1973 Dodge Monaco, Starting, Insufficient Voltage to ECU

1970FuryConv

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My 1973 Dodge Monaco has factory electronic ignition, with 5 prong ECU and dual ballast resistor. At least from Rick Ehrenberg, the set up was available 1972 to 1979, so hope someone's got an answer.

Problem: the engine has no ignition, no spark, when the key is turned to start and the engine is cranking. I proved this with a timing light. It starts, sometimes, when I release the key from start and slowly turn it back to run.

Question: for power input to the 5 prong ECU during cranking, what is the minimum voltage required for the ECU to operate the ignition system?

Condition: voltage drop, key at start, during cranking
*voltage at battery 10.9 V
*voltage at ignition switch battery input, red 12gauge wire, 10.7 V
*voltage at ignition switch start wire output, Brown 14gauge wire, 10.6 V
*voltage at ignition coil positive and the ballast resistor, start wire, Brown 14gauge wire, 10.6 V
*voltage at ignition run and aux ECU power on opposite side of dual ballast resistor, 8.1 V. I tried a new dual ballast resistor and none of the readings changed. With a 5 prong resistor, the ECU gets power from 2 sources, 2 spades on ECU side of dual ballast resistor. Both power sources are 8.1 V.

Resistance across ignition run from ballast resistor to ECU .4 ohms

My theory is that the ECU is not getting enough voltage to operate ignition system during starting.
To prove my theory, I connected a jumper wire from the brown start wire at the ballast resistor to the ignition run wire at the opposite side of the ballast resistor. I turned the key to start and the car started immediately. Another point is, when the engine is warm, the car will start with the key in the start position, and no jumper wire. The jumper wire is only needed when the engine is cold. A 3rd point is, the car runs completely normal when driven.

Possible solutions
*buy a third dual ballast resistor
*convert to a single ballast resistor and a later 4 prong ECU.

Thanks for any and all help understanding the voltage requirements of a 5 prong ECU and resolving my starting problem.
PXL_20220105_165027193.jpg

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Davea Lux

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You do not appear to have a bad ballast resistor. In the start position, the resistor is bypassed and there should be battery voltage 12.6 to 13 volts to the coil on a fully charged battery. If in doubt the ideal resistance readings from the Master Tech series are below. Try checking the voltage to the coil lead with the coil lead disconnected from the coil, a shorted coil will draw excessive load and drop the voltage reading. Also when checking the ohms of the resistor, be sure all leads are disconnected to remove any load draws from the reading. For the complete Master Tech article on electronic ignition go to www.imperialclub.com you can access the electronic ignition series in the Literature section from the home page. If you have not already done so, check the wiring to be sure the protection circuit to the ECU is hooked to the proper side of the dual ballast resistor. One side is .55 ohms, the run circuit, the other side is 5 ohms, the protection circuit for the ECU.

Dave

Ballast resistor.jpg
 
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I installed an aftermarket electronic ignition on my '68 Chrysler and it would not start while cranking, but would start (or try to) when you released the key from the start position and moved it to the run position. It pretty much did just what you are experiencing. In my experience this is opposite of what happens when a ballast resistor is bad. In my case, the problem was caused by the gap between the pickup and the part that turns inside the distributor (I don't know the proper name of it). Anyway, that gap has to be very tiny. You adjust it with a brass feeler gauge. I think the gap is supposed to be .008" but I've heard of people setting them even to .005" with good results. If it still doesn't start properly after checking this adjustment, I would think that maybe the pickup is going bad or the ECU.
 

thethee

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At this point I'd say it can only be the ECU itself. Do you have a spare you can swap in temporarily? If you use a 4 prong you don't have to get another ballast resistor
 

1970FuryConv

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I installed an aftermarket electronic ignition on my '68 Chrysler and it would not start while cranking, but would start (or try to) when you released the key from the start position and moved it to the run position. It pretty much did just what you are experiencing. In my experience this is opposite of what happens when a ballast resistor is bad. In my case, the problem was caused by the gap between the pickup and the part that turns inside the distributor (I don't know the proper name of it). Anyway, that gap has to be very tiny. You adjust it with a brass feeler gauge. I think the gap is supposed to be .008" but I've heard of people setting them even to .005" with good results. If it still doesn't start properly after checking this adjustment, I would think that maybe the pickup is going bad or the ECU.
hi Bronze Turbine,

@WissaMan and I thought the same thing. And I appreciate you giving me the info. 1st I reset the gap on my original distributor, which did not help. Then I thought the original distributor had too much up and down play in the shaft, such that the reluctor and the pickup coil weren’t properly aligned. I bought a rebuilt distributor from member Halifax hops. Ray knows what he’s doing. The car does run better once it started, but it still has the same problem. I think the problem is elsewhere. Thanks again, Ben
 

1970FuryConv

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At this point I'd say it can only be the ECU itself. Do you have a spare you can swap in temporarily? If you use a 4 prong you don't have to get another ballast resistor
Hi Theo,
I messaged Rick Ehrenberg, who at least used to be the tech writer for Mopar Action magazine. He’s a big eBay seller now. He thinks the same thing as you and thinks that I should go to a new 4 prong ECU. Of course, he offers one for sale, so I’ll probably buy his. As to the 2nd 2 issues he raised, I replaced the ignition coil with no change. As stated above, I reset the gap on my 1st electronic distributor, no change. I bought a rebuilt distributor from Halifax hops, again no change. Thanks for confirming Rick’s advice! Ben

Rick’s eBay message

I don't think ballast is the issue. I think this (ECU) will fix it, no wiring or ballast changes needed. Plug 'n' play.
for MOPAR HR7500 Electronic Ignition Module Dodge Plymouth Chrysler MAX RELIABLE | eBay
Of course, the proof of the pudding is in the eating! Also possible to have shorted turns in coil, or reluctor gap too wide.
Rick
 

1970FuryConv

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You do not appear to have a bad ballast resistor. In the start position, the resistor is bypassed and there should be battery voltage 12.6 to 13 volts to the coil on a fully charged battery. If in doubt the ideal resistance readings from the Master Tech series are below. Try checking the voltage to the coil lead with the coil lead disconnected from the coil, a shorted coil will draw excessive load and drop the voltage reading. Also when checking the ohms of the resistor, be sure all leads are disconnected to remove any load draws from the reading. For the complete Master Tech article on electronic ignition go to www.imperialclub.com you can access the electronic ignition series in the Literature section from the home page. If you have not already done so, check the wiring to be sure the protection circuit to the ECU is hooked to the proper side of the dual ballast resistor. One side is .55 ohms, the run circuit, the other side is 5 ohms, the protection circuit for the ECU.

Dave

View attachment 511842
hi Dave,

I agree that the ballast resistor is bypassed with current going to the coil directly from the ignition switch, when the key is in start position. I checked resistance across the start wire circuit and found .3 ohms. I don’t understand how with the engine actually cranking over and the starter drawing power, 12 V can still get to the ignition coil. My battery goes down to 11.0 V with the starter cranking.

Ballast resistor: bench testing both ballast resistors that I have used, the resistance is 1.2 to 1.3 ohms on the primary side and 5.0 ohms on the secondary ECU side. The wires are hooked to the correct side of the dual ballast resistor. My concern given the figures in the article is that the resistance on the primary side should be .55 ohms and I’ve got 1.2 ohms. I wonder if that difference is enough to shut down the computer ECU because power to the ECU does not bypass the ballast resistor during start up. The specifications page in the factory shop manual does quote resistance primary (coil side) .50 – .60 ohms. Given my bench resistance tests, do you think my ballast resistor is causing the cold start problem? Thanks, Ben
 

Davea Lux

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11.0 V should be enough to power the ECU. Did you let the resistor warm up prior to the test or were those cold numbers? The resistance will drop as the unit warms up. Given that the "start" circuit appears to be dead, that part of the ECU may be burned out assuming that it is getting voltage in the start position.

Dave
 

1970FuryConv

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11.0 V should be enough to power the ECU. Did you let the resistor warm up prior to the test or were those cold numbers? The resistance will drop as the unit warms up. Given that the "start" circuit appears to be dead, that part of the ECU may be burned out assuming that it is getting voltage in the start position.

Dave
Thanks, Dave
the ECU is currently getting 8.1 V at start engaged, cranking, no ignition.

The resistance tests that I performed on the 2 ballast resistors were on the bench and completely cold.

Thank you for the heads up on the start up circuit of the ECU. I did not know that was different from the run circuit.

I ordered new ECU and matching single ballast resistor from Rick Ehrenberg yesterday (link above). I talked to Halifax hops extensively on the phone last night. He says that Rick’s ECU is a quality ECU. He also says that some ECUs can run on voltage of 9 V, but it doesn’t sound like the 8.1 V that my ballast resistors are allowing to the ECU is allowing ECU to function.

I also don’t understand how any ECU can get 11 V if the coil side of the ballast resistor is designed to reduce charge to 9 V. It would seem, during start/cranking, that the ECU would get 9 V when charge goes the opposite direction from the start circuit through the coil side of the resistor and to ignition run which powers the ECU. If you can offer further enlightenment on the subject I’d be grateful. Thanks Ben.
 

1970FuryConv

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THE FIX
Ballast Resistor & ECU HIREV 7500

Ballast Resistor dual: disconnect 4 connectors. BR dual: remove, 7/16 ss, 6 ext, ¼ R. BR cold: dual 1.6/5.3 ohms, trash. Primary side s/b .5 ohms

BR new, single, cold: .9 ohms
BR1.jpg

BR new, install: 7/16 ss, 6 ext, ¼ R. Install ignition run and start. Cut off end of primary jumper and leave aux power to ECU DG/R disconnected. Not used with 4-prong ECU.
BR2.jpg

ECU, 5-prong, remove: elec conn, PHSD. Bolts: 7/16 ss, 3 ext LS, 6 ext RS, ¼ R
PXL_20220204_180736635.jpg

Coil primary resistance: 1.8 ohms. Must be over 1.4 ohms by Ehrenberg directions

ECU Base: resistance to ground .5 ohms., ground wire loose from base, .2 ohms. Running without good ground destroys ECU instantly per Ehrenberg.

New ECU, HIREV 7500: run without BR destroys ECU in short time.
PXL_20220204_180642424.jpg

Sil-Glyde: apply to 4 prongs, Q-tip. Also, end of each hole in connector
PXL_20220204_182343506.MP.jpg

New ECU: 7/16 ss, 3 ext, ¼ R. Connector does not seat as low as 70 Fury, but does install, PHSD
PXL_20220204_183608546.MP.jpg

PXL_20220204_183639840.jpg

Cold Start: Started 1st turn of key, in start position!!!!! Idle completely normal.

Idle charging before test drive: BR: Ignition Run 13.98 V, Ignition Start 11.48 V, Coil + 11.45V, Battery 14.23 V

Test Drive: normal.

Idle charging after test drive: Ballast Resistor: Ignition Run 14.0 V, Ignition Start 11.9 V, Coil + 11.86 V, Battery 14.22 V

Engine Off, key at ign run: Ballast Resistor: Ignition Run 12.05 V, Ignition Start 9.61 V, Coil + 9.57 V, Battery 12.57 V

Battery 12.8 V, ignition off
 

thethee

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Awesome, glad it's fixed!
If all else checks out, it's gotta be the ECU
 
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1970FuryConv

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Started again today, 35°F, 1st turn of key. Now I have to deal with a shorted door jam switch, so I can keep my battery connected. It's always something!:BangHead::BangHead::BangHead:
 
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