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06-30-2012
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#1 |
 
Join Date: Jan 2012
Location: Wisconsin
Posts: 886
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How-to Use HVAC Display as Tachometer
Did you know you can check engine RPM on your HVAC (C68) display? Why? Well, my tach is way off and I wanted to see how far. I ordered a Tech 1 which didn’t work, so I’m wondering how could I see how far off the tach was.
As I was troubleshooting an AC problem, I discovered that in addition to displaying HVAC fault codes, the HVAC display could show other parameters too. In my 1990 FSM, this is on page 1C1-5 To display fault codes from the HVAC system, press and hold both [fan up] and [fan down] buttons simultaneously for 5 seconds. Pressing the [fan auto] button after that will display any HVAC fault codes (eg, I had code 09 – low Freon). Now, if you press [fan up] you can move through a number of additional parameters. Parameter 6 is engine RPM (RPM / 25). After pressing [fan up] to display “6,” press [fan auto] to see the value. At idle my tach showed just a tad above 1000 RPM. Parameter 6 on the HVAC showed 26. So 26 * 25 = 650 RPM. (As a noob, I keep looking for some information that I can share to help pay back all the great info I get here – I searched and looked in solutions, and I didn’t see this … but, it’s probably posted and many of you may know this) Last edited by ZBrink; 1 Week Ago at 05:12 AM. Reason: Title Edit |
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07-01-2012
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#2 |
 Join Date: Aug 2011
Location: Evergreen, Co.
Posts: 344
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Re: Use HVAC display as tach
Yes, I just found this too, and I was surprised that my tach was dead on accurate at idle 3k and 5k too. I believe parameter 16 is engine temp in Celsius too.
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Bert Rankin 1991 ZR1 Steel Blue / Steel Blue #1029 Haibeck chip, slotted rotors, camber brace, C-beam plates, OBX exhaust, Nitto NT555 tires [SIGPIC][/SIGPIC] |
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07-01-2012
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#3 |
 
Join Date: Dec 2003
Location: S. Florida
Posts: 2,649
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Re: Use HVAC display as tach
Reputation points added for an informative post.
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--- It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat. Theodore Roosevelt - April 23, 1910 |
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07-01-2012
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#4 |
 
Join Date: Jan 2009
Location: Portland Oregon metro area (Washington side)
Posts: 3,206
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Re: Use HVAC display as tach
One parameter that is not in the table of the 90MY FSM is #16 which is the water temp in deg C.
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Scott  Vett owner since 1979._It's about the car and the people Last edited by scottfab; 11-11-2012 at 12:10 AM. Reason: corrected F to C and added "90MY FSM" |
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07-01-2012
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#5 |
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Join Date: Nov 2011
Location: Dunbarton NH
Posts: 7,536
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Re: Use HVAC display as tach
Hey, that's pretty cool. Bonus, my tach appears to be right on.
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07-01-2012
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#6 | |||
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Join Date: Jan 2012
Location: Wisconsin
Posts: 886
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Re: Use HVAC display as tach
Quote:
Quote:
Quote:
 Here's the page from my FSM for the published parameters ... 
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07-01-2012
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#7 |
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Join Date: Mar 2009
Location: Orlando, FL
Posts: 1,821
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Re: Use HVAC display as tach
Here's the chart from the 1991 FSM
Parameter 16 is actually the coolant temp in C, not F. 
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2004 Z06/Z16 LeMans Commemorative Edition 1991 ZR-1 #302 White/Gray (sold) 1991 ZR-1 #1147 Red/Saddle (sold) |
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07-01-2012
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#8 | |
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Join Date: Jan 2009
Location: Portland Oregon metro area (Washington side)
Posts: 3,206
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Re: Use HVAC display as tach
Quote:
(an option on the dash). On my SW version it reads out in F. (this is wrong, I was reading my notes in the manual and it says "16 is the water temp not in F". so much for my hand writing.)
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Scott Vett owner since 1979._It's about the car and the people Last edited by scottfab; 07-02-2012 at 05:50 PM. Reason: corrected info |
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07-01-2012
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#9 | |
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Join Date: Jan 2009
Location: Portland Oregon metro area (Washington side)
Posts: 3,206
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Re: Use HVAC display as tach
Quote:
Mine reads in F. But I will double check later this week. I made a note the manual that it was F.
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Scott Vett owner since 1979._It's about the car and the people |
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07-01-2012
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#10 |
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Join Date: Jun 2012
Location: fort worth
Posts: 182
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Re: Use HVAC display as tach
My '90 reads in Celcius. It's a late production car.
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