Engine/Transmission (1994 - 1998) Stanco Relay fix for failed heater grids.

[Got Smoke?]

Here is the write up with pictures that I promised. Many know I haven't had heater grids all winter, and I've only been procrastinating on getting this done as well.



Anyways I ordered two Stanco Relays #120-902 from Allied Electronics last week. Phone #1-800-433-5700. Total came to $35 shipping and all..... 1/2! the cost of the factory pieces.



I did the following while following directions of a past post made by JoeBioDiesel.



First thing I did was remove the battery & intake horn and tie the intercooler tube up away for a little more elbow room.

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Three sheet 10MM sheet metal screws are all that holds the bracket in place. Just simply disconnect the 4 small push on contectors, and unbolt the 4 13mm nuts holding the pos. cables on. Here's the old relays and mount

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Here's a side by side comparaison. The new Stanco's are a lot more beefier.

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Simply push the tabs back holding the old relays on and they will fall right off.

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Next cut the tabs off with some wire cutters, and grind the remains off. You may have to grind a little more too to fit the relays over the bracket. We did.

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Hold the new relays in place with the indention on the back of the relay in the indention in the bracket, and fold the brackets back around the OEM mount.

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Here's a top view of the new relays on the OEM mount.

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Here's the mount reinstalled... .

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NOW I HAVE HEATER GRIDS! Oo.

 

[Tardog]

Nicely explained.

 

[ABorchard]

When mine went out I used Ford starter relays from Napa, $8. 56 each.

Been working great for four winters.

 

[McCRam]

Thanks for the info. I'll use this if mine go out.

 

[Jim Fulmer]

Wanted to bring this back to the top! Mark's fix is a good one! I ran into this over the last few months trying to start my( a little low compression) truck and it was getting painful but was fine when it was plugged in. Well the relay fix was dead on and she starts quite well now! The only issue I have is the heaters don't run long enough... . I might need manual control as they turn off by say a minute.



Jim

 

[Signal73]

I used this fix for the relay setup in the fall and so far so good! Truck was starting like crap without grid help, Now without plugging in 10° starts arent a problem.

Jim

 

[Howard Durand]

Jim Fulmer,

It would be easy enough to override the timer for the grids but I don't think you would want to keep either of them turned on much longer than they are now.

 

[Jim Fulmer]

Jim Fulmer,

It would be easy enough to override the timer for the grids but I don't think you would want to keep either of them turned on much longer than they are now.

There times when I would like them to turn on and they don't, but when you fire it up they cycle as then the air temp is low enough to send the signal. If it does this you can look down at the air temp gauge and it is too high for them to turn on.



Jim

 

[Howard Durand]

Jim Fulmer,



The PCM wants to see a manifold temperature of 59 degrees F or lower for the grid heaters to cycle when the ignition switch is turned on. The colder it is, the longer it stays on. After the engine starts, they continue to cycle on the "post-heat" cycle for a set period of time.

Try unplugging the sensor before starting, the heaters should cycle with the switch on no matter what the ambient temperature is.

If they still don't cycle, you may have a PCM problem. Possibly need to re-program the PCM.



I just tried this on mine and, with the sensor unplugged, the heater comes on an stays on for the maximum time. When it shuts off, the wait to start light blinks on and off.



Hope this helps.



Howard

 

[Jim Fulmer]

Thanks Howard, I think I'm just going to try that little... ... trick the system.



Jim

 

[Jim Fulmer]

Alright you win the gold metal, it works like you were talking about, So what is the maximum time based on... . the temp scale in the shop manual?



Can I interrupt one of the wires and make them come on when I want to via switch?



Mine is a little different than a stocker truck, there are times when I wish they would cycle so it can start easier... ... ... I hate beating on a starter or 6-8 seconds. Once it's warm or if the grids come on it starts well.



Jim

 

[Howard Durand]

Jim,

The PCM provides ground to turn on the two relays. You could do the same thing with a switch connected between either one and ground.



I don't know how long each grid can be turned on without damaging them. The way I read the manual, if it is cold enough, both grids will be turned on at the same time. For higher temperatures, it cycles them alternately one at a time.

The grids are basically a dead short when energized. Their resistance increases as they heat up and at some point will either quit getting any hotter or burn in two. I don't know which comes first. I am guessing that they would probably drag the battery voltage down before actually burning in two.



The PCM has it's own idea about how long to keep them on, either for the pre-heat cycle or the post-heat cycle. It uses manifold temperature, engine rpm, vehicle speed and time in it's calculations and, since I haven't had any problems, I try to believe it knows what it is doing. I just wish it understood transmission shifting a little better.



To make them do what you want, you could use a momentary type pushbutton switch that is only on as long as the button is being held in.

Or you could use an electronic timer, operated by a push button, that would hold the heater on for a preset amount of time. Either way you would not be able to forget and leave it on.



I don't think you would need to cut the wires that go to the PCM, just connect your switch to the ground side of either relay in parallel with the PCM. Then it can be controlled both ways.

 

[Jim Fulmer]

Almost a year old

Had to bring it back to the top for an update! I've got another truck with a (no cycling grids) issue. If you pull the plug off in the intake temp sensor it will go into default mode and do a full cycle but the PCM is not turning them on for a normal cycle when selected.



I remember reading along time ago that the PCM sends a 5 VDC signal to the relay's to turn on, my thinking is he should check this signal with a multimeter!



Is there anything that I'm not thinking about right now that I can tell him to check or try.



On a side note mine busted right off the other day with the temps in the 20's after sitting for 3 weeks.



Jim

 

[Howard Durand]

Jim,



If I understand the circuit right, when you pull the plug off the temperature sensor, the PCM will see maximum resistance across the sensor wires which would correspond to a very low intake temperature. This would cause the PCM to initiate a grid heating cycle.



The operating coils of the grid relays are both continuously supplied with 12 volts whenever the ignition switch is turned on. When the PCM sees an intake temperature at or below 59* F, it provides ground to each of the grid relays, turning them on and off alternately, until the cycle is complete.



The feed to the temperature sensor is probably the 5 volts you are referring to. The temperature sensor is a heat sensitive variable resistor and causes the voltage across it to vary with changes in temperature. The voltage fed to it from the PCM is modified by the sensor and returned to the PCM where it controls the internal circuit that actuates the grids.



What you should see at the PCM terminals for the relays would be either 12 volts when there is no call for the heaters and zero volts when they are turned on.



I have to admit that I have not verified all of this but, based on the electrical diagram in my service manual, I believe that is how it works.

Since the grids do turn on when disconnecting the cable, I would suspect the sensor itself of being bad.

 

[Jim Fulmer]

Since the grids do turn on when disconnecting the cable, I would suspect the sensor itself of being bad.

That's what I said so he over nighted' one in and changed it... ... no joy! I guess I'm going to end up going over there and fixing it.



Jim

 

[Howard Durand]

Dumb question, was it cold enough for it to turn on when he changed out the sensor?



A good test of the system would be to get a 50,000 ohm resistor and insert it into the plug. That would simulate a temperature of about 15* F. The grids should cycle.



If I had this problem and it turned out to be a PCM failure, I would use a switch with a timer to control them.



Another thought is the possibility of contamination. Make sure the plug is clean inside as well as where the wires connect at the PCM.

 

[Jim Fulmer]

It was definitely cold enough, he's got my service manual now to do some troubleshooting, we'll see what happens and go from there.



Jim

 

[nutter3]

Posts like this

Are why I keep renewing my membership. My truck has been starting like crap all winter, unless it was plugged it, then it would start just fine.

Well I got digging around today and found no voltage at the air heater element.

Got 12 volts going from the battery to both relays,

12 volts from the PCM (on the orange/white and yellow/white) when the key is turned on and the wait to start light is on. This drops to 1. 95VDC when the wait to start light goes out. Is that correct?

I put 12 volts to the relay to try switching it and it did not work. So I figure it must be the relays. I ordered the two Stanco ones from allied electronics.



Thanks for the great information!

 

[lowboy]

Opposite heater question

I'm not trying to hijack the thread... . but I've got the opposite problem. The heaters cycle when there is no way they should. Best I can remember, this truck (my Dad's '98 12V) has done this from day one. Is there something in the FAQs which will show me how to wire an "interupt" switch into the circuit so that I can keep the heaters from cycling?



Thanks,

Don

 

[GAmes]

Mine cycled on and stayed when it was in warranty. It boiled the batteries into oblivion and finally burned up the relay. My wife took it to the dealer in Newport News, Va and the low life charged her for parts. (FAIR WARNING TO VIRGINIANS!) I now have Stanco relays with the ground wires wired to a toggle switch. I have found that if I forget to turn them off or turn on the switch by accident it will overload the alternator and it goes off line. I have to shut the engine off, wait a couple seconds and restart, then all is well.