72 Rally Pack Question
April 10th, 2019, 07:06 AM
#1
Joe Ralph
Thread Starter
Join Date: May 2017
Location: Lansing, Michigan
Posts: 685
72 Rally Pack Question
Not sure If this has been covered before ... but can I run two gauges off of one sending unit? My thoughts are this.... I want to keep the original gauges working - BUT I would like to run a new set of oil pressure / temp / and volts just to monitor the car for a few and see how it’s doing - after driving it like that for awhile I hope to remove the new gauges and get a better idea how the rally gauges react to changing circumstances etc - thoughts?
April 10th, 2019, 04:59 PM
#2
Registered User
Join Date: Sep 2011
Location: Plano, TX
Posts: 1,844
Short answer: Volts and tach yes. Others, nope. I'm also assuming you're talking about electronic gauges with senders - not mechanical gauges - for your aftermarket ones.
Long answer: pressure and temp gauges work by making a voltage divider circuit with the bottom leg of the divider is a variable resistor in the sender, and the top half of the divider is a fixed, known, resistor in the gauge itself. Splicing an additional standard gauge onto an existing sender wire will wildly change the divider circuit and neither gauge will read correctly. There's a million different configurations, which is why you always have to keep senders with the gauges they came with. You can actually find specs on gas level senders, and there's four or five common configurations there alone!
You can do this in a non-standard (read:complicated) way. I added an arduino to my cutlass to do a few things and needed water temp and a few other bits. I set up the arduino to have a much higher impedance than a standard gauge or sender, so the presence of the arduino doesn't materially affect the existing gauge. You can hack your way into this by putting a voltmeter onto the sender line and doing some testing to establish the voltage to temp curve, then just watch the voltmeter.
....
Anyway, no. You need to add the additional senders with the aftermarket kit. This is a real headache with oil pressure, so I recommend using mechanical gauges and putting a 1/8" NPT tee on the oil pressure tap so you can run the rally pack sender and the mechanical line. Make sure to use a copper line instead of the POS plastic tubes.
Long answer: pressure and temp gauges work by making a voltage divider circuit with the bottom leg of the divider is a variable resistor in the sender, and the top half of the divider is a fixed, known, resistor in the gauge itself. Splicing an additional standard gauge onto an existing sender wire will wildly change the divider circuit and neither gauge will read correctly. There's a million different configurations, which is why you always have to keep senders with the gauges they came with. You can actually find specs on gas level senders, and there's four or five common configurations there alone!
You can do this in a non-standard (read:complicated) way. I added an arduino to my cutlass to do a few things and needed water temp and a few other bits. I set up the arduino to have a much higher impedance than a standard gauge or sender, so the presence of the arduino doesn't materially affect the existing gauge. You can hack your way into this by putting a voltmeter onto the sender line and doing some testing to establish the voltage to temp curve, then just watch the voltmeter.
....
Anyway, no. You need to add the additional senders with the aftermarket kit. This is a real headache with oil pressure, so I recommend using mechanical gauges and putting a 1/8" NPT tee on the oil pressure tap so you can run the rally pack sender and the mechanical line. Make sure to use a copper line instead of the POS plastic tubes.
April 10th, 2019, 06:11 PM
#3
Joe Ralph
Thread Starter
Join Date: May 2017
Location: Lansing, Michigan
Posts: 685
Thank you for taking the time to reply - my Rallye gauges are wired wrong - They are upgraded from idiot lights and the idiot that installed them (me) ran them straight like the idiot lights so pegging and what not - so i’ve got my next project - once I get them corrected I plan on doing more conventional methods (I.e. oil pressure testing at various rpms and comparing it to the rally gauges at same Rpms - and temp testing versus rally gauges to get an idea how the gauges read) thanks again
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