| transmission rpm weirdness [message #278315] |
Sun, 24 May 2015 19:15  |
jayrabe
Messages: 509
Registered: June 2009
Location: Portland, OR
Karma: 0
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Just back from a 3800mi trip to SW, through Utah, Arizona, and Colorado.
On return trip, I was driving 45-55 most of the way from Boise to Portland, because my exh header gaskets were leaking badly and I didn't want to push it.
Just got 3.70 gears installed and at about 45 I was running 2400 rpm. But once in awhile I'd notice the rpm up to about 3000. Engine speed didn't noticeably change, and speed per GPS was the same. Also, watching the tach, I noticed a few times it glitched up and down 200-400rpm.
Anybody with any ideas on why it would do that?
Thanks,
Jay
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| Re: transmission rpm weirdness [message #278317 is a reply to message #278315] |
Sun, 24 May 2015 19:35  |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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jayrabe wrote on Sun, 24 May 2015 20:15Just back from a 3800mi trip to SW, through Utah, Arizona, and Colorado.
On return trip, I was driving 45-55 most of the way from Boise to Portland, because my exh header gaskets were leaking badly and I didn't want to push it.
Just got 3.70 gears installed and at about 45 I was running 2400 rpm. But once in awhile I'd notice the rpm up to about 3000. Engine speed didn't noticeably change, and speed per GPS was the same. Also, watching the tach, I noticed a few times it glitched up and down 200-400rpm.
Anybody with any ideas on why it would do that?
Thanks,
Jay
Jay,
I have seen this before on other GM vehicles with aftermarket tachometers.
Who made the tachometer you have and did you install it??
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: transmission rpm weirdness [message #278319 is a reply to message #278315] |
Sun, 24 May 2015 19:52 |
bobby5832708
Messages: 237
Registered: November 2006
Location: Winter Springs FL
Karma: 3
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Do you have a tach filter installed between the tachs coil wire and the coil?
A few years ago when I added a tach to my GMC I tried two different tachs, a cheap digital one and a VDO analog one. They both 'worked' but were not really stable like thay should be, sometimes they worked OK but they always jumped around and gave wildly inaccurate readings at times. I found several diagrams on the internet for tach filters, none of which worked for me. Then I came across the schematic for the filter GM supposedly used back in the day. Both tachs worked properly with the GM-spec filter. I currently have the cheap digital tach in my GMC using this filter and it has been working fine for years now.
Here's the diagram for the tach filter that works for me, your mileage may vary:
Coil ------------/\/\/\/--------o------/\/\/\/-------o-------------- Tach
5.6k | 10k |
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___ ___
.15uF 100pF
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----------o------------
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_____
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_
Edit: well, the diagram looks fine on the page where I type it in but it does not display properly on the forum page. If you want, PM me your email address and I will send you a text file, maybe that will display properly.
Bob Heller
2017 Winnebago 29VE
Winter Springs FL
[Updated on: Sun, 24 May 2015 19:56] Report message to a moderator |
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| Re: transmission rpm weirdness [message #278339 is a reply to message #278315] |
Mon, 25 May 2015 00:20 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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jayrabe wrote on Sun, 24 May 2015 17:15Just back from a 3800mi trip to SW, through Utah, Arizona, and Colorado.
On return trip, I was driving 45-55 most of the way from Boise to Portland, because my exh header gaskets were leaking badly and I didn't want to push it.
Just got 3.70 gears installed and at about 45 I was running 2400 rpm. But once in awhile I'd notice the rpm up to about 3000. Engine speed didn't noticeably change, and speed per GPS was the same. Also, watching the tach, I noticed a few times it glitched up and down 200-400rpm.
Anybody with any ideas on why it would do that?
Thanks,
Jay
Mine did this and it turned out that the plug in the conning tower of the distributor cap where the tach plugs in was the problem. The plug was shoved up beside the blade and not actually connected to the blade. I was surprised it stayed in there as long as it did,
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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