Hello Brickboarders,

My venerable 1994 940T has been boiling over lately. The temp gauge reads slightly over normal(1:00 on the gauge) and the cooling fan should be on. The fan and relays are good-it works fine with the air conditioner on and also responds to the OBD tests, both speeds.Car doesn't boil over with the fan on with AC, BTW. I've measured both sides of the ECT sensor (the rear sensor) at about 300 ohms with the engine partly warm.
Any ideas?

94 940T has 251K miles, 1996 960 has 181K, 2007 BMW 328 has 75K :)

The ECU controls the high and low speeds of the engine cooling fan by grounding the white/black and blue/white wires coming off of the fan relay. These wires connect to the ECU through a three pole connector behind the right strut tower. The connector has a third wire in it, green/red if I remember correctly.

Find this connector and make sure it is connected. If it is connected, disconnect it and first ground the white/black wire (low speed, I think). Ground the side going to the fan. See if you get fan operation. Leaving the white black wire grounded, ground the blue white wire (should be high speed) and see if you get high speed operation. If you don't get fan operation, you need to check the connections at the AC pressure switches, where these wires connect.

Since the car runs fine, your ECT sensor is probably good. Since the fan runs with AC, you know the fusible link from the battery is good.

Let me know if you need pictures. We own two 1994Ts, one 1995 944T, and one 1993 944T.
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john

Since the fan runs with AC, I know those relays, connections and the fan are OK. The ECU turned the fan on (at both speeds) during the OBD tests as I explained earlier. The problem has to be the sensor- unless the car is boiling over before the fan comes on because of a faulty pressure cap. Hmmm... That temp gauge means little with these cars. I'll try a cap first-sensor is a lot harder to replace and more expensive.

"The problem has to be the sensor- unless the car is boiling over before the fan comes on because of a faulty pressure cap. Hmmm... That temp gauge means little with these cars." sredwine

Amen to that.

The cooling system on the 940 is a completely different animal than the 240.
My son's 92 Model 940 and my wife's 94 Model 940 share similiar traits......they will boil over if the coolant cap is removed.

The temperature gage will indicate normal operating temps, and the engine will not seem to be overly hot when it happens.

I was going to check the anti-freeze level on my wife's car the other day.
She had just gotten home, and with the engine still running I loosened the cap.
The coolant started to boil immediately, and if I hadn't tightened the cap, the coolant would have boiled out.

Both of the cars act exactly the same way.

The coolant operates very close to it's boiling point, and if the pressure is reduced, the coolant will begin to boil.
The fact that I live at a elevation of 2400ft above sea level plays a part too.

I suppose that I could increase the concentration level of the Anti-freeze.
This would raise the boiling point.
Or I could reduce the operating temperature of the engine by installing a lower temperature thermostat.
Either or both would give a larger margin between boiling over and not boiling over.
One thing for sure, unlike the trusty old Model 240, you cant drive a 940 Series Volvo with the radiator cap loose....unless you want to lose all the coolant in a hurry.

This is from my own experience, and someone else's may be different.

steve

Hi,

When I sent you a note before, the figure of 75 ohms was stated by Spook. I have been unable to find this value anywhere on the web, what I did find was this.

32F (0C)-- about 6000 ohms within a range of +/- 10%
68F(20C) -- about 2300 ohms
104F(40C) -- about 1300 ohms
140F(60C) -- about 600 ohms
176F(80C) -- about 300 ohms
212F -- about 190 ohms

Based on these numbers it appears that the value should be somewhere between 190 and 300 ohms to turn on the cooling fan. Last time you tried a 75 ohms and the fan immediately went on, try values like 185, 220 and 270 ohms. The 185 ohms is made up of the two 220 ohm resistors in parallel with the 75 ohm resistor combination from the last time.

If the fan runs with values close to 190 ohms, it may mean that the contacts on cable to the sensor are bad. Unfortunately to confirm the status of the sensor you will have to measure the sensor while the engine is extremely hot. Hot as in the motor is about to boil over.

Gus

Hi,

I would get two 150 ohm resistors from radio shack, wire them in parallel. Then plug them into the ECT sensor cable, start the car. If the fan runs the sensor is bad and if it does not run, the problems is elsewhere.

The 300 ohms is a huge difference from the 75 ohms that it takes to turn the fan on.

Gus

Dear sredwine,

Hope you're well. The Engine Coolant Temperature (ECT) sensor's resistance should be 780 Ohms at 20⁰C (68⁰F) and 75 Ohms at 90⁰C (194⁰F). This is from Volvo Tech Manual TP3502201 (p. 30). The reading you report - 300 Ohms - might be correct, but without knowing coolant temperature, it is hard to be sure.

The auxiliary fan should run only in "extreme" situations: high ambient temps (above 32⁰C/90⁰F) and stop-start traffic (little cooling via airflow). If fan operation is what prevents boil-overs in conditions less than "extreme", it suggests the thermostat may be opening too slowly, allowing heat to build-up in the engine (which then shows as a "high" reading on the instrument cluster gauge).

When the thermostat opens, the extra-hot coolant expands, causing a boil-over. When was the thermostat last changed?

Hope this helps.

Yours faithfully,

Spook

Is your car equipped with Regina or Jetronic fuel injection?

steve

Jetronic-I think all the turbos had Bosch; I believe non-turbos were more likely to be Rex-Regina.

The NA 940s also came with LH 2.4.
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john

Not that familiar with the 900s, but don't you have a sensor in the radiator that triggers the fan? I didn't think the ECT sensor has anything to do with triggering the fan.
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Post Back. That's whats makes this forum work.

That car has no sensor in the radiator-I think only 740s had those. Most if not all 940s use the ECT and the ECU to control the fan relays.

The later LH (non-Regina) 900s used the ECU and the A/C press-stats (on the lower condenser rail) to operate the fan. The ECU gets its heads up to operate the fan from the ECT on the block (the one you checked). In the 700/900 FAQ Engine OBD section, use Test Mode 3 to determine if the ECU is even capable of operating the fan. I'm guessing it will fail. Your most likely problem would be the fan control relay located below the right headlamp. Other possibilities are open/damaged wiring.
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Dave -940's, prev 740/240/140/120 How time flies when you're having fun

Dave, I already did the OBD fan test-the ECU ran the fan at both speeds, as I said in the original post. I am wondering if I should jump the ECT to ground (simulating a really hot engine) to see if the ECU will turn on the fan. I am not sure which of the two sensors in that housing would be the correct one. I also don't know what the ECU will do if I do that. I would do it with the car warmed up.

Anybody have more ideas?

On your 94 Model 940 Turbo there are two systems that can operate the radiator cooling fan.

A 2 pole (2 speed fan) cooling fan relay sits on top of the radiator. This relay has two independant sets of magnetic coils and normally open contacts.
One is for high speed operaton of the fan, the other is for low speed operation.

The primary of the relay coils and contacts remain powered at all times.
The relay(s) are activated by the completion of the grounding circuit thru the relay coils. This causes the relays to turn on the fan (high or low speed).

There are two different systems that can cause the fan to operate.....cause the relays to energize

The first is the Jetronic module which responds to the Air Conditioning. It closes the ground connection on either of the cooling fan relays (depending on demand) to start the fan. This part seems to be working OK.

The second is a pair of pressure sensors located in front of the radiator.
One is a high speed pressure sensor, the other is a low speed pressure sensor.
Either sensor can start the fan by closing the ground connection on the cooling fan relay(s).

The only common point between the two relays (when using the perssure sensors to operate the fan) is a common grounding point for both relay coils.
I would look for and confirm that this ground is good.

Remember, the Jetronic module provides the ground reference for these relays when using the Air conditioning.

When cooling is called for by the radiator pressure switches, the ground reference for the relays is provided by a common bonding point to the car chassis.
Follow these ground wires and make sure that the ground is intact.


hope this helps
steve

The pressure switches you're referring to are on the AC condenser, not the radiator, and thus can't be part of the problem-since the fan comes on when the AC is activated. The ECT sensor (pair) sends engine coolant temp info to both the Jetronic and the ignition computers. I need help in troubleshooting this system, since it seems to be the area involved. For what it's worth, I've removed the sensor connector and sprayed contact cleaner into it. There are two sensors in that same housing-I'm not sure which is the one whose readings are needed. One thing really puzzles me-if the sensor signal is not getting to the ECU, why does the car run well? Gas mileage could be a little better, but with 250K+ on the clock, I don't expect great mileage!

Dear sredwine,

Hope you're well. Volvo Tech Manual TP3904202 (Wiring Diagrams, 940, 1994), shows separate coolant sensors on turbo-equipped and non-turbo engines. The coolant temp sensor for engine management (for the Engine Control Unit [ECU] and Ignition Control Unit [ICU]) is towards the rear of the head, on the driver's side (North American models). The coolant temperature sensor for the instrument cluster's temp gauge is at the front of the head.

The ECT for engine management has two wires. The green/white wire runs to the ECU (Position #13). The Red-Black wire runs to the ICU (Position #2).

The ECT might just be starting to fail, i.e., it may be working sluggishly. It can be checked by immersing it in water at 20⁰C (68⁰F), or checked in place, if the ambient temp is 20⁰C (68⁰F). At that temperature, resistance should be 780 Ohms. If it is not, then I'd replace this sensor. If it is the factory-original unit, it does not owe you a penny.

If my guess is correct, complete sensor failure will mean deterioration in fuel mileage. If I recall correctly, a faulty/failing ECT will cause a run-rich condition.

Hope this helps.

Yours faithfully,

Spook

Just measured both sections of the ECT sensor-2.8K ohms.Sounds like a new sensor will cure my problems!

Let me amend the above. I looked at a 1994 sensor resistance table from a Volvo manual and it states the range at 68 deg F should be 2250 to 2780 ohms (or about that)-looks like my sensor is in spec! It's about 75 deg F here now. I don't know where that lower reading I have seen quoted comes from.

I got a 75 ohm resistor and subbed it for the sensor. As soon as I turned the key to the run position the fan came on. It also stayed on for a while after I turned off the key. That seems to say my sensor is bad. Will get one and try it.