so fast...

I then replaced the bushings in the accelerator pedal bracket, so it's nice and smooth now.

Didn't get a chance to crank it because I ran out of time that day.

I cranked it today, and blipped the throttle by hand on the throttle body. I thought I saw a tiny spark at the throttle return spring where it attaches to the rear wall. I touched the spring lightly with my finger and burned the crap out of it! The engine had been running only about 10 seconds at this point.

What gives?

Theory: The new accelerator pedal bushings improved the electrical connection to the body. The new grounding cable isn't making good enough contact to the engine case, so the engine is still preferring to ground through the throttle cable.

Assumption: A 4ga cable connected from the engine case straight to the body where the battery connects is an equal (if not superior) ground compared to the transmission ground strap.

I still plan on inspecting and possibly replacing the transmission ground strap, but I'm curious as to why I burned my finger...

Ideas? Advice?
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Racer Chris
post Aug 8 2017, 07:21 PM
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The problem is where you have your ground attached to the crankcase.
Make the connection more direct, with the ring terminal in contact with the transmission case. The reason that the trans case is better is that the most current draw occurs when the starter runs - and the starter mounts on the transmission.
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GeorgeRud
post Aug 8 2017, 08:20 PM
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I wonder if running the ground strap from a starter mounting bolt directly back to the battery negative terminal would provide even better cranking performance?
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The Cabinetmaker
post Aug 8 2017, 08:28 PM
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Electricity follows the path of least resistance.
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Spoke
post Aug 9 2017, 05:09 AM
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QUOTE(clapeza @ Aug 8 2017, 09:14 PM) *

I cranked it today, and blipped the throttle by hand on the throttle body. I thought I saw a tiny spark at the throttle return spring where it attaches to the rear wall. I touched the spring lightly with my finger and burned the crap out of it! The engine had been running only about 10 seconds at this point.



Did the arcing happen as the engine was cranking or running? If while running, that means the alternator is not grounded and battery charging current is flowing through the spring and cable.

The grounding of the alternator goes through the 4 small studs and nuts that hold the fan shroud to the engine case since the alternator is bolted to the fan shroud. You may need to add a ground strap from the alternator case to the engine case.

You should fix or replace the transmission ground cable and remove the cable from the engine case to battery. The basic 914 engine grounding scheme is perfectly fine if all the components are in good shape and the connections are clean, shiny, and secure.
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clapeza
post Aug 9 2017, 06:26 AM
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QUOTE(Spoke @ Aug 9 2017, 07:09 AM) *

QUOTE(clapeza @ Aug 8 2017, 09:14 PM) *

I cranked it today, and blipped the throttle by hand on the throttle body. I thought I saw a tiny spark at the throttle return spring where it attaches to the rear wall. I touched the spring lightly with my finger and burned the crap out of it! The engine had been running only about 10 seconds at this point.


Did the arcing happen as the engine was cranking or running? If while running, that means the alternator is not grounded and battery charging current is flowing through the spring and cable.



It was while running.
Yikes! (IMG:style_emoticons/default/yikes.gif)

Looks like my to-do list just grew.
(x) Remove, clean, replace transmission ground strap.
(x+1) Inspect alternator mounting bolts, and possibly install new ground from alternator case to engine case.
...
(x+50) DRIVE IT!

I applaud the collective knowledge and helpfulness of this board. Thanks, guys!!
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cali914
post Aug 9 2017, 09:29 AM
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Yeah I heard if you don't have a transmission ground strap the grounding usually takes place through the clutch cable, and after time it will become brittle and snap.
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clapeza
post Aug 9 2017, 09:56 AM
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QUOTE(cali914 @ Aug 9 2017, 11:29 AM) *

Yeah I heard if you don't have a transmission ground strap the grounding usually takes place through the clutch cable, and after time it will become brittle and snap.

I can see mine, but I'm not sure how well it's connected at both ends, and how much corrosion it has. As The Cabinetmaker said, electricity will take the path of least resistance. A nice thick ground strap that's corroded and crusty loses to a nice shiny thin throttle or transmission cable every time.

BTW, does anyone have a BTDT guide to removing and replacing the transmission ground strap? It obviously has to be done with the engine in the car, but besides groping around blindly, is there a better way?
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Eric_Shea
post Aug 9 2017, 10:03 AM
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The factory has a solution. It should really only take a few minutes with a 13mm socket and a wire brush. These things are "914 Guy" cheap:

https://www.delcity.net/store/Ground-Straps...MIaAqF2EALw_wcB
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Mikey914
post Aug 9 2017, 10:04 AM
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QUOTE(cali914 @ Aug 9 2017, 08:29 AM) *

Yeah I heard if you don't have a transmission ground strap the grounding usually takes place through the clutch cable, and after time it will become brittle and snap.

I had the ground strap. It had corrosion. It actually stretches as you are driving then snap. Symptoms are it starts grinding (as not fully disengaged) the it goes.
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clapeza
post Aug 9 2017, 12:10 PM
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QUOTE(Eric_Shea @ Aug 9 2017, 12:03 PM) *

The factory has a solution. It should really only take a few minutes with a 13mm socket and a wire brush. These things are "914 Guy" cheap:

https://www.delcity.net/store/Ground-Straps...MIaAqF2EALw_wcB


I went to my local AutoZone over the weekend, and asked them for a "ground strap". They looked at me like it was joke, like "muffler bearings" or "blinker juice." Not one, but TWO of them gave me a glazed look.

The (slightly) older one said,"I've never heard of anything like that." (IMG:style_emoticons/default/confused24.gif)

It wasn't until I pulled up a Google image search for "ground strap" did they believe me!

I got a similar response at O'Reilly.

I went to NAPA. They closed at 3pm on Saturday. They have a ground strap like the one in the link. What length is closest to factory?
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Spoke
post Aug 9 2017, 12:55 PM
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When you started your car, did the starter strain to start the car or did it turn over nicely?

If it turned over nicely, likely your transmission ground strap is ok.

If the spring heats up while running, then your alternator is grounding through the spring. Did you have the engine out and do some painting on the fan shroud? If you add a ground strap to the alternator, attach the other end to the engine case, not the fan shroud. The alternator already is grounded to the fan shroud.

BTW, for a transmission ground strap, I picked up a standard starter cable like this one at my FLAPS. I think the one I picked up was either 2ga or 4ga 12 inches long. It was longer than the original so I looped it once.

Starter Cable



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Spoke
post Aug 9 2017, 01:05 PM
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Here's a pic of the ground strap I'm using. I think it's 1 ft and 2ga.


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clapeza
post Aug 9 2017, 01:13 PM
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QUOTE(Spoke @ Aug 9 2017, 02:55 PM) *

When you started your car, did the starter strain to start the car or did it turn over nicely?

If it turned over nicely, likely your transmission ground strap is ok.

If the spring heats up while running, then your alternator is grounding through the spring. Did you have the engine out and do some painting on the fan shroud? If you add a ground strap to the alternator, attach the other end to the engine case, not the fan shroud. The alternator already is grounded to the fan shroud.

BTW, for a transmission ground strap, I picked up a standard starter cable like this one at my FLAPS. I think the one I picked up was either 2ga or 4ga 12 inches long. It was longer than the original so I looped it once.

Starter Cable


The starter has always sounded and run strong. That's kinda why all the discussion around the transmission ground strap always seems to come back to talk about how well the starter pulls - this puzzled me. Mine seems fine. That's what led to the idea of the extra ground for the engine and not straight to the transmission.

I have not had the engine out of the car.

The new ground cable I bought looks exactly like the one you pictured. It's the one from the rearmost case bolt in the engine compartment going straight to the battery's ground post.

I'll look at the transmission ground and also look into a supplemental ground for the alternator. I finally got her off the jack stands. I guess she missed them!!

Thanks, Spoke!
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Spoke
post Aug 9 2017, 03:18 PM
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QUOTE(clapeza @ Aug 9 2017, 03:13 PM) *

It's the one from the rearmost case bolt in the engine compartment going straight to the battery's ground post.


This is the cable you attached, correct? There should only be one grounding cable from the battery to the chassis. All other ground straps for components should connect to the chassis at some point including the engine/transmission/starter/alternator. If this cable does go from the engine rearmost case bolt to the battery negative terminal, it should be removed.

The only ground strap for the engine should go from the end of the transmission to the grounding lug on the bottom of the trunk floor. The starter is also grounded to the transmission to complete the starter current loop.

After you make any changes to the grounding, start the car and see if the wire gets hot. If it does, get your voltmeter and measure the voltage from one side of the spring to the other. Measure as well the voltage from the vehicle chassis to the spring. Need to know if there's a voltage between the spring and chassis. The alternator should be grounded to the engine but weird things do seem to be happening here.
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Tom_T
post Aug 9 2017, 04:18 PM
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Clean the connections/studes & cable/strap, &/or replace if too far gone/corroded, at your main transaxle case ground strap to the R trunk underside. That is the primary ground route & will be the usual cause for acc. &/or clutch wire grounding when it's bad. I don't think that your alt. cable install is adequate.

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Tom
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N_Jay
post Aug 9 2017, 04:27 PM
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You grounded the engine, good for stating, but left the rest of the car floating so every other power using device grounded to the body needed to find a way back to the engine and battery.
You need a strap to the body also.

Don't worry about multiple ground connections. You are not going to create "ground loops" or any other problem by having an extra ground connection, as long as the three main connections are good. Battery Neg, Car Chassis, and engine/transmission case.
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Larmo63
post Aug 9 2017, 04:32 PM
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This isn't rocket science.......

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clapeza
post Aug 9 2017, 06:48 PM
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QUOTE(Spoke @ Aug 9 2017, 05:18 PM) *

QUOTE(clapeza @ Aug 9 2017, 03:13 PM) *

It's the one from the rearmost case bolt in the engine compartment going straight to the battery's ground post.


This is the cable you attached, correct? There should only be one grounding cable from the battery to the chassis. All other ground straps for components should connect to the chassis at some point including the engine/transmission/starter/alternator. If this cable does go from the engine rearmost case bolt to the battery negative terminal, it should be removed.



I'm sorry, I wasn't clear. The new ground cable connects the engine case to the chassis. It's connected to the same stud on the passenger side of the engine compartment that the negative terminal from the battery connects to. It is not connected only to the negative terminal of the battery, if that makes a difference.

But I'm going to disconnect it, and get back to basics and move forward.

As soon as I refresh/refurb/replace the transmission ground strap, I'll do the voltage tests you suggest. With any luck, I'll be able to get to this Friday afternoon.

Again, thanks everyone for the helpful and constructive comments. (...and not calling me an idiot!)
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clapeza
post Aug 12 2017, 08:14 PM
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I replaced the transmission ground strap with a new one. It starts up just as easily as it did before.

It continues to warm up the throttle cable while running, complete with sparking where the throttle return spring connects to the rear wall of the engine compartment. I measured the voltage between the spring and the chassis. It read 9.2v. With the engine not running, it was reading 0.2v.

It's definitely got some amperage going through that spring - the shiny finish has dulled/burned off, and it's not as "springy" as before.

I've put the car back on ramps, and I'll see about running a new heavy duty ground from the alternator to the engine case tomorrow.
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