TI ignition resistor values
Thread Starter
Le Mans Master
Joined: Dec 2006
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TI ignition resistor values
R7 and R8 are built into the wiring harness. R7 is the resistor wire between the ignition switch and the system. R8 is the resistor wire between the amp and the coil. I know the values are .43 ohm and .68 ohm. What I cannot find on GM documents is which is which. Actually, I have seen prints with the values listed both ways. Does anyone have a GM document showing these values?
I am 90% sure I know the answer, but would like to see something in black and white.
Has anyone ever taken careful measurements off of their harness?
I am 90% sure I know the answer, but would like to see something in black and white.
Has anyone ever taken careful measurements off of their harness?
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
R7 and R8 are built into the wiring harness. R7 is the resistor wire between the ignition switch and the system. R8 is the resistor wire between the amp and the coil. I know the values are .43 ohm and .68 ohm. What I cannot find on GM documents is which is which. Actually, I have seen prints with the values listed both ways. Does anyone have a GM document showing these values?
I am 90% sure I know the answer, but would like to see something in black and white.
Has anyone ever taken careful measurements off of their harness?
I am 90% sure I know the answer, but would like to see something in black and white.
Has anyone ever taken careful measurements off of their harness?
My ohm meter range stops at .1
My measurement for your requested R values are:
R8 (coil+ > Amp plug | white>black) = .6
R7 (Ign plug > Amp plug | white>pink = .9
Am guessing you already have below diagram:
If you have additional documentation please post.
Thread Starter
Le Mans Master
Joined: Dec 2006
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Interesting. Not what I expected. I did expect R8 to be .68 ohms. The .9 ohms on R7 is twice what I would have thought.
All the schematics I have found show a total of the two to be around 1-1.1ohms.
Is this an original or a repop harness? (just curious).
I REALLY appreciate you taking the time to measure these values.
All the schematics I have found show a total of the two to be around 1-1.1ohms.
Is this an original or a repop harness? (just curious).
I REALLY appreciate you taking the time to measure these values.
Last edited by Big2Bird; Mar 21, 2018 at 02:46 PM.
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
Interesting. Not what I expected. I did expect R8 to be .68 ohms. The .9 ohms on R7 is twice what I would have thought.
All the schematics I have found show a total of the two to be around 1-1.1ohms.
Is this an original or a repop harness? (just curious).
I REALLY appreciate you taking the time to measure these values.
All the schematics I have found show a total of the two to be around 1-1.1ohms.
Is this an original or a repop harness? (just curious).
I REALLY appreciate you taking the time to measure these values.
Where are you getting the values you're expecting?
BTW: I measured each several times and cleaned the contact/wire to eliminate any resistance due to oxidation. Sorry my meter isn't more accurate but seldom necessary in this hobby.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Per the Chevrolet Chassis Service Manual, in the 'Specifications' section at the back of the book, and in the IGNITION COIL table, the two resistors are listed as .43 ohm and .68 ohm, just as you posted. But it doesn't indicated which is which.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
Last edited by 7T1vette; Mar 21, 2018 at 04:41 PM.
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
Per the Chevrolet Chassis Service Manual, in the 'Specifications' section at the back of the book, and in the IGNITION COIL table, the two resistors are listed as .43 ohm and .68 ohm, just as you posted. But it doesn't indicated which is which.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Per the Chevrolet Chassis Service Manual, in the 'Specifications' section at the back of the book, and in the IGNITION COIL table, the two resistors are listed as .43 ohm and .68 ohm, just as you posted. But it doesn't indicated which is which.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
However, with the measured values provided above, I would suggest that R8 is .43 ohm and R7 is .68 ohm. Measuring small resistances requires a 4-wire evaluation, so that the resistance in the test leads from the ohmmeter do not factor into the readings. When using a 2-lead ohmmeter, the 'actual' values measured will be slightly higher than the REAL value of those resistances.
2) I have found errors in GM manuals over the years.
3)There is also a service bulletin addressing changes from 65-66.
Let's see what Mike says about it. We have discussed this before, but much info has been lost to obscurity.
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Thread Starter
Le Mans Master
Joined: Dec 2006
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Melting Slicks
Joined: May 2007
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Tech Contributor
Joined: Jun 2004
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
1) A Wheatstone bridge would be more accurate, but his relative measurement speaks for itself.
2) I have found errors in GM manuals over the years.
3)There is also a service bulletin addressing changes from 65-66.
Let's see what Mike says about it. We have discussed this before, but much info has been lost to obscurity.
2) I have found errors in GM manuals over the years.
3)There is also a service bulletin addressing changes from 65-66.
Let's see what Mike says about it. We have discussed this before, but much info has been lost to obscurity.
Sitting at my desk I would have designed it the other way around (for the reasons we discussed earlier), but perhaps there are some in-vehicle issues that persuaded GM to do it the other way. Whatever the reason, I will defer to the information supplied by Fred and PA man.
Tech Contributor
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Don't know if this post is directed to me or not. (My point above was that I would have swapped the values of R7 and R8, not the position of R8.)
Thread Starter
Le Mans Master
Joined: Dec 2006
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Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 8,837
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1) Can you borrow another meter to test it again?
2)Can someone else post their harness values?
3)Mike, what would his posted values do to the circuit?
4)What if the values of the resistance wire changed with age?
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
My readings taken from the harness includes the lead in wire for those resistance wires as I'm not unwrapping the original harness.
For R8 (coil resistance wire) and R7 (run wire) those readings are proportional to the values given by PAmotorman above and the specs in service manual: That is; R8 is lower than R7 by roughly the same amount/ratio
R8 = .6 v .5 v .43
R7 = .9 v .7 v .68
Further: if you check the Chevrolet Service News April 1964; Ignition System-3, Figure 3 trouble diagnosis procedure you'll find in the flow chart that the Spec for replacing the Coil resistance wire is "(approx. .45 ohm) and the spec for replacing the run wire is (approx. .7 ohm).
Finally, there's a very in depth troubleshooting article by John P. Means "More on Transistor Ignition Troubleshooting" that gives the harness values for R8 & R7 as @.5 & @.7 respectively.
So consensus is R8<R7
Where R8= @.43 and R7 = @.68 ohms
Last edited by Hammerhead Fred; Mar 22, 2018 at 10:34 AM.
Melting Slicks
Joined: May 2007
Posts: 2,415
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i used my tester to repair 100s of amps back in the day. i did most all the dealers and corvette shops in the area as i believe i was the only one with a tester. i tested some amps that were not bad so the owners needed to look elsewhere. most of the time it was a bad ground wire at the amp.
Thread Starter
Le Mans Master
Joined: Dec 2006
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Things to consider:
My readings taken from the harness includes the lead in wire for those resistance wires as I'm not unwrapping the original harness.
For R8 (coil resistance wire) and R7 (run wire) those readings are proportional to the values given by PAmotorman above and the specs in service manual: That is; R8 is lower than R7 by roughly the same amount/ratio
R8 = .6 v .5 v .43
R7 = .9 v .7 v .68
Further: if you check the Chevrolet Service News April 1964; Ignition System-3, Figure 3 trouble diagnosis procedure you'll find in the flow chart that the Spec for replacing the Coil resistance wire is "(approx. .45 ohm) and the spec for replacing the run wire is (approx. .7 ohm).
Finally, there's a very in depth troubleshooting article by John P. Means "More on Transistor Ignition Troubleshooting" that gives the harness values for R8 & R7 as @.5 & @.7 respectively.
So consensus is R8<R7
Where R8= @.43 and R7 = @.68 ohms
My readings taken from the harness includes the lead in wire for those resistance wires as I'm not unwrapping the original harness.
For R8 (coil resistance wire) and R7 (run wire) those readings are proportional to the values given by PAmotorman above and the specs in service manual: That is; R8 is lower than R7 by roughly the same amount/ratio
R8 = .6 v .5 v .43
R7 = .9 v .7 v .68
Further: if you check the Chevrolet Service News April 1964; Ignition System-3, Figure 3 trouble diagnosis procedure you'll find in the flow chart that the Spec for replacing the Coil resistance wire is "(approx. .45 ohm) and the spec for replacing the run wire is (approx. .7 ohm).
Finally, there's a very in depth troubleshooting article by John P. Means "More on Transistor Ignition Troubleshooting" that gives the harness values for R8 & R7 as @.5 & @.7 respectively.
So consensus is R8<R7
Where R8= @.43 and R7 = @.68 ohms
IF all those statements are true, then that would mean your harness is out of spec.
I find that hard to believe.
The difference "could" be ideal engineering design vs acceptable production parameters. Higher total values in the circuit would increase service life.
Mr. Means may have misinterpreted his conclusion as well.
The test panel I am building utilizes variable resistors, so I can test various values to see what happens. The newer TI amp boards may even prove robust enough to lower these vales, or reverse them as "may have been intended," but proved excessive for the old germanium transistors.
Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
i used my tester to repair 100s of amps back in the day. i did most all the dealers and corvette shops in the area as i believe i was the only one with a tester. i tested some amps that were not bad so the owners needed to look elsewhere. most of the time it was a bad ground wire at the amp.
