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Re: [GMCnet] OEM temp gauge readings [message #357745 is a reply to message #357738] Sun, 16 August 2020 11:17 Go to previous messageGo to previous message
powwerjon is currently offline  powwerjon   United States
Messages: 849
Registered: March 2013
Karma:
Senior Member
I have to ask what temperature sender your using. If you still have the stock unit then you need to change it out to a NAPA TS-6469 unit. Check out the following information.

Temperature Sender
This may be old news for most but doesn't hurt to be told. The original
temp. sender on all GMC Motorhomes at :
1/4 is 225 degrees,
1/2 is 250 degrees,
3/4 is 270 degrees, and
H is 280 degrees.
This has caused a lot of engine damage when engine starts to overheat. Go to NAPA and get a
TS-6469 sender and it will show:
1/4= 180 degrees,
1/2 = 215 degrees,
3/4 = 240 degrees, and
H = 255 degrees.
Since the standard pressure cap raises antifreeze solution boiling point to 250 degrees this will be more accurate readings. I don't know why they did this but it is a serious problem. Info from GMC Motorhome News published by Cinnabar, Dec. '95 issue. I did this and it agrees exactly with my VIP digital
readout.

If you are using the 7 blade metal fan then a 2747 ish is an acceptable fan clutch. I run one of the newer 10 blade fiberglass fans and I found that I needed to us a 2797 ish fan clutch as the 2747 doesn’t have enough clutch to turn it. I know this from personal experience.

EXAMPLE: I had installed one of John Biwersi 10 blade fiberglass units. We drove the coach out to AZ to spend the winter and of course the temps were cool and cold on the way out. In the spring when we were coming back we decided to go to Page, AZ to see the Glen Canyon Dam and sandstone caves and canyons. The trip to Flagstaff was uneventful as the temperature was in the low 70’s, Going north on 89 into the high desert the temps were in the High 80’ to low 90’s and we were motoring along when I noticed that water temp was rising to 1/2 on the gauge. Remember that I said that I use the TS-6489 temp sender. Well I said maybe I got a bad fan clutch, so I stopped at the next rest area that happened to appear about 2 miles down the road. I alway carry a spare fan clutch and proceeded to change it out and go on our way north on 89. About 15 minutes later the same temp rise happened again. Once again much was in our favor as we entered Cameron AZ and they had an AutoZone and they had a 2797 rated fan. 45 minutes later new fan clutch and we were on our way again to Page, AZ. I run a 180 thermostat and it was locked in the rest of the trip and I still have that fan clutch install and yes I do have a spare 2797 type fan clutch in storage. What I learned from this experience was that the 10 blade fan requires more fan clutch engagement than the 7 blade fan. I gave my 2747 type units to my traveling GMC friend. He had a clutch failure so he got 2 fan clutches, one to install and one for a on the road spare.

J.R. Wright
GMC Great Laker MHC
GMCGL Tech Editor
GMC Eastern States Charter Member
GMCMI
78 GMC Buskirk 29.5’ Stretch
75 GMC Avion (Under Reconstruction)
Michigan


> On Aug 16, 2020, at 11:11 AM, Chris S. via Gmclist wrote:
>
> Hello all:
> Not too long ago our fan clutch completely locked up and we needed a replacement. I am not entirely sure which clutch it had. We ended up putting on
> an AC Delco 15-4947 after reading on it here, there was a thread by Ken Burton where he did some research and found this seemed to be the best
> replacement regarding the specs of the fan and setup on the GMC, and also after reading success and failures of Hayden clutches here too.
>
> I haven't put many miles on the coach with this new clutch but this past week we were in temps up to about 88 degrees out. (In MI, that's what we
> call HOT :) ). With the old clutch in this temp range , the needle would barely pass the quarter mark on the gauge, but it would also kick in quite
> often even on the highway and I'd hear the fan. Now, with the 15-4947 clutch, I noticed the gauge has gone as high as just between the quarter and
> half mark while on the highway, and I never hear the fan roar like the old one. But, when getting off the highway or slowing to 55 MPH or so, the
> needle settles back to the quarter mark.
>
> I believe I've read that once it hits the half mark on the gauge, it's already overheating. Should I be concerned with the readings I'm getting?
>
> Thanks in advance as always!
> Chris
>
>
> --
> Chris S. -
> 77 Kingsley, 3.70 FD, mostly OEM -
> S.E. Michigan
>
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