I have a relay (actually, I have 3 of them that all behave the same way) that won’t close using the ground to the PCM. When I supply ground from any other source, it functions properly. I have verified that the ground is actually a ground (it gives me 12 volts on the voltmeter when the engine is warmed up). Why won’t that ground operate the relay?
It’s impossible to say without knowing what kind of car is involved, exactly which relay circuit is suspect, and and not having wiring schematic in hand.
The part about 12 volts on the ground circuit raises the question of how this figure was arrived at; assuming I’m reading this right.
Are you saying that a cooling fan is not coming on when the engine is warmed up?
95 cutlass supreme. I’m concerned with the cooling fan #1 relay. I measured 13.7 volts across the switching circuit after the car warmed up (0 when cold, which tells me the PCM is closing the ground after warm-up). Even when I use jumper wires (to rule out loose pins) they won’t close the relay. When I use the positive wire and ANY other ground, the relay functions properly. I’m about to just jump it and delete the relay.
The AutoZone wiring schematic (which sometimes can be a bit questionable) shows the circuitry varies based on the VIN letter code. Which code does your car have and have you verified that all fuses are good?
One variant shows 3 relays and 4 fuses involved in cooling fan operation so that one becomes a bit murkier but still not overly complicated.
Do you have two fans or just one fan? Usually when there are three relays, there are two fans. When there is one fan, there is usually only one relay.
Does the fan come on when you turn on the AC? It should.
As for why the relay is acting the way you say it is, I’d have to see exactly how you are taking the readings. Do you have a stand off for the relay in order to take those voltage readings with the relay in the circuit? Are you taking the readings with the relay in the circuit?
BTW, the fan does not come on when the vehicle reaches operating temperature, it comes on when the coolant temperature approaches overheating. For example, it the normal operating temperature is 180F, the fan may not come on until the coolant reaches 205F. The fan does not operate continuously after warm up.
Sorry for the confusion. There are only two relays for the cooling system, and a third that I bought, expecting that the other two might be bad. There are two fans. I measured the voltage at the socket-pins without the relay installed. I don’t have a “standoff” but I might rig one with some jumpers. Regardless of what temp the coolant is, I’m guessing the fan is supposed to be on because the ground from the pcm isn’t a ground until after the engine warms up, then it closes the ground. All I have to go off of for temperature is the notoriously unreliable dashboard gauge. I have let it idle until the temp gauge got all the way to the red, so I would have expected at least the fan to come on (never has). AC is busted on an unknown level, except that the seller told me it would cost more to fix than the car is worth (she was asking 1000 for it, so that’s my benchmark). I don’t know that the AC would turn on the fan even if the fans/relays worked.
Are you sure the PCM supplies the ground? The PCM’s weren’t as sophisticated back then. Sometimes the ground came directly from the temp sensor in the bottom tank of the radiator or wherever the ECTS is located.
Anyway, first see if you can jumper the “contact” terminals and if the fan runs when you do. One relay should turn on only one fan, the other relay should turn on both fans. The one that only turns on one fan is the one one controlled by the coolant temperature.
If that works, then you will need to make stand offs to do any further troubleshooting.