Temperature gauge woes

[pabusnut]

I have been chasing temperature gauge troubles for a while now.  I replaced both the sending unit (2X now) and the gauge(original spec parts from Luke).  I have 24VDC at the gauge and at the engine(when i unhook the lead to the sending unit).  Bus is a 1973 GMC 4905 with an 8V71, manual trans.

The problem is the gauge pegs at 260 after running a few minutes at high idle.  It does go up from the lowest indicated temperature--100 the progressively to 260--not all at one shot.  I know the engine is not hot, and not even warm yet, as I can lay my hand on the thermostat housing and it is barely warm.  I am having trouble wrapping my brain around what the problem could be.

After I shut the engine down, I checked the resistance on the sending unit, and I had 667 ohms.  Since I didn't know what it is supposed to be I am not sure that is correct.

Assuming there is nothing wrong with either the sender or the gauge, what are the other possible things that could cause this problem.


Thanks!

Steve Toomey
PAbusnut

[gus]

Steve,

Check the resistance of the gage wire while disconnected at both ends. It is probably pretty high. I don't know the correct sender resistance, you need to check the manufacturer for that and to see if it is 24v.

I just went through this with my 4107 and finally changed to a digital gage which now works fine. My old analog gage was erratic at best and always kept me worried about a non-existent overheating problem.

Sounds to me as if you have the wrong sensor (12v) or corroded wiring/connections, a common problem with older GMs. I've had to clean up a lot of my corroded terminal connections and cut off some corroded wiring ends. I also replaced the steel terminal nuts with brass. Check these connections at the terminals on the driver's electrical panel. You will need a wiring diagram to find out the correct connection number for the heat gage if you are using the old wiring.

The other possibility is air in the system but that has never been a problem in my 8V71.

[pabusnut]

Thanks Gus,

I know there is no air in the system--well at least at that point, as it gushed out while I quickly replaced the sensor.

I will start tracing the wires back, since I do have all the MX manuals.  I guess it wouldn't take much additional resistance to make the gauge read high.  Out of town this week, but I will find it next week.

Steve

[wildbob24]

Steve,

Make sure you didn't use any thread sealer on the sender. This is a very common error and will inhibit with the sender's ground connection.

Bob


Sent from my iPad using Tapatalk HD

[robertglines1]

2nd above post on tape or sealer. Fooled many.

[chessie4905]

Get 40 foot pieces of automotive wire and bypass the on bus wiring to see if it is an electrical resistance problem. This suggestion is for the less electrical/electronic inclined among us. The wiring can be used later for other uses.

[zubzub]

Get 40 foot pieces of automotive wire and bypass the on bus wiring to see if it is an electrical resistance problem. This suggestion is for the less electrical/electronic inclined among us. The wiring can be used later for other uses.

+1 on this
I have an extension cord running the length of the bus for trouble shooting.  When in doubt I bypass everything and see what is working then backtrack.  Extension cord is good for trouble shooting gives you 3 color coded wires.

[gus]

I use teflon plumbing putty on all pipe thread connections, especially steel to steel. The natural tightening of a NPT connection will ensure good continuity.

I don't use teflon tape on the bus for fear it will find its way into valves.

[Len Silva]

VDO and just about every gauge manufacturer says not to use ANY sealant on the thread.  Senders use a special tapered thread to insure a good seal.  The sender is not voltage sensitive, only the gauge is.  You should know what the sender resistance should be at various temperatures.  This is available from the manufacturer.

Here is a typical resistance chart from VDO.  You can see that there are a large number of different resistances shown.  It is important that you know the make and model of the gauge you are using.

Note that temperature gauge senders usually have the higher resistance when cold.
http://www.nsifleet.com/pdf/TemperatureResistanceChart.pdf

[chessie4905]

GM states in the service manual to NOT put any sealant on sending unit threads.

[pabusnut]

Well,

I guess I made a mistake by using sealer on the threads, but I don't think too much actually got on, because the coolant was gushing out like 'old faithful' while I took the old sender and installed the new one.  I did check the resistance between the center post of the sender and the body of the sender vs. the thermostat housing and found no difference.  I used a liquid sealer that is rated for everything including gasoline, natural gas, diesel, etc.

I will start cleaning/refurbishing all the connections this week.  I think my probem may be a short somewhere, if the resistance does go down as the temp goes up.  I will have to find a chart for the sender/gauge combo I have.

Steve Toomey
PAbusnut

[Geoff]

Even though the rule of thumb is not to put sealer on the threads of the sending unit, what normally happens if you do is the male and female threads will cut through and make the ground connection anyway.  If you used thread sealer and put the sending unit on hand tight, you may not get a connection.  Put the ohm meter on the outside nut of the sending unit and to ground on the bus and you will see if the sending unit is grounded or not.

[gus]

I always check engine to sender resistance after using teflon putty and have never found even the slightest resistance.

You will become a believer once you try to remove a steel fitting from a steel water jacket! Brass is much better but I use putty even then.

[Scott Crosby]

Sending units are fragile and can be damaged internally by over tightening the wire hold down nut, and allowing the stud to spin.  My oil pressure unit was damaged like that, showed 10 psi when off and negative psi when running.  Very strange reading. 

[pabusnut]

Update:  I have cleaned all connections, added new star washers, and replaced the steel nuts with brass ones at all the intermediate connections.  Saw some minor corrosion, but nothing major. Cleaned the quick disconnect connectors between the engine panel and the engine.  I pulled the (NOS) gauge from the dash and did a little checking which confuses me now.  The gauge connects to the wiring harness via a two-prong T-type push on connector.  I checked the resistance between each of the prongs sticking out of the back of the gauge and the gauge body, and found that I had somewhere between 250-400 ohms, when I expected to see ZERO. 
My plan for tonight is to run a 40 ft extension cord from the sending unit to the gauge and see what happens when I start it up.

Steve