Re: Voltage drop during cranking
[Re: AndyF]
#2512038
06/23/18 02:38 AM
06/23/18 02:38 AM
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Joined: Aug 2011
Posts: 2,842 Pattison Texas
CSK
master
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master
Joined: Aug 2011
Posts: 2,842
Pattison Texas
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are you using a relay connected direct to the battery for the ECU ?
1968 Charger COLD A/C Hilborn EFI 512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim 2020 T/A 392 Stock 11.79 @ 114.5
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Re: Voltage drop during cranking
[Re: AndyF]
#2512054
06/23/18 07:55 AM
06/23/18 07:55 AM
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Joined: Jan 2003
Posts: 2,902 A shed in England
Tig
master
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master
Joined: Jan 2003
Posts: 2,902
A shed in England
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Now that I have a data logger (Sniper EFI system) I can see stuff that I haven't seen before. For example, I can see the system voltage while cranking. When the engine is hot the start slows down a little bit so I looked at the data logger and the voltage dips down to about 9 volts before the engine fires. That seems like it might be a tad low since the ignition system might get a little weak at that point.
Anyone else with a data logger know what their voltage is during a hot start? I'm not sure if I have a problem or not. If I do have a problem I'm not sure if it is starter heat soak or perhaps a voltage drop in the wiring. I've done this with a grid, like yourself we had around 9v when cranking at best. I put this down to being just the way things are and played with start rpm timings etc to get it as good as I could. Motor would turn lazy, kick back etc. I've since totally rewired the car and run an earth "ring" around the car with bus bars to pick up earth points for stuff. I've also run a heavy gauge earth cable direct to the starter. The difference is amazing, motor spins quick hot or cold and last time I checked it we had 11V + cold cranking. I have to put this down to a better earth and less voltage drop on the earth side, bad earths also get worse when hot. I'd at least run a good ground wire to the starter to see if it makes any difference.
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials. 9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge. RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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Re: Voltage drop during cranking
[Re: AndyF]
#2512072
06/23/18 10:14 AM
06/23/18 10:14 AM
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Joined: Feb 2008
Posts: 710 Central TEXAS!!!!
sr4440
super stock
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super stock
Joined: Feb 2008
Posts: 710
Central TEXAS!!!!
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Andy, that's too big of a drop. Preform a voltage drop test and find out where the issue(s) are located.
Joe
Without Data, you’re just another guy with an opinion.
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Re: Voltage drop during cranking
[Re: AndyF]
#2512136
06/23/18 12:48 PM
06/23/18 12:48 PM
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Joined: Jan 2003
Posts: 2,902 A shed in England
Tig
master
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master
Joined: Jan 2003
Posts: 2,902
A shed in England
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Just for Info, the MSD grid is only measuring and logging supply voltage (to the grid itself). So theoretically the voltage could have been different across the starter motor.
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials. 9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge. RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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Re: Voltage drop during cranking
[Re: AndyF]
#2512172
06/23/18 02:09 PM
06/23/18 02:09 PM
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Joined: Jan 2003
Posts: 19,304 Las Vegas
Al_Alguire
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 19,304
Las Vegas
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My TS car was having issues until we ran a ground cable up to the starter itself. Just was not happy any other way. Likely the 60MM cam huge spring load and babbit bearings are all partly the cause. Ive never had to do this before but this engine is just not happy any other way
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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Re: Voltage drop during cranking
[Re: AndyF]
#2512243
06/23/18 05:16 PM
06/23/18 05:16 PM
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Joined: Jan 2003
Posts: 4,267 Morrow, OH
markz528
master
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master
Joined: Jan 2003
Posts: 4,267
Morrow, OH
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Andy - you should measure the voltage drop on the ground circuit. Measure from the negative battery post to the starter housing. I personally would use my Fluke 87 and use the min/max memory function at the fast sampling rate, but just watching it is fine. Between your ground cable and the body ground, the drop should be minimal. I would expect the drop to definitely be under 1 volt. Unfortunately I don't remember what mine came in at.
67 Coronet 500 9.610 @ 139.20 mph 67 Coronet 500 (street car) 14.82 @ 94 mph 69 GTX (clone) - build in progress......
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Re: Voltage drop during cranking
[Re: AndyF]
#2512255
06/23/18 05:52 PM
06/23/18 05:52 PM
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Joined: Dec 2005
Posts: 11,408 Fulton County, PA
CMcAllister
Mr. Helpful
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Mr. Helpful
Joined: Dec 2005
Posts: 11,408
Fulton County, PA
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Voltage drop is the voltage lost from the battery to the starter. It can be measured by actually measuring the voltage from the hot post on the battery to the starter stud while cranking.
Check the voltage at the battery while cranking. If it is also that low, the battery is weak. If the drop is actually a couple of volts, that's something other than a cable being a size or two too small.
9 is pretty low. I maintain a lot better than that running 2ga. from the back of the car. Check connections and check the drop across the master switch if you have one. Those things don't last forever. Also check starter current draw.
If the results don't match the theory, change the theory.
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