Temp gauge incorrect
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Cargo Master
Joined: Mar 2003
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Temp gauge incorrect
I have a '67 Bronco w/ 351W that I have installed new VDO gauges. This has been a non running project that has been pieced together with many other Broncos,(most years are very similar), so I do not have a "before" to reference to.
The problem is the temp gauge is reading correctly when in the KEY ON ENGINE OFF and when in ACCESSORY , but when the engine is started and running, the temp gauge immediately maxes out.
The gauges are new and I have used the sending unit supplied with the gauges.
It must be grounding out when the engine starts somehow. Any ideas on where to look???
Regards
Ken
The problem is the temp gauge is reading correctly when in the KEY ON ENGINE OFF and when in ACCESSORY , but when the engine is started and running, the temp gauge immediately maxes out.
The gauges are new and I have used the sending unit supplied with the gauges.
It must be grounding out when the engine starts somehow. Any ideas on where to look???
Regards
Ken
i ain't rite
Joined: Mar 2005
Posts: 65,448
Likes: 5,517
From: Marlboro Mental Hospital.
disconnect the wire from the sender, and see if it still shorts out. if it does, it is in the wiring. if not it is in the sender.
are you using the original wiring, or did you run new wires?
are you using the original wiring, or did you run new wires?
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Cargo Master
Joined: Mar 2003
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You know, after sleeping on it, I came to the same ideas you just mentioned.
No, I never installed a ground to the body. This is the first thing I'm doing this weekend.
I used the original red/white wire. I think if the body ground doesn't improve the reading, I will run a brand new wire to the sending unit.
When I disconnect the sender, the gauge pegs left (cooler). When I hold wire directly to engine block, the gauge pegs right (250deg.)
When cool, the sender has 660 ohms resistance which is near the 700 ohms manufacturer specifications.
Alot of times electricity doesn't make sense, and that is what is throwing me. Sender acts as if grounded out with engine running, but if I kill the engine and return key back to on, the temp gauge is fairly accurate.
Time for some more investigating.
Thanks Guys!
Regards
Ken
No, I never installed a ground to the body. This is the first thing I'm doing this weekend.
I used the original red/white wire. I think if the body ground doesn't improve the reading, I will run a brand new wire to the sending unit.
When I disconnect the sender, the gauge pegs left (cooler). When I hold wire directly to engine block, the gauge pegs right (250deg.)
When cool, the sender has 660 ohms resistance which is near the 700 ohms manufacturer specifications.
Alot of times electricity doesn't make sense, and that is what is throwing me. Sender acts as if grounded out with engine running, but if I kill the engine and return key back to on, the temp gauge is fairly accurate.
Time for some more investigating.
Thanks Guys!
Regards
Ken
i ain't rite
Joined: Mar 2005
Posts: 65,448
Likes: 5,517
From: Marlboro Mental Hospital.
that is a strange one. lets see if i have the facts correct.
koeo, everything is the way it should be. cold engine cold on gauge, hot engine reads proper temp.
koer, wire off sender,gauge reads cold. wire on block, gauge reads overheat, wire on sender gauge reads overheat.
i would try these three steps to try to isolate the problem
replace sender. if that don't fix it,
i would run a wire direct from gauge to sender. if that don't fix it,
run a power wire from battery to gauge.
koeo, everything is the way it should be. cold engine cold on gauge, hot engine reads proper temp.
koer, wire off sender,gauge reads cold. wire on block, gauge reads overheat, wire on sender gauge reads overheat.
i would try these three steps to try to isolate the problem
replace sender. if that don't fix it,
i would run a wire direct from gauge to sender. if that don't fix it,
run a power wire from battery to gauge.
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Cargo Master
Joined: Mar 2003
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Likes: 296
Originally Posted by tjc transport
that is a strange one. lets see if i have the facts correct.
koeo, everything is the way it should be. cold engine cold on gauge, hot engine reads proper temp.
koer, wire off sender,gauge reads cold. wire on block, gauge reads overheat, wire on sender gauge reads overheat.
i would try these three steps to try to isolate the problem
replace sender. if that don't fix it,
i would run a wire direct from gauge to sender. if that don't fix it,
run a power wire from battery to gauge.
koeo, everything is the way it should be. cold engine cold on gauge, hot engine reads proper temp.
koer, wire off sender,gauge reads cold. wire on block, gauge reads overheat, wire on sender gauge reads overheat.
i would try these three steps to try to isolate the problem
replace sender. if that don't fix it,
i would run a wire direct from gauge to sender. if that don't fix it,
run a power wire from battery to gauge.
I won't get a chance to work on it til this weekend, seeing how it has open headers and I don't want to urk the neighbors at night time in order to get engine to temp.
I'll report back with the cause and fix as a follow up when I get there.
Regards
Ken
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Reason I asked, the gauge seems normal until the ignition is working.
My suggestion would be, get a voltage reading on the power side of the gauge with the key on (when the gauge works normally,) then fire it up and see what the voltage signal looks like that way. If it acts strangely, switch the meter to the AC scale and see if you get a steady reading. My suspicion is that you are getting some feedback from your Mallory unit.
My suggestion would be, get a voltage reading on the power side of the gauge with the key on (when the gauge works normally,) then fire it up and see what the voltage signal looks like that way. If it acts strangely, switch the meter to the AC scale and see if you get a steady reading. My suspicion is that you are getting some feedback from your Mallory unit.
i ain't rite
Joined: Mar 2005
Posts: 65,448
Likes: 5,517
From: Marlboro Mental Hospital.
thats what i was thinking also. thats why i suggested running a power feed off the battery to eliminate that possibility. i just could not think of the proper way to put into words while the painkillers were nice and fresh yesterday.
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From: Virginia
There is a simple test for the gauge. Turn the key to run without starting the engine. Take the wire off the sending unit, and the gauge should read full scale low or high. Then take the sending unit wire and ground it to the engine block. The gauge should read full scale the opposite direction.
If it passes this test, then the wiring and the gauge are ok.
If it passes this test, then the wiring and the gauge are ok.
i ain't rite
Joined: Mar 2005
Posts: 65,448
Likes: 5,517
From: Marlboro Mental Hospital.
Originally Posted by Franklin2
Yea, his second post says he already did the test anyway. I need to pay more attention. 
it's worked for me these past 3 months.
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Cargo Master
Joined: Mar 2003
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It has three terminals.
Here is the schematic. It is wired just as it shows.
https://www.ford-trucks.com/user_gal...id=111828&.jpg
Just as a side note:
All the other gauges work correctly at all times. Wouldn't the ignition screw with all of them if that was a problem?
Regards
Ken
Here is the schematic. It is wired just as it shows.
https://www.ford-trucks.com/user_gal...id=111828&.jpg
Just as a side note:
All the other gauges work correctly at all times. Wouldn't the ignition screw with all of them if that was a problem?
Regards
Ken
Last edited by McLeod; Mar 29, 2006 at 11:19 PM.