Electronic Ign. Control Unit (ECU) Heat condition

All,
I am troubleshooting a hot-start problem on another thread in this forum section, and during my time under the hood, I have noticed that my ECU gets REALLY hot after only about 10-mins of driving? (By HOT, I mean it will burn your fingers like if you touched the engine block.) Is this normal? Could a failing/hot ECU in ANYWAY impact the ignition signals being sent to the ignition relay and hence affect hot-cranking?
BACKGROUND: My car is a 1974 Duster 360 (originally a slant-6 car) with OEM style electronic ignition system. I have been troubleshooting hot-starting problems since I bought the car about 2-years ago that I describe in details in another post. In summary: with a OEM style starter, I'd get a lot of chatter and VERY slow cranking from the starter whenever the engine was hot (5-10 mins of driving). This starter looked to be well-used and was contacting a header pipe so I opted to do the following: a) changed to a high-torque mini-starter, b) crimped the closest header pipe for more clearance and wrapped starter with a heat shield, c) checked all battery cables fm trunk to engine bay & cleaned/re-fit all connections, d) upgraded to a red-top Optima battery, & e) installed a new ignition relay.
NOW: I get absolutely NOTHING(no noise/clicks at all) when I try to start a hot engine but, after the engine cools for a while, the starter will chatter and crank slowly. After fully cooling off(35-45mins), it cranks strong and fires up instantly... Any help is appreciated.

I think I'd inspect, clean, and tighten up the bulkhead connections.
Is the "chatter" from the starter?

As Dave mentioned check all grounds/connections and condition of wiring.
Good luck.

Had the same no start issue with the wife’s 70 Dart. It was the solenoid (?) on the starter. Swapped the starter which solved the problem.

Yes, they get hot. That metal piece on the top is a heat sink for the big transistor.

Yes. If its failing in hot conditions if will effect hot start and run. Original ECUs are powered slightly differently during startup than run.

scott mattingly said:

I'd get a lot of chatter and VERY slow cranking from the starter whenever the engine was hot (5-10 mins of driving). This starter looked to be well-used and was contacting a header pipe so I opted to do the following: a) changed to a high-torque mini-starter, b) crimped the closest header pipe for more clearance and wrapped starter with a heat shield, c) checked all battery cables fm trunk to engine bay & cleaned/re-fit all connections, d) upgraded to a red-top Optima battery, & e) installed a new ignition relay.
NOW: I get absolutely NOTHING(no noise/clicks at all) when I try to start a hot engine but, after the engine cools for a while, the starter will chatter and crank slowly. After fully cooling off(35-45mins), it cranks strong and fires up instantly... Any help is appreciated.

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Sounds like a power issue. In other words the starter, regardless of which one you have, doesn't seem to have enough power (volts and current).
Need at least some voltage measurements to guess what's going on.

You can buy an ecu for $20 at parts store. Pretty cheap trouble shooting if you value your time.

Maybe the positive cable assembly doesn't like to be hot. Ohm it out with a good meter. Don't forget to subtract the meter lead resistance, which should only be tenths of an ohm.

Positive cables have gotten pricey lately, and the ones after 71 or so sometimes are tricky to find.

Demonic said:

Positive cables have gotten pricey lately, and the ones after 71 or so sometimes are tricky to find.

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Modified set up.

scott mattingly said:

all battery cables fm trunk to engine bay

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Maybe very modified.

Engine grounded properly?
Wire-gauges thick enough from trunk to enginebay?

Ballast resistor? For a Mopar box ballast resistance plus coil resistance is 2 to 3 Ohms. What what coil? What is battery voltage running? All questions relate to ignition primary current, that is what heats ECU.

KitCarlson said:

Ballast resistor? For a Mopar box ballast resistance plus coil resistance is 2 to 3 Ohms. What what coil? What is battery voltage running? All questions relate to ignition primary current, that is what heats ECU.

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Guys,
Really appreciate all the feedback. I ALSO strongly believe I am simply not getting sufficient voltage to the started/relay circuits under hot conditions. I will take several voltage and resistance readings as suggested, re-check all ground and + connections points, and see what I have. Will probably replace the ECU as a matter of good practice along the way, though right now the car starts quick(when cold), and runs smooth across the rev range. Probably just replace the ballast resistor as well...(I do have a slight stumble just off idle but I am pretty sure that is Carb issue--came with a Demon 800 cfm --which I think is too big). The + Battery cable from trunk to starter looks new and is 2ga which looks like a pretty "healthy" size (but I have never had a trunk mount before). The 18" ground cable running just under the trunk to a frame mount is also 2Ga. Engine ground consists of an ~16-18" cable (slightly smaller than the 2Ga size) running from front of engine block to a radiator bolt.

scott mattingly said:

Guys,
Really appreciate all the feedback. I ALSO strongly believe I am simply not getting sufficient voltage to the started/relay circuits under hot conditions. I will take several voltage and resistance readings as suggested, re-check all ground and + connections points, and see what I have. Will probably replace the ECU as a matter of good practice along the way, though right now the car starts quick(when cold), and runs smooth across the rev range. Probably just replace the ballast resistor as well...(I do have a slight stumble just off idle but I am pretty sure that is Carb issue--came with a Demon 800 cfm --which I think is too big). The + Battery cable from trunk to starter looks new and is 2ga which looks like a pretty "healthy" size (but I have never had a trunk mount before). The 18" ground cable running just under the trunk to a frame mount is also 2Ga. Engine ground consists of an ~16-18" cable (slightly smaller than the 2Ga size) running from front of engine block to a radiator bolt.

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I had a bad issue with a big block that was wired like yours, the negative starter cable went thru the frame. I changed it to a full length cable from the neg battery post all the way to the engine block, cured all issues, after I fried 2 starters.
The neg cable needs the be a least the same gauge as the positive cable or you will have problems. Make sure you remove all the paint from under the connection points and add a little dielectric grease at the mating surfaces.

If starters are deprived voltage, current increases, that increases drop....a run away condition. Get some long clip leads monitor starter voltage at starter. Hot and cold. Then use leads at +battery post to starter to look for drop cranking. Same -post to engine. Cold and hot.

Not much frame cross section, steel poor conductor compared to copper.

It would be really helpful if you're OK with combining your electrical issues into this thread.
ref: Car won't even crank...
While both are '74s, yours is modified.

scott mattingly said:

Voltage at battery is usually strong & better than 12V

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If this was voltage under load, then we could be more confident about the condition of charge.

scott mattingly said:

Voltage at the regulator from the large +Battery post to either the ground tab of the regulator or the engine block is ~12.4V.

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With everything off, 12.4 Volts is probably OK. Not outstanding, and not bad.

Kit suggested taking posting photos. That would be really helpful. Not just here but also with the other connections such as the ballast resistor and ECU.
With a PC, take a digital photo and drop and drag onto post. It can be done with cell phones too. Someone else can help with that if needed.
Here's one from way back showing a fairly stock '74.
Not Getting 12 Volts at Coil

scott mattingly said:

I was measuring the 8V between the regulator +Battery post to the actual IGN wire that I pulled of the regulator.

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I'll answer this below where you correct the description.

scott mattingly said:

Any idea what that voltage should ideally be at each key position for that wire?

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The system voltage should be the same in every supply wire that is connected to a power source.
If the battery is at 12.5 V, then everything in the system shown below is at 12.5 Volts

But in our cars there are two power sources tied together.
So when the alternator is running it supplies power at the regulated set point, around 14 Volts. (As long as it can meet the demand; otherwise voltage drops)
All of the wires shown above should be or close to the alternator output voltage.

Picture it like pressure in your house plumbing with all of the taps shut. Or like pressure from an air compressor. Attach a system with two or three hoses to the compressor. Pressure in the hoses is the same as at the compressor.

scott mattingly said:

I expect they might change when key is fully twisted

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When the key is in RUN, power is made available to ignition and accessory feeds.


Here's a catch: When the switch is in run, the ignition and alternator are turned on. In other words they draw power when the key is in run. This is important to know. Any time the power is flowing out of the battery, its voltage will drop. The most extreme case is when the car is started, battery voltage may go down to 11, 10 or even 9 volts depending on the starter's current draw and battery condition.

Fully twisted clockwise connects ignition switch feed to the ignition 2 (start) wire(s )and the starter relay wire.

scott mattingly said:

My error. 8V measured between the big bolt on the Ign Relay where the +battery lead connects and the small wire that connect to the IGN tab on the ignition relay that I pull off and push the voltmeter probes into. I am only checking this with the key either off OR turned on.

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If this is the start wire to the relay, it should have no voltage except when the key is in Start position. In start, it should have the same voltage as the battery. In other words roughly 12 Volts above ground but will depend on the starter load and batttery condition. If you measure from battery positive to the connection (back probe the connector while connected), the voltage difference should be very small. Certainly under a volt; preferable a 1/2 Volt or less IMO.

scott mattingly said:

Might just be a bad idea to have the trunk mounted battery though I do like how it cleans up the engine bay.

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If the battery tray is still in place, its an easy thing to do.
Placement in trunk is generally done to shift weight for racing.
Disadvantage is a the long runs of wire and at least for non-glasmat batteries, venting them out of the trunk.

Mattax said:

Here's a catch, in run position the ignition and alternator are turned on. In other words they draw power when the key is in run. This is important to know. Any time the power is flowing out of the battery, its voltage will drop.

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Using orange arrows to indicate current flow with key switch in RUN, engine off.


All power comes from battery. Voltage does not drop until it reaches a device with resistance. As current passes through those devices, voltage drops.

With the engine running, alternator provides the power. But in terms of voltage, same deal.
In an ideal world, if the alternator output is at 14.1 Volts, then everything up to the devices will be at 14.1 Volts.

Mattax said:

Using orange arrows to indicate current flow with key switch in RUN, engine off.
View attachment 1715382114

All power comes from battery. Voltage does not drop until it reaches a device with resistance. As current passes through those devices, voltage drops.

With the engine running, alternator provides the power. But in terms of voltage, same deal.
In an ideal world, if the alternator output is at 14.1 Volts, then everything up to the devices will be at 14.1 Volts.
View attachment 1715382121

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All,
I continue to appreciate (& be amazed by) the amount of advice and level of detail you have all provided. (I owe you all a drink sometime...) sorry for length of this note.
--Current situation(same): Car still starts strong when cold and will do nothing when the key is twisted after car is hot (~10-minute drive). After cool-down, it starts normally again. Here are some updates on my troubleshooting.
1) Wrapped about 2/3 of the header pipes on the driver side all around/under the starter to minimize heat-soak.
2) Checked for voltage at the IGN lead to the Starter relay with key Off and in On position = 0V. (Need to get a helper so I can check when key is fully turned to Start.)
3) When car is hot (and turning the key results in nothing at all), the car will CRANK STRONG and start fine if I turn key to On and JUMPER THE Starter relay between the Pos+ post and the square Sol post just under it.
4) I have not taken any Ohm readings on the Ballast resistor, but it is very hot after a short drive (I expect this is normal.)
5) Idling Voltage reading taken between the Starter Relay Pos+ post and engine block = 13.9V.
6) Though I am not sure of what the correct readings should be, I decided to check the Voltage regulator (VR) internal resistance between the two pins with the alternator plug removed. With multi-meter set to 2000K-Ohm setting, I get: COLD reading = ~1850; HOT reading= ~1250. Are these values O.K.? I am just too ignorant to understand if/how the VR resistance would impact the signal to the starter relay when the key is turned. Does it?
7) NEXT Steps: a) Carry a screwdriver with me whenever I take the car for a drive so I can start it by shorting across the Starter relay(Like I stated in (3) above); b) Get some voltage readings at the Ign wire leading to the Starter relay when the key is twisted for both Hot & Cold conditions; c) check the resistance of the Ballast resistor; d) look for references to help determine if the VR is good or bad.(The battery is charging.)
FOOTNOTE: I don't think I mentioned this before, but FYI. The Pos+ lead from my trunk-mounted battery goes directly to my starter. A large (maybe 3-gage) wire connects between that same +lug and the Pos+ post on the starter relay. Most wiring diagrams show a separate Pos+ lead from the battery to the starter relay, but since my wiring set-up still puts both these connections in parallel, I think it is O.K. (Right?Wrong?)
-THX for hanging in there with me on this...-Scott

A separate feed from battery to relay post helps reduce voltage drop and starter electrical noise when cranking. I think fusible link connects at post along with most electrical loads. Having electronics "ON" when cranking may lead to early failure.

While the long battery leads may have low resistance, they have inductance too. The battery normally serves as good filter, just like a capacitor. The long leads changes that. To have any chance with electronics, best to feed power from battery terminal connection, not starter post as it exists now.

scott mattingly said:

2) Checked for voltage at the IGN lead to the Starter relay with key Off and in On position = 0V. (Need to get a helper so I can check when key is fully turned to Start.)

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That or a pair of alligator clips, or roll down the window and stick your hand in and turn the key - assuming a column mounted key

scott mattingly said:

3) When car is hot (and turning the key results in nothing at all), the car will CRANK STRONG and start fine if I turn key to On and JUMPER THE Starter relay between the Pos+ post and the square Sol post just under it.

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So Run position works and start does not.
Don't know exactly what you are jumpering. Don't know what you have. Don't know if the warning or interlocks are on it.
Best guess is you either providing power to the relay or just direct feeding the starter solenoid.

scott mattingly said:

I am just too ignorant to understand if/how the VR resistance would impact the signal to the starter relay when the key is turned. Does it?

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There's other threads on how votlage regulators works. Bottom line is it controls flow to the alternator rotor.

scott mattingly said:

5) Idling Voltage reading taken between the Starter Relay Pos+ post and engine block = 13.9V.

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OK. If you are testing for regulation issues, you need to increase throttle and see if voltage follows rpm. If you are looking for voltage drops, then measure at the alternator. Not sure why you're looking at this but it doesn't hurt.

scott mattingly said:

I don't think I mentioned this before, but FYI. The Pos+ lead from my trunk-mounted battery goes directly to my starter.

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Pretty typical with batteries in the trunk.
You need to draw a diagram of what you've got and/or post photos.

Starting circuit was not shown in anything posted above. But that is what you need to check out to find the problem.
Schematically this is all that should be involved - although yours may have seatbelt interlock or warning buzzer tied in.
Ignition switch provides power to the relay and a neutral safety switch completes the connection to ground.
The relay then internally connects power to the starter solenoid.
Coil and ECU get power power from the Ignition 2 wire(s).

(Ignition start and run) = IGN1 + IGN2, for HEI without ballast resistor.

All,
Wanted to get a follow-up to all who have contributed. SUMMARY: Problem was/is my Neutral Safety Switch.
MEASUREMENTS: With a friend's help, We pulled the NSS wire off its Ignition Relay connection and checked voltage btwn it and ground. COLD in Neutral = 0 Volts; HOT in Neutral = ~11 Volts. So...when it was hot and I turned the key, I was putting ~11-12 volts across both sides of the Ign. Relay and hence it would not close.
SOLUTION: I have simply taken the NSS out of the equation and placed a small jumper wire from the NSS lug on the Ign. Relay directly to ground. Now the car will start (Hot or Cold) in any gear. It still cranks slowly when hot (though I have a heat shield on the starter and wrapped the header pipes near the starter), but think I might just need a slight timing adjustment......
I also crawled under the car and tracked down what I think is the NSS (Circular electrical switch about 5/8' in diameter mounted just behind the small shift lever arm on the driver's side of the trans. case.) Wires from this switch lead up and over the transmission casing but I have not been able to see what they are connected to. They do seem to be connected somewhere (are not loose), but I need to get car on a lift to check further. Still not sure why this switch works correctly when cold, but not when car/trans was hot...Maybe there is some sort of adjustment? (FYI, my hot-start problems started after I had the trans. rebuilt at a local shop.)