THANK YOU!!! Fixed my '87 El Camino tach quick fast and in a hurry. I didn't have 200K ohm resistor on hand and as tempting as it was to order 100 of them for only $6.95 with free prime shipping, I just used 2 100K ohm resistors in series. Idle shows about 750 RPM which for me at the moment is "certified good enough". Thanks again!
On the small white IC chip the black you see in 2 places on the top of the chip going to the pins are carbon film resistors . If you were to remove the chip by unsoldering all 16 leads you would find 2 more carbon film resistors going across to other leads of the chip on the underneath side of it. The Carbon film has clear lacquer over them. If you look closely at the carbon film you will see a small thin line or cut going partially down the middle of it which is how AC Delco changed the resistance of the carbon film between the two leads to calibrate the tach. There are over 40 different configurations of this white IC chip with different resistances going to different IC leads. Just so folks know not every GM tach that has the little white IC chip will have the same carbon film resistors between the leads as shown in the video and even if configured the same the resistances may not be the 200 Ohms. Great job fixing the tach. I will usually put a small potentiometer across these leads so I can adjust calibration up around 5 or 6K RPM. AC Delco put out a tachometer repair service bulletin back in the early 80's showing all the schematics of these boards and the configurations and the layouts of the little white IC resistor chips.
On mine, the tach calibration resistor was on that resistor chip, and it had almost opened up completely. You could see how the resistor film had deteriorated with the naked eye. The target range on mine (6 cylinder) was somewhere around 250k Ohms.
THANK YOU!!! Fixed my '87 El Camino tach quick fast and in a hurry. I didn't have 200K ohm resistor on hand and as tempting as it was to order 100 of them for only $6.95 with free prime shipping, I just used 2 100K ohm resistors in series. Idle shows about 750 RPM which for me at the moment is "certified good enough". Thanks again!
@@N68172 outstanding. !! I've fixed several GM tacks with this method. Amazing, isn't it? 👍
On the small white IC chip the black you see in 2 places on the top of the chip going to the pins are carbon film resistors . If you were to remove the chip by unsoldering all 16 leads you would find 2 more carbon film resistors going across to other leads of the chip on the underneath side of it. The Carbon film has clear lacquer over them. If you look closely at the carbon film you will see a small thin line or cut going partially down the middle of it which is how AC Delco changed the resistance of the carbon film between the two leads to calibrate the tach. There are over 40 different configurations of this white IC chip with different resistances going to different IC leads. Just so folks know not every GM tach that has the little white IC chip will have the same carbon film resistors between the leads as shown in the video and even if configured the same the resistances may not be the 200 Ohms. Great job fixing the tach. I will usually put a small potentiometer across these leads so I can adjust calibration up around 5 or 6K RPM. AC Delco put out a tachometer repair service bulletin back in the early 80's showing all the schematics of these boards and the configurations and the layouts of the little white IC resistor chips.
On mine, the tach calibration resistor was on that resistor chip, and it had almost opened up completely. You could see how the resistor film had deteriorated with the naked eye. The target range on mine (6 cylinder) was somewhere around 250k Ohms.
OL= open test leads
Nice