major voltage drop

[john h]

team,
I was poking around trying to figure out where I have 12 volt power in start and run positions so I could power an EFI upgrade.  I was specifically looking at the wire for the choke.  I put my meter on it and I have 11.7 Volts in key on.  I pulled the main coil wire and had someone crank. my voltage at the choke wire drops to 7.2 V  I also did the same at the blue wire right were it goes into the ballast resistor.  I get 11.7 until I crank, then drops to 7.2 immediately after cranking for 2-3 seconds. same thing at the brown IG2 wire right at the ballast.  The battery has full charge and the starter turns very fast.  I never have issues starting the car when it's cold.  after it's run up to temp and sat for 20 minutes, it has to crank quite a bit to start but I assumed that was fuel in the bowl boiled out.  it always cranks fast. warm or cold.  I'm pretty sure that I won't be able to run any EFI computer with that much voltage drop in the system.  I have the standard orange ECU with newer engine harnesses.  another odd thing is that I have power at both the brown wire IGN 2 and the Blue Wire IGN 1 at key on. any ideas of what to start looking for?

Thanks.  John

[734406pk]

All good info. Where do you have your ground test lead connected? Voltage drop while cranking is normal, but your voltage readings are low to start with (Key on engine off).

[john h]

All good info. Where do you have your ground test lead connected? Voltage drop while cranking is normal, but your voltage readings are low to start with (Key on engine off).

I just had the test ground sort of wedged against the carb.  Now that you brought that up, I maybe should have made a "good"connection to the battery.  I should probably head out tomorrow after work and do a proper test. maybe I don't have as much drop as I thought.

[734406pk]

I think your right. Connect your ground lead directly to the battery negative terminal and retest. You may have a ground fault issue here. Post back if you need help with the diagnosis.

[john h]

I think your right. Connect your ground lead directly to the battery negative terminal and retest. You may have a ground fault issue here. Post back if you need help with the diagnosis.

thank you for that idea.  Hopefully, that will help me sleep tonight and not worry about what I may have to fix all night and all day tomorrow at work. 

[734406pk]

thank you for that idea.  Hopefully, that will help me sleep tonight and not worry about what I may have to fix all night and all day tomorrow at work.

No problem! That's why C-C.com members are here to help!

[john h]

I got some testing done, but tonight I made sure my test lead was  securely clamped to the negative battery post.  I ran multiple scenarios.  I have to say though that my meter is not the most expensive or most sensitive.  it's just a cheap one.   here are my tests:

Battery 12.6 volts
Ignition in on position:   Blue ballast wire 11.74V
                                       Green Ballast wire 11.74
                                       choke wire 11.65v
disconnect ECU 4 pin
Ignition on:                     Blue Ballast wire 12.25V
                                       Green Ballast wire 12.25V
                                       choke wire             12.25V
                                       blue wire at ECU connector 12.27
                                       green wire at ECU connector  12.27

ECU wire connected-Crank 3-4 Seconds:
                                       Choke wire 8.3V
                                       Blue Ball wire 8.28V
                                       Green Ball wire 8.28V
                                       Brown Ballast wire 10.4V

ECU not connected-Crank 3-4 seconds:
                                       Brown Ballast wire 11.6V
                                       Blue Ballast wire 8.95V
                                       Choke wire   8.95V
                                       ECU BLue wire 8.96V
                                       ECU Green Wire 8.96v    Batt starting to get a bit weak at this point down around 12.1 volts but still cranks starter strong. put 2amp 12v charger on
so, I can't tell if I have a big issue or not.  One thing I do know is it's better without the ECU powered, and I'm taking all that out since I bought EFI with timing control today. the only thing still needed is the Voltage regulator.  I also noticed the ballast resistor is not tight to the firewall the self tapping bolt hole is wobbled out but in there does that ballast need a great ground?

Thanks John

[734406pk]

John- nice job on the voltage readings! Your battery voltage is taking a major dive while cranking. The absolute lowest voltage you should see is 9.6 v at the battery. Either you have a weak battery that can't deliver the required amperage or the starter amp draw is so high that it pulls the battery down below the threshold. I think the easiest thing to do is to take the battery to Autozone etc and have it tested. For an EFI system, i would want at least 950 CCA at 0 degrees battery rating. If your battery is tested and meets this spec, look at checking the starter motor for excessive amp draw due to shorted windings or (i hope not) excess engine rotational drag. It should not take more than 50 FtLbs to rotate the crankshaft unless you have some VERY high compression ratio. This is a general spec. Good luck and post back your results.

[john h]

John- nice job on the voltage readings! Your battery voltage is taking a major dive while cranking. The absolute lowest voltage you should see is 9.6 v at the battery. Either you have a weak battery that can't deliver the required amperage or the starter amp draw is so high that it pulls the battery down below the threshold. I think the easiest thing to do is to take the battery to Autozone etc and have it tested. For an EFI system, i would want at least 950 CCA at 0 degrees battery rating. If your battery is tested and meets this spec, look at checking the starter motor for excessive amp draw due to shorted windings or (i hope not) excess engine rotational drag. It should not take more than 50 FtLbs to rotate the crankshaft unless you have some VERY high compression ratio. This is a general spec. Good luck and post back your results.

Thanks for the ideas.  I will pull the battery.  IT's an autoZone battery and likely still has some warranty left.  I guess I did not think to test the battery at crank.  It seems like the power going to the ECU is robbing the system since it drops so much when the 4 pin is connected.  I wonder if that unit is failing??  Like I mentioned before, it does not affect the starting of the car and once everything is running, everything gits around 13.5-14V.  I just know that EFI is very sensitive.  Does the ballast need to be grounded properly?  the bolt is not tight.  I also noticed I have a 4 pin connector on the ECU but 2 dual Ballast  I thought the single ballast dropped the green wire out, but that has power at the 4 pin connector.   

[734406pk]

I don't think that the ECU is robbing power, but i think you have some excess voltage drop to the ignition blue wire and/ or ground circuit. It is easy to test, but you need to test the battery first so we are not thrown a "curve ball". Don't be concerned at this time with post-ballast voltage and grounding the ballast (there is none just a fastener). Get the battery tested for 900CCA minimum and we can go from there. Good Luck!

[Chryco Psycho]

 I would also be checking every connection point in the circuit such as the firewall bulkhead connector in & out , the plug for the ign switch , corrosion at any of these point will drop voltage  to the ecu & coil , you can also test power into the ign switch at the base of the column & power out red wire in & blue wire out to see if there is any loss through the ign swtich itself

[john h]

further testing is going to have to wait a bit.  I just dropped the car off for a new new ring and pinion.  The new rebuild was making more noise than the old worn ring and bearings.  Bearings were better I guess but the ring was awful.  I pulled it all and put it back together the first time but the drive line shop said they would do everything for my trouble.  They claim a defective Yukon gear set.   they said even if they screwed up the tolerances, it would not be that loud.  Anyway, long story longer, I did not want to pull too much of the wiring apart testing because I knew it was going in tonight and I would not have it for a few days. 

Thanks for the ideas guys.  I will check it all this coming weekend.

John