hot relay

mr.flat6 · 10-27-2002, 01:48 PM

1981 911sc

on my fuse box there are 5 black round relays & i red one.
one of the black ones is always hot to the touch. the others are cool.
comments? is this normal?

tx!
john

Ove · 06:10 PM

Which relay is the hot one? What electric part does it control?
The fuelpump? Do you have a fuse in this circut?

In my car there are only two (horn and fuelpump).
The fuelpump relay is warm, but not hot after a drive.
Don't think any relay should get that hot. Try to replace it with one of the other relays to see if that makes a difference.

The reason it becomes hot is the resistance in the circut. You either have a lot of current (too much) going through it or there is a lot of resistance. I would check for corrotion around the relay pins. The corrotion makes it hard for electricity to pass through, and things will heat up. Spread each relay pin a little bit so they make good contact. You may have seen melted wire insulation on 911s near the screws attaching the wires to the fuse holders in the fuse box. Same problem here, wires corrode, resistance increases and they heat up.

[edit: The red relay is for the fuelpump. Check that the red one actually is the one that operates the pump. Not sure if the black ones get hotter than the red. On my car I have a black one to power the pump, and it is not too hot.]

Doug Zielke · 10-27-2002, 02:58 PM

Ove is on the right track here....check *every* connection for corrotion.

911pcars · 04:54 PM

"....The reason it becomes hot is the resistance in the circut. You either have a lot of current (too much) going through it or there is a lot of resistance."

Slight correction in the above statement. If there is increased resistance in the relay, there will be less current flow, not more (Ohm's Law: I=E/R). The increased heat is due to increased current flow in one of the two circuits within the relay. The control circuit is the low current path that connects to the switch. Closing this circuit energizes the coil of wire inside the relay and causes the contacts to close. Increased current flow (more heat) in the control circuit is usually caused by a short circuit in the coil windings or in the switch/wire path.

The contact circuit connects the source voltage (usually the battery) to the load (accessory). If this circuit contains a short (bare wire touches ground or a malfunction in the load), current flow increases which will overheat the entire circuit, including the relay. A fuse usually protects an overheated circuit, but may not if the current flow isn't quite high enough. Some circuits in a Porsche are not fuse protected at all!! (duh). If relay contact surfaces are worn or poor, there may be insufficient area to carry the necessary current flow and thus will overheat; sort of like using 10 gauge wire instead of proper battery cables to jump start a car. On the other hand, if there is excessive resistance in the contact circuit of a relay (corrosion, loose connection, etc.), there will be less available voltage (and current) and the load may not work correctly (dim bulb, slow motor, etc.). Excessive resistance in the control circuit may prevent the relay from working at all (no click - no contact).

Sherwood Lee
http://members.rennlist.org/911pcars

10/30 correction:
" Increased current flow (more heat) in the control circuit is usually caused by a short circuit in the coil windings or in the switch/wire path.
"
Depending on the location of the malfunction, a short circuit in the switch/wire path may cause the relay to energize all the time. Any overheating here is due to the relay being constantly ON (no extra current flows through the relay as the control circuit coil winding is the load).

Wil Ferch · 10-30-2002, 08:34 AM

AFAIK...he red relays do NOT have a higher load carrying capacity compared to the black ones ( where do we get this stuff ???).

The red relays have a diode in the circuit...a sort of electrical "check valve" that prevents reverse current...that's all !

Anyone else...feel free to chime in if wrong

--Wil Ferch

mr.flat6 · 10-30-2002, 08:50 AM

if that's the case for the red ones. why don't the black ones have the diode/reverse check too?

tx
john

Wil Ferch · 10-30-2002, 04:17 PM

Simple:
----Because the circuits they ( blacks) are installed in..don't require back flow protection. upshot? you can use red ones where black ones existed in the first place....but not vice versa.
OK ?

---Wil Ferch

Doug Zielke · 10-30-2002, 05:17 PM

The diode prevents arcing at the relay contacts due to back-induced EMF from high current motor loads (like the fuel pump).

Ove · 06:09 PM

Thanks for correcting me on the relays, Wil,
Leaves me with one question though. On my car there is no red relay, the previous owner obviously substituted it with a black one. Was going to replace it anyway, but thought the only risk in using it temporarely was blowing the relay because it was weaker somehow. Now I wonder if there is a risk of current "backfiring" from the fuelpump and damaging other electrical components on its way. If not, what is the purpose of using the diode in this circut, and what damage could occur if I don't have one?

[edit: Seems like Doug allready answered my question. Thanks. ]

Sherwood,
I wrote:
"....The reason it becomes hot is the resistance in the circut. You either have a lot of current (too much) going through it or there is a lot of resistance."
And you commented:
"Slight correction in the above statement. If there is increased resistance in the relay, there will be less current flow, not more (Ohm's Law: I=E/R)."
I didn't say that there would be more current flow if the resistance increased.

Wil Ferch · 10-31-2002, 05:01 AM

Ove:
Taking all the responses together...it looks like you want to put in a red relay where it is intended to be..that might cure the problem.

--Wil Ferch

targa80 · targa80's Garage

The purpose of the diode is to prevent relay chatter (momentary contact) of the contacts. When the power is turned off and the circuits instantaneous voltage is collapsing a reverse current is created causing the coil to close the contacts momemtarily. Arching can occur in circuits with high current demands reducing the life of the relay contacts. The diode shorts the reverse current past the coil preventing the chatter. All of the electronic equipment I work on uses a diode in the relay coil circuits specifically for that purpose.