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'84 Temp Gauge/Fan Switch



Somebody Help!

I guess this 'Vette isn't wired up like I'm used to. I have a sending unit mounted into the driver's side cyl. head (single wire) and one mounted into the front of the intake manifold (two wire). I plugged in a manual type gauge in place of the sending unit in the head, just to see if the gauge in the car was accurate. What surprised me is that the gauge in the car still worked! Which one is for the gauge and which one is the switch for the fan relay? The previous owner has done some "creative" wiring to get the fan to work and I just want to get it like it's supposed to be.

The original problem is that the engine temperature is basically proportional to engine speed, the higher the RPM the higher the temperature, driving or just sitting in the garage. This is with the fan running. I have replace the thermostat and I know that the passages are not restricted. I have not experienced this particular problem before. Is this a water pump problem?


Hi there,
The first thing is how the system works, as your front sensor, with the 2 wires is the sensor for your dash gauge. This is a power feed in, and a signal back to the gauge cluster.
This single pigtail is for the ECM to see when the temperature gets to 228, so it turns on the fan to cool the engine. This simply closes, and the ECM sees this, and engages the relay, therefor the fan.
NOw, temperature issues the faster you drive could be a number of things, do you have obstructions in the radiator, or the condenser?? I cannot tell you how many times I have had to blow out debris from the radiator, and the like. Bags, and garbage get caught up there all the time. I would check this first.
Next, are you sure that you are getting correct coolant flow from the pump, as I would look at this next. Remove the belt, and see if the pulley moved laterally, like a bad bearing. This could also give you some insight if it is good or not.
These are thoughts, and let us know how you make out.
Best to you, c4c5
Hi c4c5,
Thanks for the info. But......the sensor mounted in the head(the single wire) is not just open or closed. It operates like a typical temperature transducer, temp goes up, resistance goes down. If I remember correctly it measures about 1k ohms at room temperature and around 200 ohms at roughly 200 degrees F, typical of a sending unit for a temp gauge. Now I don't see that this is a problem since the fan relay engages when the coil of the relay sees about 200 ohms to ground (I tested this last night).

According to the Chilton's manual I have, the sensor for the fan relay(as well as the AC fan switch) does not go to the ECM but directly to the fan relay. Now I know that these books are not always 100% accurate, but this one has been correct so far. The AC switch does check out to connect to the relay, but niether of the temp sensors do. I think the previous owner has messed up the temp sensor's wiring.

I don't have a service manual yet with detailed wiring diagrams. Can you tell me what pin on the ECM the single-wire temp sensor connector goes to?

Thanks for the help

c4c5specialist said:

This single pigtail is for the ECM to see when the temperature gets to 228, so it turns on the fan to cool the engine. This simply closes, and the ECM sees this, and engages the relay, therefor the fan.

This is incorrect.......the 84 model DOES NOT have ECM Fan control!!!!!!! (ECM control started on TPI/85 model) But the wiring is essentially the same..........

The fan switch (on the passenger side between cylinders 6 & 8) is directly wired to the relay located on the drivers side wheelwell (kinda hidden under the brake reservoir area) which is routed to the fan.......

The old relay (long and flat) can be replaced by the upgraded more reliable relay.....pic attached is courtesy of Ecklers(its their part #seen at top)

PS...Bill......is your color ORIGINAL?????? Maroon not red ??????
If so it may be a one of a kind car painted for GM as a sample 86 color!!!!!!! They were sold in your area after the GM paint show!!!!!! If u think its original email me I have more info on the special colors!
As per bulletin 85-13-12, on the revised schematics of the cooling fans, depending on the
VIN, you could have 2 different systems, and I was assuming, at my error, that you had the later design.
It was not my intention to mislead in this situation, Corvette84, and I made a mistake, my bad.
Regrets, c4c5
No...I dont accept you're telling him that ....U are devious and wanted to drive him totally nuts......I know how u work!!!!!
U ver Bad!! :t

Well..duh.....just in case some of u dont get it...I was being sarcastic in above statement!!! (They actually did a survey and found that those of us in the North east have a very sarcastic nature that other Americans find confusing and foreigners actually find insulting.....we wacky Yanks!!!!)

Maybe we need a "read with sarcasm" icon?!?!?!

Thanks for the info guys.

But...my setup doesn't have a sensor in the passenger's side head. Only the one on the driver's side and one in the front of the intake. With a little continuity testing I see that the sensor on the driver's side connects to the ECM (it has to know engine temp to go into closed loop, right?) but the sensor in the front of the intake does not connect to the ECM or the other three connectors that go into the "cockpit", yet this one must control the temp gauge since the gauge still works with the other one disconnected. ???? Is that confusing or what!

c4c5, you mentioned cooling fan schematics. Do you have these schematics? I don't have a service manual yet and I am just trying to get it working good enough to get the emission's test and tags. I will replace all of this stuff later.

Corvette1984, I don't know if the color is original. I looked into it and I see that there were two reds available, a "RED" and "LIGHT RED". From what I can tell the "RED" as they called it was more of a maroon color, and the "LIGHT RED" was more of a true red. The paint codes on the sticker indicate a two-tone silver.

By looking at the paint itself, it does look original. I can find no evidence whatsoever of any silver or gray paint anywhere, although the interior is silver/gray and black. It also a little bubbling and cracking between the doors and rear wheels, which I would think would have been repaired if the car was re-painted. More info on this would be great if you have it!

For the 84 Get the haynes book OK schematics 84-87 88 up SUCK.

The problem is you don't have the sensor on the
side of the block.

The cooling fan relay control line ( at the relay is Dk green/white) ground for operation.

Which MAGICALLY changes to a brown wire when it crosses through CRKT 335 at the bulk head connector

There SHOULD be a brown wire taped up in the harness(cut, hidden, lost in space) on the passenger's side of the engine compartment which the connector is a round female connector.Which SHOULD buzz out to pin B of the fan relay.

If you do not have a sensor? what's in the head where the hole is? brass plug, broken sensor, wood dowel pounded in?

BTW change the factory sesnor to the mid america type fan on at 200 held on till 180, instead of the factory sensor fan on @ 232 off at 231

Haynes? The Haynes book that I have is NOT accurate when it comes to wiring schematics. The Chilton's book I have has been correct so far, but just isn't detailed enough.

The passenger's side head has a hex plug in it, which unfortunately has been stripped pretty bad. I may have to remove the head to get it out.



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