VDO Temp Gauge issue - Defender Source
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  #1  
Old August 5th, 2014, 03:45 PM
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Anthony Nelson
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VDO Temp Gauge issue

aftermarket VDO Temp gauge in a 97 NAS D90 reads higher than scangauge. like by 20* at points. seems better when moving vs standing still. 180* on the scan gives me 200 on the gauge. recently replaced the waterpump and went back to a 195* OEM thermostat. Temps never read higher than about 196-199* at idle on a hot day, while gauge climbs to 210*.

i know there is a ground issue, like needle moves up a little with lights on, but has anyone else seen this and how did it get addressed.
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  #2  
Old August 5th, 2014, 03:48 PM
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Quote:
Originally Posted by sloanfiske View Post
aftermarket VDO Temp gauge in a 97 NAS D90 reads higher than scangauge. like by 20* at points. seems better when moving vs standing still. 180* on the scan gives me 200 on the gauge. recently replaced the waterpump and went back to a 195* OEM thermostat. Temps never read higher than about 196-199* at idle on a hot day, while gauge climbs to 210*. i know there is a ground issue, like needle moves up a little with lights on, but has anyone else seen this and how did it get addressed.
It's a ground issue. Run a fresh ground.
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  #3  
Old August 5th, 2014, 03:49 PM
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There may be some real difference as the sensors are in different places. Most likely it is a ground issue. Clean and check the gauge ground at the gauge and where it grounds to the body. Make sure the engine ground is solids with clean connections.

Could also be a bad sender or gauge. You just never know.
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  #4  
Old August 5th, 2014, 04:59 PM
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Thanks. Just off the gauge to body/bulkhead behind the dash ok?
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  #5  
Old August 5th, 2014, 05:16 PM
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If it were a ground issue wouldn't the gauge read low? If the wire for the sending unit is run directly to ground the gauge should read fully hot. I've run into this with another VDO setup on a D90. It read about 15-20 degrees higher than the ECU.
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  #6  
Old August 5th, 2014, 05:43 PM
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No it's a ground because his reading changes with extra electrical load. I had this on my 97. In my case I probably went overboard with the fix. I ran a dedicated larger wire between the sender and the gauge, increased the size of the grounds to my dash that feed the gauges and tied them all back to both the firewall and the frame and finally added another separate ground strap from the frame to the engine on the opposite side to the existing. Not sure if it needed all that but it's rock solid under any load conditions now....
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Old August 5th, 2014, 05:43 PM
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Run 16ga wires from the sender & a separate wire from the gauge to the ground lug on the firewall in the engine compartment. This got mine reading correctly.
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Old August 5th, 2014, 05:59 PM
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So, I pulled out the stock gauge from the shelf just for giggles. Stock gauge is 15.4 ohm. VDO is 22.7 ohm. No wonder. As the VDO was plugged into the OEM harness, that's probably the culprit. Tossed the stock one in and hooked up the scangauge. Sits just left of center at 180 and dead center at 190. Crawls a fraction past at 195, when the TStat opens and comes back to center or just left. On another note, when I took it around town to check all of this Mumbo Jumbo, I ran into a early 70s NATO 109 with all the junk off the top and the front windshield pushed forward. Older guy...has owned the truck for 25 years. Super awesome.

------ Follow up post added August 5th, 2014 06:03 PM ------

Here's the 109. My son took this picture a few weeks ago. It's pretty rad. I had never seen it in town. Turns out the guy just sold another 3 door 109. He's holding onto this one.
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  #9  
Old August 5th, 2014, 06:08 PM
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Huh? The VDO needs a completely different range sender than the stock gauge. Are you not using a VDO sender?
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  #10  
Old August 5th, 2014, 06:13 PM
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It's a vdo sender as well. PO put it in. I have an extra stock sender, but I just gave this a shot. Would need to splice in the old style connector to replace the sender.
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Old August 5th, 2014, 06:42 PM
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Any thoughts?
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  #12  
Old August 5th, 2014, 07:03 PM
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Here is the solution to your issues!

Quote:
Originally Posted by sloanfiske View Post
aftermarket VDO Temp gauge in a 97 NAS D90 reads higher than scangauge. like by 20* at points. seems better when moving vs standing still. 180* on the scan gives me 200 on the gauge. recently replaced the waterpump and went back to a 195* OEM thermostat. Temps never read higher than about 196-199* at idle on a hot day, while gauge climbs to 210*.

i know there is a ground issue, like needle moves up a little with lights on, but has anyone else seen this and how did it get addressed.
VDO electric gauges generally require a (+) wire, a (-) wire, and a wire from the sending unit.

If any of these wires share with other electrical devices of any kind the gauge readings are inaccurate.
Also if you mismatch a sending unit with a specific voltage range that is not fully compatible with the voltage range of the gauge, you'll get inaccurate readings.

If your gauge and sending unit are matched and functioning properly it will work accurately if you do the following:

(+) solution via a 12V relay:
Run a wire from the ignition switch to the 86 position on the 12V relay.
Ground 85 position on the 12V relay.
Run a wire from the 12V brown circuit on the starter or starter solenoid to the 30 position on the 12V relay.
Run 87 to the (+) gauge lead.

(-) solution via a new ground wire to a grounding source, NOT the (-) battery pole (not shared with anything).

From the sender: make sure the wire connection is good and is not interfered with any other metal objects.
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Old August 5th, 2014, 07:14 PM
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Awesome. Thanks guys. I'll give this all a shot.
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  #14  
Old August 5th, 2014, 07:28 PM
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The power side does not matter the gauge is designed for variable input voltage. What is important is that there is no voltage loss from the gauge to the sender and the gauge to ground and between the Gaige ground and the sender ground.
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Old August 5th, 2014, 08:23 PM
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It's a single wire from sender to gauge. Assuming now just ground from gauge needs to be changed and see if that addresses issue first I suppose.
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Old August 5th, 2014, 09:14 PM
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Yes, the gauge AND sender Must be made for each other and use the proper wire gauge too.
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  #17  
Old August 5th, 2014, 09:19 PM
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VDO is in. With correct sender. I'll re ground the gauge and see what happens.

Thanks again all.
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Old August 5th, 2014, 10:18 PM
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Quote:
Originally Posted by Red90 View Post
The power side does not matter the gauge is designed for variable input voltage.
Sort of, but not always true...

There will always be some fluctuations on the (+) side, BUT
according to VDO, when a power draw fluctuates the available voltage on the (+) side,
the gauge reading will vary, particularly when more than 1 device is wired to run off the (+) circuit feeding the gauge and if the voltage drops below the gauge threshold, the gauge readings will vary.

The best case wiring was described above via a relay in my post above.
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  #19  
Old August 5th, 2014, 11:00 PM
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The gauge draws basically no power so your wiring would need to be seriously messed up to have a serious loss from the loom to the gauge.
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  #20  
Old August 6th, 2014, 10:08 PM
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16G wire thru the firewall to bulkhead didn't do the trick under operating load. But if the ignition was in second position with the engine off, gauge matched the scangauge.

hmmm.
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